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  25 </div>
  26 <div class="section">
  27 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  28 <a name="proto.users_guide"></a><a class="link" href="users_guide.html" title="Users' Guide">Users' Guide</a>
  29 </h2></div></div></div>
  30 <div class="toc"><dl class="toc">
  31 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started">Getting Started</a></span></dt>
  32 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end">Fronts Ends: Defining
  33       Terminals and Non-Terminals of Your EDSL</a></span></dt>
  34 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form">Intermediate
  35       Form: Understanding and Introspecting Expressions</a></span></dt>
  36 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end">Back Ends: Making Expression
  37       Templates Do Useful Work</a></span></dt>
  38 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples">Examples</a></span></dt>
  39 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.resources">Background and Resources</a></span></dt>
  40 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.glossary">Glossary</a></span></dt>
  41 </dl></div>
  42 <h4>
  43 <a name="proto.users_guide.h0"></a>
  44       <span class="phrase"><a name="proto.users_guide.compilers__compiler_construction_toolkits__and_proto"></a></span><a class="link" href="users_guide.html#proto.users_guide.compilers__compiler_construction_toolkits__and_proto">Compilers,
  45       Compiler Construction Toolkits, and Proto</a>
  46     </h4>
  47 <p>
  48       Most compilers have front ends and back ends. The front end parses the text
  49       of an input program into some intermediate form like an abstract syntax tree,
  50       and the back end takes the intermediate form and generates an executable from
  51       it.
  52     </p>
  53 <p>
  54       A library built with Proto is essentially a compiler for an embedded domain-specific
  55       language (EDSL). It also has a front end, an intermediate form, and a back
  56       end. The front end is comprised of the symbols (a.k.a., terminals), members,
  57       operators and functions that make up the user-visible aspects of the EDSL.
  58       The back end is made of evaluation contexts and transforms that give meaning
  59       and behavior to the expression templates generated by the front end. In between
  60       is the intermediate form: the expression template itself, which is an abstract
  61       syntax tree in a very real sense.
  62     </p>
  63 <p>
  64       To build a library with Proto, you will first decide what your interface will
  65       be; that is, you'll design a programming language for your domain and build
  66       the front end with tools provided by Proto. Then you'll design the back end
  67       by writing evaluation contexts and/or transforms that accept expression templates
  68       and do interesting things with them.
  69     </p>
  70 <p>
  71       This users' guide is organized as follows. After a <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started" title="Getting Started">Getting
  72       Started guide</a>, we'll cover the tools Proto provides for defining and
  73       manipulating the three major parts of a compiler:
  74     </p>
  75 <div class="variablelist">
  76 <p class="title"><b></b></p>
  77 <dl class="variablelist">
  78 <dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.front_end" title="Fronts Ends: Defining Terminals and Non-Terminals of Your EDSL">Front Ends</a></span></dt>
  79 <dd><p>
  80             How to define the aspects of your EDSL with which your users will interact
  81             directly.
  82           </p></dd>
  83 <dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form" title="Intermediate Form: Understanding and Introspecting Expressions">Intermediate
  84         Form</a></span></dt>
  85 <dd><p>
  86             What Proto expression templates look like, how to discover their structure
  87             and access their constituents.
  88           </p></dd>
  89 <dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end" title="Back Ends: Making Expression Templates Do Useful Work">Back Ends</a></span></dt>
  90 <dd><p>
  91             How to define evaluation contexts and transforms that make expression
  92             templates do interesting things.
  93           </p></dd>
  94 </dl>
  95 </div>
  96 <p>
  97       After that, you may be interested in seeing some <a class="link" href="users_guide.html#boost_proto.users_guide.examples" title="Examples">Examples</a>
  98       to get a better idea of how the pieces all fit together.
  99     </p>
 100 <div class="section">
 101 <div class="titlepage"><div><div><h3 class="title">
 102 <a name="boost_proto.users_guide.getting_started"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started" title="Getting Started">Getting Started</a>
 103 </h3></div></div></div>
 104 <div class="toc"><dl class="toc">
 105 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto">Installing
 106         Proto</a></span></dt>
 107 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.naming">Naming
 108         Conventions</a></span></dt>
 109 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.hello_world">Hello
 110         World</a></span></dt>
 111 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator">Hello
 112         Calculator</a></span></dt>
 113 </dl></div>
 114 <div class="section">
 115 <div class="titlepage"><div><div><h4 class="title">
 116 <a name="boost_proto.users_guide.getting_started.installing_proto"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto" title="Installing Proto">Installing
 117         Proto</a>
 118 </h4></div></div></div>
 119 <h6>
 120 <a name="boost_proto.users_guide.getting_started.installing_proto.h0"></a>
 121           <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.getting_proto"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.getting_proto">Getting
 122           Proto</a>
 123         </h6>
 124 <p>
 125           You can get Proto by downloading Boost (Proto is in version 1.37 and later),
 126           or by accessing Boost's SVN repository on SourceForge.net. Just go to
 127           <a href="http://svn.boost.org/trac/boost/wiki/BoostSubversion" target="_top">http://svn.boost.org/trac/boost/wiki/BoostSubversion</a>
 128           and follow the instructions there for anonymous SVN access.
 129         </p>
 130 <h6>
 131 <a name="boost_proto.users_guide.getting_started.installing_proto.h1"></a>
 132           <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.building_with_proto"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.building_with_proto">Building
 133           with Proto</a>
 134         </h6>
 135 <p>
 136           Proto is a header-only template library, which means you don't need to
 137           alter your build scripts or link to any separate lib file to use it. All
 138           you need to do is <code class="computeroutput"><span class="preprocessor">#include</span>
 139           <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>. Or, you might decide to just include
 140           the core of Proto (<code class="computeroutput"><span class="preprocessor">#include</span>
 141           <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>) and whichever contexts and transforms
 142           you happen to use.
 143         </p>
 144 <h6>
 145 <a name="boost_proto.users_guide.getting_started.installing_proto.h2"></a>
 146           <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.requirements"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.requirements">Requirements</a>
 147         </h6>
 148 <p>
 149           Proto depends on Boost. You must use either Boost version 1.34.1 or higher,
 150           or the version in SVN trunk.
 151         </p>
 152 <h6>
 153 <a name="boost_proto.users_guide.getting_started.installing_proto.h3"></a>
 154           <span class="phrase"><a name="boost_proto.users_guide.getting_started.installing_proto.supported_compilers"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.installing_proto.supported_compilers">Supported
 155           Compilers</a>
 156         </h6>
 157 <p>
 158           Currently, Boost.Proto is known to work on the following compilers:
 159         </p>
 160 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 161 <li class="listitem">
 162               Visual C++ 8 and higher
 163             </li>
 164 <li class="listitem">
 165               GNU C++ 3.4 and higher
 166             </li>
 167 <li class="listitem">
 168               Intel on Linux 8.1 and higher
 169             </li>
 170 <li class="listitem">
 171               Intel on Windows 9.1 and higher
 172             </li>
 173 </ul></div>
 174 <div class="note"><table border="0" summary="Note">
 175 <tr>
 176 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
 177 <th align="left">Note</th>
 178 </tr>
 179 <tr><td align="left" valign="top"><p>
 180             Please send any questions, comments and bug reports to eric &lt;at&gt;
 181             boostpro &lt;dot&gt; com.
 182           </p></td></tr>
 183 </table></div>
 184 </div>
 185 <div class="section">
 186 <div class="titlepage"><div><div><h4 class="title">
 187 <a name="boost_proto.users_guide.getting_started.naming"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming" title="Naming Conventions">Naming
 188         Conventions</a>
 189 </h4></div></div></div>
 190 <p>
 191           Proto is a large library and probably quite unlike any library you've used
 192           before. Proto uses some consistent naming conventions to make it easier
 193           to navigate, and they're described below.
 194         </p>
 195 <h6>
 196 <a name="boost_proto.users_guide.getting_started.naming.h0"></a>
 197           <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.functions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.functions">Functions</a>
 198         </h6>
 199 <p>
 200           All of Proto's functions are defined in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
 201           namespace. For example, there is a function called <code class="computeroutput"><span class="identifier">value</span><span class="special">()</span></code> defined in <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
 202           that accepts a terminal expression and returns the terminal's value.
 203         </p>
 204 <h6>
 205 <a name="boost_proto.users_guide.getting_started.naming.h1"></a>
 206           <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.metafunctions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.metafunctions">Metafunctions</a>
 207         </h6>
 208 <p>
 209           Proto defines <span class="emphasis"><em>metafunctions</em></span> that correspond to each
 210           of Proto's free functions. The metafunctions are used to compute the functions'
 211           return types. All of Proto's metafunctions live in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span></code>
 212           namespace and have the same name as the functions to which they correspond.
 213           For instance, there is a class template <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;&gt;</span></code> that you can use to compute the
 214           return type of the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> function.
 215         </p>
 216 <h6>
 217 <a name="boost_proto.users_guide.getting_started.naming.h2"></a>
 218           <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.function_objects"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.function_objects">Function
 219           Objects</a>
 220         </h6>
 221 <p>
 222           Proto defines <span class="emphasis"><em>function object</em></span> equivalents of all of
 223           its free functions. (A function object is an instance of a class type that
 224           defines an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
 225           member function.) All of Proto's function object types are defined in the
 226           <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span></code> namespace and have the same
 227           name as their corresponding free functions. For example, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span></code> is a class that defines a function
 228           object that does the same thing as the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> free function.
 229         </p>
 230 <h6>
 231 <a name="boost_proto.users_guide.getting_started.naming.h3"></a>
 232           <span class="phrase"><a name="boost_proto.users_guide.getting_started.naming.primitive_transforms"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.naming.primitive_transforms">Primitive
 233           Transforms</a>
 234         </h6>
 235 <p>
 236           Proto also defines <span class="emphasis"><em>primitive transforms</em></span> -- class types
 237           that can be used to compose larger transforms for manipulating expression
 238           trees. Many of Proto's free functions have corresponding primitive transforms.
 239           These live in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
 240           namespace and their names have a leading underscore. For instance, the
 241           transform corresponding to the <code class="computeroutput"><span class="identifier">value</span><span class="special">()</span></code> function is called <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code>.
 242         </p>
 243 <p>
 244           The following table summarizes the discussion above:
 245         </p>
 246 <div class="table">
 247 <a name="boost_proto.users_guide.getting_started.naming.t0"></a><p class="title"><b>Table&#160;33.1.&#160;Proto Naming Conventions</b></p>
 248 <div class="table-contents"><table class="table" summary="Proto Naming Conventions">
 249 <colgroup>
 250 <col>
 251 <col>
 252 </colgroup>
 253 <thead><tr>
 254 <th>
 255                   <p>
 256                     Entity
 257                   </p>
 258                 </th>
 259 <th>
 260                   <p>
 261                     Example
 262                   </p>
 263                 </th>
 264 </tr></thead>
 265 <tbody>
 266 <tr>
 267 <td>
 268                   <p>
 269                     Free Function
 270                   </p>
 271                 </td>
 272 <td>
 273                   <p>
 274                     <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code>
 275                   </p>
 276                 </td>
 277 </tr>
 278 <tr>
 279 <td>
 280                   <p>
 281                     Metafunction
 282                   </p>
 283                 </td>
 284 <td>
 285                   <p>
 286                     <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;&gt;</span></code>
 287                   </p>
 288                 </td>
 289 </tr>
 290 <tr>
 291 <td>
 292                   <p>
 293                     Function Object
 294                   </p>
 295                 </td>
 296 <td>
 297                   <p>
 298                     <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span></code>
 299                   </p>
 300                 </td>
 301 </tr>
 302 <tr>
 303 <td>
 304                   <p>
 305                     Transform
 306                   </p>
 307                 </td>
 308 <td>
 309                   <p>
 310                     <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code>
 311                   </p>
 312                 </td>
 313 </tr>
 314 </tbody>
 315 </table></div>
 316 </div>
 317 <br class="table-break">
 318 </div>
 319 <div class="section">
 320 <div class="titlepage"><div><div><h4 class="title">
 321 <a name="boost_proto.users_guide.getting_started.hello_world"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_world" title="Hello World">Hello
 322         World</a>
 323 </h4></div></div></div>
 324 <p>
 325           Below is a very simple program that uses Proto to build an expression template
 326           and then execute it.
 327         </p>
 328 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
 329 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 330 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 331 <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">;</span>
 332
 333 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span>
 334
 335 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
 336 <span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
 337 <span class="special">{</span>
 338     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span>
 339     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
 340 <span class="special">}</span>
 341
 342 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 343 <span class="special">{</span>
 344     <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="string">"hello"</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="string">" world"</span> <span class="special">);</span>
 345     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 346 <span class="special">}</span>
 347 </pre>
 348 <p>
 349           This program outputs the following:
 350         </p>
 351 <pre class="programlisting">hello, world
 352 </pre>
 353 <p>
 354           This program builds an object representing the output operation and passes
 355           it to an <code class="computeroutput"><span class="identifier">evaluate</span><span class="special">()</span></code>
 356           function, which then executes it.
 357         </p>
 358 <p>
 359           The basic idea of expression templates is to overload all the operators
 360           so that, rather than evaluating the expression immediately, they build
 361           a tree-like representation of the expression so that it can be evaluated
 362           later. For each operator in an expression, at least one operand must be
 363           Protofied in order for Proto's operator overloads to be found. In the expression
 364           ...
 365         </p>
 366 <pre class="programlisting"><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="string">"hello"</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="string">" world"</span>
 367 </pre>
 368 <p>
 369           ... the Protofied sub-expression is <code class="computeroutput"><span class="identifier">cout_</span></code>,
 370           which is the Proto-ification of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code>.
 371           The presence of <code class="computeroutput"><span class="identifier">cout_</span></code> "infects"
 372           the expression, and brings Proto's tree-building operator overloads into
 373           consideration. Any literals in the expression are then Protofied by wrapping
 374           them in a Proto terminal before they are combined into larger Proto expressions.
 375         </p>
 376 <p>
 377           Once Proto's operator overloads have built the expression tree, the expression
 378           can be lazily evaluated later by walking the tree. That is what <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>
 379           does. It is a general tree-walking expression evaluator, whose behavior
 380           is customizable via a <span class="emphasis"><em>context</em></span> parameter. The use of
 381           <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code>
 382           assigns the standard meanings to the operators in the expression. (By using
 383           a different context, you could give the operators in your expressions different
 384           semantics. By default, Proto makes no assumptions about what operators
 385           actually <span class="emphasis"><em>mean</em></span>.)
 386         </p>
 387 <h6>
 388 <a name="boost_proto.users_guide.getting_started.hello_world.h0"></a>
 389           <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_world.proto_design_philosophy"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_world.proto_design_philosophy">Proto
 390           Design Philosophy</a>
 391         </h6>
 392 <p>
 393           Before we continue, let's use the above example to illustrate an important
 394           design principle of Proto's. The expression template created in the <span class="emphasis"><em>hello
 395           world</em></span> example is totally general and abstract. It is not tied
 396           in any way to any particular domain or application, nor does it have any
 397           particular meaning or behavior on its own, until it is evaluated in a
 398           <span class="emphasis"><em>context</em></span>. Expression templates are really just heterogeneous
 399           trees, which might mean something in one domain, and something else entirely
 400           in a different one.
 401         </p>
 402 <p>
 403           As we'll see later, there is a way to create Proto expression trees that
 404           are <span class="emphasis"><em>not</em></span> purely abstract, and that have meaning and
 405           behaviors independent of any context. There is also a way to control which
 406           operators are overloaded for your particular domain. But that is not the
 407           default behavior. We'll see later why the default is often a good thing.
 408         </p>
 409 </div>
 410 <div class="section">
 411 <div class="titlepage"><div><div><h4 class="title">
 412 <a name="boost_proto.users_guide.getting_started.hello_calculator"></a><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello
 413         Calculator</a>
 414 </h4></div></div></div>
 415 <p>
 416           "Hello, world" is nice, but it doesn't get you very far. Let's
 417           use Proto to build a EDSL (embedded domain-specific language) for a lazily-evaluated
 418           calculator. We'll see how to define the terminals in your mini-language,
 419           how to compose them into larger expressions, and how to define an evaluation
 420           context so that your expressions can do useful work. When we're done, we'll
 421           have a mini-language that will allow us to declare a lazily-evaluated arithmetic
 422           expression, such as <code class="computeroutput"><span class="special">(</span><span class="identifier">_2</span>
 423           <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span>
 424           <span class="special">*</span> <span class="number">100</span></code>,
 425           where <code class="computeroutput"><span class="identifier">_1</span></code> and <code class="computeroutput"><span class="identifier">_2</span></code> are placeholders for values to be
 426           passed in when the expression is evaluated.
 427         </p>
 428 <h6>
 429 <a name="boost_proto.users_guide.getting_started.hello_calculator.h0"></a>
 430           <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.defining_terminals"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.defining_terminals">Defining
 431           Terminals</a>
 432         </h6>
 433 <p>
 434           The first order of business is to define the placeholders <code class="computeroutput"><span class="identifier">_1</span></code> and <code class="computeroutput"><span class="identifier">_2</span></code>.
 435           For that, we'll use the <code class="computeroutput"><a class="link" href="../boost/proto/terminal.html" title="Struct template terminal">proto::terminal&lt;&gt;</a></code>
 436           metafunction.
 437         </p>
 438 <pre class="programlisting"><span class="comment">// Define a placeholder type</span>
 439 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
 440 <span class="keyword">struct</span> <span class="identifier">placeholder</span>
 441 <span class="special">{};</span>
 442
 443 <span class="comment">// Define the Protofied placeholder terminals</span>
 444 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
 445 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span>
 446 </pre>
 447 <p>
 448           The initialization may look a little odd at first, but there is a good
 449           reason for doing things this way. The objects <code class="computeroutput"><span class="identifier">_1</span></code>
 450           and <code class="computeroutput"><span class="identifier">_2</span></code> above do not require
 451           run-time construction -- they are <span class="emphasis"><em>statically initialized</em></span>,
 452           which means they are essentially initialized at compile time. See the
 453           <a class="link" href="appendices.html#boost_proto.appendices.rationale.static_initialization" title="Static Initialization">Static
 454           Initialization</a> section in the <a class="link" href="appendices.html#boost_proto.appendices.rationale" title="Appendix C: Rationale">Rationale</a>
 455           appendix for more information.
 456         </p>
 457 <h6>
 458 <a name="boost_proto.users_guide.getting_started.hello_calculator.h1"></a>
 459           <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.constructing_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.constructing_expression_trees">Constructing
 460           Expression Trees</a>
 461         </h6>
 462 <p>
 463           Now that we have terminals, we can use Proto's operator overloads to combine
 464           these terminals into larger expressions. So, for instance, we can immediately
 465           say things like:
 466         </p>
 467 <pre class="programlisting"><span class="comment">// This builds an expression template</span>
 468 <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">;</span>
 469 </pre>
 470 <p>
 471           This creates an expression tree with a node for each operator. The type
 472           of the resulting object is large and complex, but we are not terribly interested
 473           in it right now.
 474         </p>
 475 <p>
 476           So far, the object is just a tree representing the expression. It has no
 477           behavior. In particular, it is not yet a calculator. Below we'll see how
 478           to make it a calculator by defining an evaluation context.
 479         </p>
 480 <h6>
 481 <a name="boost_proto.users_guide.getting_started.hello_calculator.h2"></a>
 482           <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.evaluating_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.evaluating_expression_trees">Evaluating
 483           Expression Trees</a>
 484         </h6>
 485 <p>
 486           No doubt you want your expression templates to actually <span class="emphasis"><em>do</em></span>
 487           something. One approach is to define an <span class="emphasis"><em>evaluation context</em></span>.
 488           The context is like a function object that associates behaviors with the
 489           node types in your expression tree. The following example should make it
 490           clear. It is explained below.
 491         </p>
 492 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">calculator_context</span>
 493   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">&gt;</span>
 494 <span class="special">{</span>
 495     <span class="comment">// Values to replace the placeholders</span>
 496     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">args</span><span class="special">;</span>
 497
 498     <span class="comment">// Define the result type of the calculator.</span>
 499     <span class="comment">// (This makes the calculator_context "callable".)</span>
 500     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
 501
 502     <span class="comment">// Handle the placeholders:</span>
 503     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
 504     <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;)</span> <span class="keyword">const</span>
 505     <span class="special">{</span>
 506         <span class="keyword">return</span> <span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">args</span><span class="special">[</span><span class="identifier">I</span><span class="special">];</span>
 507     <span class="special">}</span>
 508 <span class="special">};</span>
 509 </pre>
 510 <p>
 511           In <code class="computeroutput"><span class="identifier">calculator_context</span></code>,
 512           we specify how Proto should evaluate the placeholder terminals by defining
 513           the appropriate overloads of the function call operator. For any other
 514           nodes in the expression tree (e.g., arithmetic operations or non-placeholder
 515           terminals), Proto will evaluate the expression in the "default"
 516           way. For example, a binary plus node is evaluated by first evaluating the
 517           left and right operands and adding the results. Proto's default evaluator
 518           uses the <a href="../../../libs/typeof/index.html" target="_top">Boost.Typeof</a>
 519           library to compute return types.
 520         </p>
 521 <p>
 522           Now that we have an evaluation context for our calculator, we can use it
 523           to evaluate our arithmetic expressions, as below:
 524         </p>
 525 <pre class="programlisting"><span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
 526 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">45</span><span class="special">);</span> <span class="comment">// the value of _1 is 45</span>
 527 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">50</span><span class="special">);</span> <span class="comment">// the value of _2 is 50</span>
 528
 529 <span class="comment">// Create an arithmetic expression and immediately evaluate it</span>
 530 <span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">,</span> <span class="identifier">ctx</span> <span class="special">);</span>
 531
 532 <span class="comment">// This prints "10"</span>
 533 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">d</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 534 </pre>
 535 <p>
 536           Later, we'll see how to define more interesting evaluation contexts and
 537           expression transforms that give you total control over how your expressions
 538           are evaluated.
 539         </p>
 540 <h6>
 541 <a name="boost_proto.users_guide.getting_started.hello_calculator.h3"></a>
 542           <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.customizing_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.customizing_expression_trees">Customizing
 543           Expression Trees</a>
 544         </h6>
 545 <p>
 546           Our calculator EDSL is already pretty useful, and for many EDSL scenarios,
 547           no more would be needed. But let's keep going. Imagine how much nicer it
 548           would be if all calculator expressions overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> so that they could be used as function
 549           objects. We can do that by creating a calculator <span class="emphasis"><em>domain</em></span>
 550           and telling Proto that all expressions in the calculator domain have extra
 551           members. Here is how to define a calculator domain:
 552         </p>
 553 <pre class="programlisting"><span class="comment">// Forward-declare an expression wrapper</span>
 554 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
 555 <span class="keyword">struct</span> <span class="identifier">calculator</span><span class="special">;</span>
 556
 557 <span class="comment">// Define a calculator domain. Expression within</span>
 558 <span class="comment">// the calculator domain will be wrapped in the</span>
 559 <span class="comment">// calculator&lt;&gt; expression wrapper.</span>
 560 <span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
 561   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calculator</span><span class="special">&gt;</span> <span class="special">&gt;</span>
 562 <span class="special">{};</span>
 563 </pre>
 564 <p>
 565           The <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code>
 566           type will be an expression wrapper. It will behave just like the expression
 567           that it wraps, but it will have extra member functions that we will define.
 568           The <code class="computeroutput"><span class="identifier">calculator_domain</span></code> is
 569           what informs Proto about our wrapper. It is used below in the definition
 570           of <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code>.
 571           Read on for a description.
 572         </p>
 573 <pre class="programlisting"><span class="comment">// Define a calculator expression wrapper. It behaves just like</span>
 574 <span class="comment">// the expression it wraps, but with an extra operator() member</span>
 575 <span class="comment">// function that evaluates the expression.    </span>
 576 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
 577 <span class="keyword">struct</span> <span class="identifier">calculator</span>
 578   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
 579 <span class="special">{</span>
 580     <span class="keyword">typedef</span>
 581         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
 582     <span class="identifier">base_type</span><span class="special">;</span>
 583
 584     <span class="identifier">calculator</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span>
 585       <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span>
 586     <span class="special">{}</span>
 587
 588     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
 589
 590     <span class="comment">// Overload operator() to invoke proto::eval() with</span>
 591     <span class="comment">// our calculator_context.</span>
 592     <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">const</span>
 593     <span class="special">{</span>
 594         <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
 595         <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span>
 596         <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a2</span><span class="special">);</span>
 597
 598         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
 599     <span class="special">}</span>
 600 <span class="special">};</span>
 601 </pre>
 602 <p>
 603           The <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code>
 604           struct is an expression <span class="emphasis"><em>extension</em></span>. It uses <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;&gt;</span></code>
 605           to effectively add additional members to an expression type. When composing
 606           larger expressions from smaller ones, Proto notes what domain the smaller
 607           expressions are in. The larger expression is in the same domain and is
 608           automatically wrapped in the domain's extension wrapper.
 609         </p>
 610 <p>
 611           All that remains to be done is to put our placeholders in the calculator
 612           domain. We do that by wrapping them in our <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> wrapper, as below:
 613         </p>
 614 <pre class="programlisting"><span class="comment">// Define the Protofied placeholder terminals, in the</span>
 615 <span class="comment">// calculator domain.</span>
 616 <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span>
 617 <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span>
 618 </pre>
 619 <p>
 620           Any larger expression that contain these placeholders will automatically
 621           be wrapped in the <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> wrapper and have our <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
 622           overload. That means we can use them as function objects as follows.
 623         </p>
 624 <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">result</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">)(</span><span class="number">45.0</span><span class="special">,</span> <span class="number">50.0</span><span class="special">);</span>
 625 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">result</span> <span class="special">==</span> <span class="special">(</span><span class="number">50.0</span> <span class="special">-</span> <span class="number">45.0</span><span class="special">)</span> <span class="special">/</span> <span class="number">50.0</span> <span class="special">*</span> <span class="number">100</span><span class="special">));</span>
 626 </pre>
 627 <p>
 628           Since calculator expressions are now valid function objects, we can use
 629           them with standard algorithms, as shown below:
 630         </p>
 631 <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">56</span><span class="special">,</span> <span class="number">84</span><span class="special">,</span> <span class="number">37</span><span class="special">,</span> <span class="number">69</span> <span class="special">};</span>
 632 <span class="keyword">double</span> <span class="identifier">a2</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">65</span><span class="special">,</span> <span class="number">120</span><span class="special">,</span> <span class="number">60</span><span class="special">,</span> <span class="number">70</span> <span class="special">};</span>
 633 <span class="keyword">double</span> <span class="identifier">a3</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">0</span> <span class="special">};</span>
 634
 635 <span class="comment">// Use std::transform() and a calculator expression</span>
 636 <span class="comment">// to calculate percentages given two input sequences:</span>
 637 <span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a1</span><span class="special">+</span><span class="number">4</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,</span> <span class="identifier">a3</span><span class="special">,</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">);</span>
 638 </pre>
 639 <p>
 640           Now, let's use the calculator example to explore some other useful features
 641           of Proto.
 642         </p>
 643 <h6>
 644 <a name="boost_proto.users_guide.getting_started.hello_calculator.h4"></a>
 645           <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.detecting_invalid_expressions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.detecting_invalid_expressions">Detecting
 646           Invalid Expressions</a>
 647         </h6>
 648 <p>
 649           You may have noticed that you didn't have to define an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">-()</span></code>
 650           or <code class="computeroutput"><span class="keyword">operator</span><span class="special">/()</span></code>
 651           -- Proto defined them for you. In fact, Proto overloads <span class="emphasis"><em>all</em></span>
 652           the operators for you, even though they may not mean anything in your domain-specific
 653           language. That means it may be possible to create expressions that are
 654           invalid in your domain. You can detect invalid expressions with Proto by
 655           defining the <span class="emphasis"><em>grammar</em></span> of your domain-specific language.
 656         </p>
 657 <p>
 658           For simplicity, assume that our calculator EDSL should only allow addition,
 659           subtraction, multiplication and division. Any expression involving any
 660           other operator is invalid. Using Proto, we can state this requirement by
 661           defining the grammar of the calculator EDSL. It looks as follows:
 662         </p>
 663 <pre class="programlisting"><span class="comment">// Define the grammar of calculator expressions</span>
 664 <span class="keyword">struct</span> <span class="identifier">calculator_grammar</span>
 665   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
 666         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
 667       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
 668       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
 669       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
 670       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
 671     <span class="special">&gt;</span>
 672 <span class="special">{};</span>
 673 </pre>
 674 <p>
 675           You can read the above grammar as follows: an expression tree conforms
 676           to the calculator grammar if it is a binary plus, minus, multiplies or
 677           divides node, where both child nodes also conform to the calculator grammar;
 678           or if it is a terminal. In a Proto grammar, <code class="computeroutput"><a class="link" href="../boost/proto/_.html" title="Struct _">proto::_</a></code> is a wildcard that matches
 679           any type, so <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span>
 680           <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span></code>
 681           matches any terminal, whether it is a placeholder or a literal.
 682         </p>
 683 <div class="note"><table border="0" summary="Note">
 684 <tr>
 685 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
 686 <th align="left">Note</th>
 687 </tr>
 688 <tr><td align="left" valign="top"><p>
 689             This grammar is actually a little looser than we would like. Only placeholders
 690             and literals that are convertible to doubles are valid terminals. Later
 691             on we'll see how to express things like that in Proto grammars.
 692           </p></td></tr>
 693 </table></div>
 694 <p>
 695           Once you have defined the grammar of your EDSL, you can use the <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code> metafunction to check
 696           whether a given expression type conforms to the grammar. For instance,
 697           we might add the following to our <code class="computeroutput"><span class="identifier">calculator</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> overload:
 698         </p>
 699 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
 700 <span class="keyword">struct</span> <span class="identifier">calculator</span>
 701   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span> <span class="comment">/* ... as before ... */</span> <span class="special">&gt;</span>
 702 <span class="special">{</span>
 703     <span class="comment">/* ... */</span>
 704     <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">const</span>
 705     <span class="special">{</span>
 706         <span class="comment">// Check here that the expression we are about to</span>
 707         <span class="comment">// evaluate actually conforms to the calculator grammar.</span>
 708         <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_grammar</span><span class="special">&gt;));</span>
 709         <span class="comment">/* ... */</span>
 710     <span class="special">}</span>
 711 <span class="special">};</span>
 712 </pre>
 713 <p>
 714           The addition of the <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code> line enforces at compile time that we
 715           only evaluate expressions that conform to the calculator EDSL's grammar.
 716           With Proto grammars, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;&gt;</span></code> and <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code> it is very easy to give the users of
 717           your EDSL short and readable compile-time errors when they accidentally
 718           misuse your EDSL.
 719         </p>
 720 <div class="note"><table border="0" summary="Note">
 721 <tr>
 722 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
 723 <th align="left">Note</th>
 724 </tr>
 725 <tr><td align="left" valign="top"><p>
 726             <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code>
 727             is part of the Boost Metaprogramming Library. To use it, just <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>.
 728           </p></td></tr>
 729 </table></div>
 730 <h6>
 731 <a name="boost_proto.users_guide.getting_started.hello_calculator.h5"></a>
 732           <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.controlling_operator_overloads"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.controlling_operator_overloads">Controlling
 733           Operator Overloads</a>
 734         </h6>
 735 <p>
 736           Grammars and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;&gt;</span></code>
 737           make it possible to detect when a user has created an invalid expression
 738           and issue a compile-time error. But what if you want to prevent users from
 739           creating invalid expressions in the first place? By using grammars and
 740           domains together, you can disable any of Proto's operator overloads that
 741           would create an invalid expression. It is as simple as specifying the EDSL's
 742           grammar when you define the domain, as shown below:
 743         </p>
 744 <pre class="programlisting"><span class="comment">// Define a calculator domain. Expression within</span>
 745 <span class="comment">// the calculator domain will be wrapped in the</span>
 746 <span class="comment">// calculator&lt;&gt; expression wrapper.</span>
 747 <span class="comment">// NEW: Any operator overloads that would create an</span>
 748 <span class="comment">//      expression that does not conform to the</span>
 749 <span class="comment">//      calculator grammar is automatically disabled.</span>
 750 <span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
 751   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calculator</span><span class="special">&gt;,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
 752 <span class="special">{};</span>
 753 </pre>
 754 <p>
 755           The only thing we changed is we added <code class="computeroutput"><span class="identifier">calculator_grammar</span></code>
 756           as the second template parameter to the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;&gt;</span></code> template when defining <code class="computeroutput"><span class="identifier">calculator_domain</span></code>. With this simple addition,
 757           we disable any of Proto's operator overloads that would create an invalid
 758           calculator expression.
 759         </p>
 760 <h6>
 761 <a name="boost_proto.users_guide.getting_started.hello_calculator.h6"></a>
 762           <span class="phrase"><a name="boost_proto.users_guide.getting_started.hello_calculator.____and_much_more"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator.____and_much_more">...
 763           And Much More</a>
 764         </h6>
 765 <p>
 766           Hopefully, this gives you an idea of what sorts of things Proto can do
 767           for you. But this only scratches the surface. The rest of this users' guide
 768           will describe all these features and others in more detail.
 769         </p>
 770 <p>
 771           Happy metaprogramming!
 772         </p>
 773 </div>
 774 </div>
 775 <div class="section">
 776 <div class="titlepage"><div><div><h3 class="title">
 777 <a name="boost_proto.users_guide.front_end"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end" title="Fronts Ends: Defining Terminals and Non-Terminals of Your EDSL">Fronts Ends: Defining
 778       Terminals and Non-Terminals of Your EDSL</a>
 779 </h3></div></div></div>
 780 <div class="toc"><dl class="toc">
 781 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.making_terminals">Making
 782         Terminals</a></span></dt>
 783 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads">Proto's
 784         Operator Overloads</a></span></dt>
 785 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions">Making
 786         Lazy Functions</a></span></dt>
 787 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain">Customizing
 788         Expressions in Your Domain</a></span></dt>
 789 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.define_operators">Adapting
 790         Existing Types to Proto</a></span></dt>
 791 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.code_repetition">Generating
 792         Repetitive Code with the Preprocessor</a></span></dt>
 793 </dl></div>
 794 <p>
 795         Here is the fun part: designing your own mini-programming language. In this
 796         section we'll talk about the nuts and bolts of designing an EDSL interface
 797         using Proto. We'll cover the definition of terminals and lazy functions that
 798         the users of your EDSL will get to program with. We'll also talk about Proto's
 799         expression template-building operator overloads, and about ways to add additional
 800         members to expressions within your domain.
 801       </p>
 802 <div class="section">
 803 <div class="titlepage"><div><div><h4 class="title">
 804 <a name="boost_proto.users_guide.front_end.making_terminals"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_terminals" title="Making Terminals">Making
 805         Terminals</a>
 806 </h4></div></div></div>
 807 <p>
 808           As we saw with the Calculator example from the Introduction, the simplest
 809           way to get an EDSL up and running is simply to define some terminals, as
 810           follows.
 811         </p>
 812 <pre class="programlisting"><span class="comment">// Define a literal integer Proto expression.</span>
 813 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">};</span>
 814
 815 <span class="comment">// This creates an expression template.</span>
 816 <span class="identifier">i</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span>
 817 </pre>
 818 <p>
 819           With some terminals and Proto's operator overloads, you can immediately
 820           start creating expression templates.
 821         </p>
 822 <p>
 823           Defining terminals -- with aggregate initialization -- can be a little
 824           awkward at times. Proto provides an easier-to-use wrapper for literals
 825           that can be used to construct Protofied terminal expressions. It's called
 826           <code class="computeroutput"><a class="link" href="../boost/proto/literal.html" title="Struct template literal">proto::literal&lt;&gt;</a></code>.
 827         </p>
 828 <pre class="programlisting"><span class="comment">// Define a literal integer Proto expression.</span>
 829 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
 830
 831 <span class="comment">// Proto literals are really just Proto terminal expressions.</span>
 832 <span class="comment">// For example, this builds a Proto expression template:</span>
 833 <span class="identifier">i</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span>
 834 </pre>
 835 <p>
 836           There is also a <code class="computeroutput"><a class="link" href="../boost/proto/lit.html" title="Function lit">proto::lit()</a></code> function for constructing
 837           a <code class="computeroutput"><a class="link" href="../boost/proto/literal.html" title="Struct template literal">proto::literal&lt;&gt;</a></code> in-place. The above
 838           expression can simply be written as:
 839         </p>
 840 <pre class="programlisting"><span class="comment">// proto::lit(0) creates an integer terminal expression</span>
 841 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span>
 842 </pre>
 843 </div>
 844 <div class="section">
 845 <div class="titlepage"><div><div><h4 class="title">
 846 <a name="boost_proto.users_guide.front_end.proto_s_operator_overloads"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads" title="Proto's Operator Overloads">Proto's
 847         Operator Overloads</a>
 848 </h4></div></div></div>
 849 <p>
 850           Once we have some Proto terminals, expressions involving those terminals
 851           build expression trees for us. Proto defines overloads for each of C++'s
 852           overloadable operators in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
 853           namespace. As long as one operand is a Proto expression, the result of
 854           the operation is a tree node representing that operation.
 855         </p>
 856 <div class="note"><table border="0" summary="Note">
 857 <tr>
 858 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
 859 <th align="left">Note</th>
 860 </tr>
 861 <tr><td align="left" valign="top"><p>
 862             Proto's operator overloads live in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
 863             namespace and are found via ADL (argument-dependent lookup). That is
 864             why expressions must be "tainted" with Proto-ness for Proto
 865             to be able to build trees out of expressions.
 866           </p></td></tr>
 867 </table></div>
 868 <p>
 869           As a result of Proto's operator overloads, we can say:
 870         </p>
 871 <pre class="programlisting"><span class="special">-</span><span class="identifier">_1</span><span class="special">;</span>        <span class="comment">// OK, build a unary-negate tree node</span>
 872 <span class="identifier">_1</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span>    <span class="comment">// OK, build a binary-plus tree node</span>
 873 </pre>
 874 <p>
 875           For the most part, this Just Works and you don't need to think about it,
 876           but a few operators are special and it can be helpful to know how Proto
 877           handles them.
 878         </p>
 879 <h6>
 880 <a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.h0"></a>
 881           <span class="phrase"><a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.assignment__subscript__and_function_call_operators"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads.assignment__subscript__and_function_call_operators">Assignment,
 882           Subscript, and Function Call Operators</a>
 883         </h6>
 884 <p>
 885           Proto also overloads <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code>, and <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>, but these operators are member functions
 886           of the expression template rather than free functions in Proto's namespace.
 887           The following are valid Proto expressions:
 888         </p>
 889 <pre class="programlisting"><span class="identifier">_1</span> <span class="special">=</span> <span class="number">5</span><span class="special">;</span>     <span class="comment">// OK, builds a binary assign tree node</span>
 890 <span class="identifier">_1</span><span class="special">[</span><span class="number">6</span><span class="special">];</span>      <span class="comment">// OK, builds a binary subscript tree node</span>
 891 <span class="identifier">_1</span><span class="special">();</span>       <span class="comment">// OK, builds a unary function tree node</span>
 892 <span class="identifier">_1</span><span class="special">(</span><span class="number">7</span><span class="special">);</span>      <span class="comment">// OK, builds a binary function tree node</span>
 893 <span class="identifier">_1</span><span class="special">(</span><span class="number">8</span><span class="special">,</span><span class="number">9</span><span class="special">);</span>    <span class="comment">// OK, builds a ternary function tree node</span>
 894 <span class="comment">// ... etc.</span>
 895 </pre>
 896 <p>
 897           For the first two lines, assignment and subscript, it should be fairly
 898           unsurprising that the resulting expression node should be binary. After
 899           all, there are two operands in each expression. It may be surprising at
 900           first that what appears to be a function call with no arguments, <code class="computeroutput"><span class="identifier">_1</span><span class="special">()</span></code>,
 901           actually creates an expression node with one child. The child is <code class="computeroutput"><span class="identifier">_1</span></code> itself. Likewise, the expression
 902           <code class="computeroutput"><span class="identifier">_1</span><span class="special">(</span><span class="number">7</span><span class="special">)</span></code> has two
 903           children: <code class="computeroutput"><span class="identifier">_1</span></code> and <code class="computeroutput"><span class="number">7</span></code>.
 904         </p>
 905 <p>
 906           Because these operators can only be defined as member functions, the following
 907           expressions are invalid:
 908         </p>
 909 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
 910 <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">_1</span><span class="special">;</span>         <span class="comment">// ERROR: cannot assign _1 to an int</span>
 911
 912 <span class="keyword">int</span> <span class="special">*</span><span class="identifier">p</span><span class="special">;</span>
 913 <span class="identifier">p</span><span class="special">[</span><span class="identifier">_1</span><span class="special">];</span>          <span class="comment">// ERROR: cannot use _1 as an index</span>
 914
 915 <span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">(</span><span class="identifier">_1</span><span class="special">);</span>   <span class="comment">// ERROR: cannot call std::sin() with _1</span>
 916 </pre>
 917 <p>
 918           Also, C++ has special rules for overloads of <code class="computeroutput"><span class="keyword">operator</span><span class="special">-&gt;</span></code> that make it useless for building
 919           expression templates, so Proto does not overload it.
 920         </p>
 921 <h6>
 922 <a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.h1"></a>
 923           <span class="phrase"><a name="boost_proto.users_guide.front_end.proto_s_operator_overloads.the_address_of_operator"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.proto_s_operator_overloads.the_address_of_operator">The
 924           Address-Of Operator</a>
 925         </h6>
 926 <p>
 927           Proto overloads the address-of operator for expression types, so that the
 928           following code creates a new unary address-of tree node:
 929         </p>
 930 <pre class="programlisting"><span class="special">&amp;</span><span class="identifier">_1</span><span class="special">;</span>    <span class="comment">// OK, creates a unary address-of tree node</span>
 931 </pre>
 932 <p>
 933           It does <span class="emphasis"><em>not</em></span> return the address of the <code class="computeroutput"><span class="identifier">_1</span></code> object. However, there is special
 934           code in Proto such that a unary address-of node is implicitly convertible
 935           to a pointer to its child. In other words, the following code works and
 936           does what you might expect, but not in the obvious way:
 937         </p>
 938 <pre class="programlisting"><span class="keyword">typedef</span>
 939     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
 940 <span class="identifier">_1_type</span><span class="special">;</span>
 941
 942 <span class="identifier">_1_type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
 943 <span class="identifier">_1_type</span> <span class="keyword">const</span> <span class="special">*</span> <span class="identifier">p</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">_1</span><span class="special">;</span> <span class="comment">// OK, &amp;_1 implicitly converted</span>
 944 </pre>
 945 </div>
 946 <div class="section">
 947 <div class="titlepage"><div><div><h4 class="title">
 948 <a name="boost_proto.users_guide.front_end.making_lazy_functions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions" title="Making Lazy Functions">Making
 949         Lazy Functions</a>
 950 </h4></div></div></div>
 951 <p>
 952           If we limited ourselves to nothing but terminals and operator overloads,
 953           our embedded domain-specific languages wouldn't be very expressive. Imagine
 954           that we wanted to extend our calculator EDSL with a full suite of math
 955           functions like <code class="computeroutput"><span class="identifier">sin</span><span class="special">()</span></code>
 956           and <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code>
 957           that we could invoke lazily as follows.
 958         </p>
 959 <pre class="programlisting"><span class="comment">// A calculator expression that takes one argument</span>
 960 <span class="comment">// and takes the sine of it.</span>
 961 <span class="identifier">sin</span><span class="special">(</span><span class="identifier">_1</span><span class="special">);</span>
 962 </pre>
 963 <p>
 964           We would like the above to create an expression template representing a
 965           function invocation. When that expression is evaluated, it should cause
 966           the function to be invoked. (At least, that's the meaning of function invocation
 967           we'd like the calculator EDSL to have.) You can define <code class="computeroutput"><span class="identifier">sin</span></code>
 968           quite simply as follows.
 969         </p>
 970 <pre class="programlisting"><span class="comment">// "sin" is a Proto terminal containing a function pointer</span>
 971 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">double</span><span class="special">(*)(</span><span class="keyword">double</span><span class="special">)</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">sin</span> <span class="special">=</span> <span class="special">{&amp;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">};</span>
 972 </pre>
 973 <p>
 974           In the above, we define <code class="computeroutput"><span class="identifier">sin</span></code>
 975           as a Proto terminal containing a pointer to the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">()</span></code> function. Now we can use <code class="computeroutput"><span class="identifier">sin</span></code> as a lazy function. The <code class="computeroutput"><span class="identifier">default_context</span></code> that we saw in the Introduction
 976           knows how to evaluate lazy functions. Consider the following:
 977         </p>
 978 <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">pi</span> <span class="special">=</span> <span class="number">3.1415926535</span><span class="special">;</span>
 979 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span>
 980 <span class="comment">// Create a lazy "sin" invocation and immediately evaluate it</span>
 981 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="identifier">sin</span><span class="special">(</span><span class="identifier">pi</span><span class="special">/</span><span class="number">2</span><span class="special">),</span> <span class="identifier">ctx</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 982 </pre>
 983 <p>
 984           The above code prints out:
 985         </p>
 986 <pre class="programlisting">1</pre>
 987 <p>
 988           I'm no expert at trigonometry, but that looks right to me.
 989         </p>
 990 <p>
 991           We can write <code class="computeroutput"><span class="identifier">sin</span><span class="special">(</span><span class="identifier">pi</span><span class="special">/</span><span class="number">2</span><span class="special">)</span></code> because the <code class="computeroutput"><span class="identifier">sin</span></code>
 992           object, which is a Proto terminal, has an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()()</span></code> that builds a node representing a function
 993           call invocation. The actual type of <code class="computeroutput"><span class="identifier">sin</span><span class="special">(</span><span class="identifier">pi</span><span class="special">/</span><span class="number">2</span><span class="special">)</span></code> is actually
 994           something like this:
 995         </p>
 996 <pre class="programlisting"><span class="comment">// The type of the expression sin(pi/2):</span>
 997 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span>
 998     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">double</span><span class="special">(*)(</span><span class="keyword">double</span><span class="special">)</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="special">&amp;</span>
 999     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_child</span><span class="special">&lt;</span> <span class="keyword">double</span> <span class="keyword">const</span> <span class="special">&gt;::</span><span class="identifier">type</span>
1000 <span class="special">&gt;::</span><span class="identifier">type</span>
1001 </pre>
1002 <p>
1003           This type further expands to an unsightly node type with a <span class="emphasis"><em>tag</em></span>
1004           type of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code> and two children: the first
1005           representing the function to be invoked, and the second representing the
1006           argument to the function. (Node tag types describe the operation that created
1007           the node. The difference between <code class="computeroutput"><span class="identifier">a</span>
1008           <span class="special">+</span> <span class="identifier">b</span></code>
1009           and <code class="computeroutput"><span class="identifier">a</span> <span class="special">-</span>
1010           <span class="identifier">b</span></code> is that the former has tag
1011           type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code> and the latter has tag type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span></code>. Tag types are pure compile-time
1012           information.)
1013         </p>
1014 <div class="note"><table border="0" summary="Note">
1015 <tr>
1016 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
1017 <th align="left">Note</th>
1018 </tr>
1019 <tr><td align="left" valign="top"><p>
1020             In the type computation above, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_child</span><span class="special">&lt;&gt;</span></code> is a metafunction that ensures
1021             its argument is a Proto expression type. If it isn't one already, it
1022             becomes a Proto terminal. We'll learn more about this metafunction, along
1023             with <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>, its runtime counterpart,
1024             <a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child" title="Controlling How Child Expressions Are Captured">later</a>.
1025             For now, you can forget about it.
1026           </p></td></tr>
1027 </table></div>
1028 <p>
1029           It is important to note that there is nothing special about terminals that
1030           contain function pointers. <span class="emphasis"><em>Any</em></span> Proto expression has
1031           an overloaded function call operator. Consider:
1032         </p>
1033 <pre class="programlisting"><span class="comment">// This compiles!</span>
1034 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)(</span><span class="number">2</span><span class="special">)(</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">)(</span><span class="number">5</span><span class="special">,</span><span class="number">6</span><span class="special">,</span><span class="number">7</span><span class="special">,</span><span class="number">8</span><span class="special">);</span>
1035 </pre>
1036 <p>
1037           That may look strange at first. It creates an integer terminal with <code class="computeroutput"><a class="link" href="../boost/proto/lit.html" title="Function lit">proto::lit()</a></code>, and then invokes it like
1038           a function again and again. What does it mean? Who knows?! You get to decide
1039           when you define an evaluation context or a transform. But more on that
1040           later.
1041         </p>
1042 <h6>
1043 <a name="boost_proto.users_guide.front_end.making_lazy_functions.h0"></a>
1044           <span class="phrase"><a name="boost_proto.users_guide.front_end.making_lazy_functions.making_lazy_functions__continued"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions.making_lazy_functions__continued">Making
1045           Lazy Functions, Continued</a>
1046         </h6>
1047 <p>
1048           Now, what if we wanted to add a <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code> function to our calculator EDSL that
1049           users could invoke as follows?
1050         </p>
1051 <pre class="programlisting"><span class="comment">// A calculator expression that takes one argument</span>
1052 <span class="comment">// and raises it to the 2nd power</span>
1053 <span class="identifier">pow</span><span class="special">&lt;</span> <span class="number">2</span> <span class="special">&gt;(</span><span class="identifier">_1</span><span class="special">);</span>
1054 </pre>
1055 <p>
1056           The simple technique described above of making <code class="computeroutput"><span class="identifier">pow</span></code>
1057           a terminal containing a function pointer doesn't work here. If <code class="computeroutput"><span class="identifier">pow</span></code> is an object, then the expression
1058           <code class="computeroutput"><span class="identifier">pow</span><span class="special">&lt;</span>
1059           <span class="number">2</span> <span class="special">&gt;(</span><span class="identifier">_1</span><span class="special">)</span></code> is
1060           not valid C++. (Well, technically it is; it means, <code class="computeroutput"><span class="identifier">pow</span></code>
1061           less than 2, greater than <code class="computeroutput"><span class="special">(</span><span class="identifier">_1</span><span class="special">)</span></code>,
1062           which is nothing at all like what we want.) <code class="computeroutput"><span class="identifier">pow</span></code>
1063           should be a real function template. But it must be an unusual function:
1064           one that returns an expression template.
1065         </p>
1066 <p>
1067           With <code class="computeroutput"><span class="identifier">sin</span></code>, we relied on
1068           Proto to provide an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()()</span></code> to build an expression node with tag
1069           type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code> for us. Now we'll need to do
1070           so ourselves. As before, the node will have two children: the function
1071           to invoke and the function's argument.
1072         </p>
1073 <p>
1074           With <code class="computeroutput"><span class="identifier">sin</span></code>, the function
1075           to invoke was a raw function pointer wrapped in a Proto terminal. In the
1076           case of <code class="computeroutput"><span class="identifier">pow</span></code>, we want it
1077           to be a terminal containing TR1-style function object. This will allow
1078           us to parameterize the function on the exponent. Below is the implementation
1079           of a simple TR1-style wrapper for the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pow</span></code>
1080           function:
1081         </p>
1082 <pre class="programlisting"><span class="comment">// Define a pow_fun function object</span>
1083 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="identifier">Exp</span> <span class="special">&gt;</span>
1084 <span class="keyword">struct</span> <span class="identifier">pow_fun</span>
1085 <span class="special">{</span>
1086     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
1087
1088     <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d</span><span class="special">)</span> <span class="keyword">const</span>
1089     <span class="special">{</span>
1090         <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pow</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span> <span class="identifier">Exp</span><span class="special">);</span>
1091     <span class="special">}</span>
1092 <span class="special">};</span>
1093 </pre>
1094 <p>
1095           Following the <code class="computeroutput"><span class="identifier">sin</span></code> example,
1096           we want <code class="computeroutput"><span class="identifier">pow</span><span class="special">&lt;</span>
1097           <span class="number">1</span> <span class="special">&gt;(</span>
1098           <span class="identifier">pi</span><span class="special">/</span><span class="number">2</span> <span class="special">)</span></code> to have
1099           a type like this:
1100         </p>
1101 <pre class="programlisting"><span class="comment">// The type of the expression pow&lt;1&gt;(pi/2):</span>
1102 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span>
1103     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">pow_fun</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
1104     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_child</span><span class="special">&lt;</span> <span class="keyword">double</span> <span class="keyword">const</span> <span class="special">&gt;::</span><span class="identifier">type</span>
1105 <span class="special">&gt;::</span><span class="identifier">type</span>
1106 </pre>
1107 <p>
1108           We could write a <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code>
1109           function using code like this, but it's verbose and error prone; it's too
1110           easy to introduce subtle bugs by forgetting to call <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>
1111           where necessary, resulting in code that seems to work but sometimes doesn't.
1112           Proto provides a better way to construct expression nodes: <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code>.
1113         </p>
1114 <h6>
1115 <a name="boost_proto.users_guide.front_end.making_lazy_functions.h1"></a>
1116           <span class="phrase"><a name="boost_proto.users_guide.front_end.making_lazy_functions.lazy_functions_made_simple_with__literal_make_expr____literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.making_lazy_functions.lazy_functions_made_simple_with__literal_make_expr____literal_">Lazy
1117           Functions Made Simple With <code class="literal">make_expr()</code></a>
1118         </h6>
1119 <p>
1120           Proto provides a helper for building expression templates called <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code>. We can concisely define
1121           the <code class="computeroutput"><span class="identifier">pow</span><span class="special">()</span></code>
1122           function with it as below.
1123         </p>
1124 <pre class="programlisting"><span class="comment">// Define a lazy pow() function for the calculator EDSL.</span>
1125 <span class="comment">// Can be used as: pow&lt; 2 &gt;(_1)</span>
1126 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="identifier">Exp</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span> <span class="special">&gt;</span>
1127 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>
1128     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>  <span class="comment">// Tag type</span>
1129   <span class="special">,</span> <span class="identifier">pow_fun</span><span class="special">&lt;</span> <span class="identifier">Exp</span> <span class="special">&gt;</span>        <span class="comment">// First child (by value)</span>
1130   <span class="special">,</span> <span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&amp;</span>           <span class="comment">// Second child (by reference)</span>
1131 <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span>
1132 <span class="identifier">pow</span><span class="special">(</span><span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">arg</span><span class="special">)</span>
1133 <span class="special">{</span>
1134     <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">&gt;(</span>
1135         <span class="identifier">pow_fun</span><span class="special">&lt;</span><span class="identifier">Exp</span><span class="special">&gt;()</span>    <span class="comment">// First child (by value)</span>
1136       <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">arg</span><span class="special">)</span>   <span class="comment">// Second child (by reference)</span>
1137     <span class="special">);</span>
1138 <span class="special">}</span>
1139 </pre>
1140 <p>
1141           There are some things to notice about the above code. We use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;&gt;</span></code>
1142           to calculate the return type. The first template parameter is the tag type
1143           for the expression node we're building -- in this case, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code>.
1144         </p>
1145 <p>
1146           Subsequent template parameters to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;&gt;</span></code> represent child nodes. If a child
1147           type is not already a Proto expression, it is automatically made into a
1148           terminal with <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>.
1149           A type such as <code class="computeroutput"><span class="identifier">pow_fun</span><span class="special">&lt;</span><span class="identifier">Exp</span><span class="special">&gt;</span></code> results in terminal that is held by
1150           value, whereas a type like <code class="computeroutput"><span class="identifier">Arg</span>
1151           <span class="keyword">const</span> <span class="special">&amp;</span></code>
1152           (note the reference) indicates that the result should be held by reference.
1153         </p>
1154 <p>
1155           In the function body is the runtime invocation of <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code>.
1156           It closely mirrors the return type calculation. <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code>
1157           requires you to specify the node's tag type as a template parameter. The
1158           arguments to the function become the node's children. When a child should
1159           be stored by value, nothing special needs to be done. When a child should
1160           be stored by reference, you must use the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">()</span></code> function to wrap the argument.
1161         </p>
1162 <p>
1163           And that's it! <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code>
1164           is the lazy person's way to make a lazy funtion.
1165         </p>
1166 </div>
1167 <div class="section">
1168 <div class="titlepage"><div><div><h4 class="title">
1169 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain" title="Customizing Expressions in Your Domain">Customizing
1170         Expressions in Your Domain</a>
1171 </h4></div></div></div>
1172 <div class="toc"><dl class="toc">
1173 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.domains">Domains</a></span></dt>
1174 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends">The
1175           <code class="literal">extends&lt;&gt;</code> Expression Wrapper</a></span></dt>
1176 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.expression_generators">Expression
1177           Generators</a></span></dt>
1178 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.inhibiting_overloads">Controlling
1179           Operator Overloads</a></span></dt>
1180 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child">Controlling
1181           How Child Expressions Are Captured</a></span></dt>
1182 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains">EDSL
1183           Interoperatability: Sub-Domains</a></span></dt>
1184 </dl></div>
1185 <p>
1186           In this section, we'll learn all about <span class="emphasis"><em>domains</em></span>. In
1187           particular, we'll learn:
1188         </p>
1189 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1190 <li class="listitem">
1191               How to associate Proto expressions with a domain,
1192             </li>
1193 <li class="listitem">
1194               How to add members to expressions within a domain,
1195             </li>
1196 <li class="listitem">
1197               How to use a <span class="emphasis"><em>generator</em></span> to post-process all new
1198               expressions created in your domain,
1199             </li>
1200 <li class="listitem">
1201               How to control which operators are overloaded in a domain,
1202             </li>
1203 <li class="listitem">
1204               How to specify capturing policies for child expressions and non-Proto
1205               objects, and
1206             </li>
1207 <li class="listitem">
1208               How to make expressions from separate domains interoperate.
1209             </li>
1210 </ul></div>
1211 <div class="section">
1212 <div class="titlepage"><div><div><h5 class="title">
1213 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.domains"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.domains" title="Domains">Domains</a>
1214 </h5></div></div></div>
1215 <p>
1216             In the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello
1217             Calculator</a> section, we looked into making calculator expressions
1218             directly usable as lambda expressions in calls to STL algorithms, as
1219             below:
1220           </p>
1221 <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">data</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">1.</span><span class="special">,</span> <span class="number">2.</span><span class="special">,</span> <span class="number">3.</span><span class="special">,</span> <span class="number">4.</span><span class="special">};</span>
1222
1223 <span class="comment">// Use the calculator EDSL to square each element ... HOW?</span>
1224 <span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">data</span> <span class="special">+</span> <span class="number">4</span><span class="special">,</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_1</span> <span class="special">);</span>
1225 </pre>
1226 <p>
1227             The difficulty, if you recall, was that by default Proto expressions
1228             don't have interesting behaviors of their own. They're just trees. In
1229             particular, the expression <code class="computeroutput"><span class="identifier">_1</span>
1230             <span class="special">*</span> <span class="identifier">_1</span></code>
1231             won't have an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
1232             that takes a double and returns a double like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">()</span></code> expects -- unless we give it one. To
1233             make this work, we needed to define an expression wrapper type that defined
1234             the <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
1235             member function, and we needed to associate the wrapper with the calculator
1236             <span class="emphasis"><em>domain</em></span>.
1237           </p>
1238 <p>
1239             In Proto, the term <span class="emphasis"><em>domain</em></span> refers to a type that
1240             associates expressions in that domain to an expression <span class="emphasis"><em>generator</em></span>.
1241             The generator is just a function object that accepts an expression and
1242             does something to it, like wrapping it in an expression wrapper.
1243           </p>
1244 <p>
1245             You can also use a domain to associate expressions with a grammar. When
1246             you specify a domain's grammar, Proto ensures that all the expressions
1247             it generates in that domain conform to the domain's grammar. It does
1248             that by disabling any operator overloads that would create invalid expressions.
1249           </p>
1250 </div>
1251 <div class="section">
1252 <div class="titlepage"><div><div><h5 class="title">
1253 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends" title="The extends&lt;&gt; Expression Wrapper">The
1254           <code class="literal">extends&lt;&gt;</code> Expression Wrapper</a>
1255 </h5></div></div></div>
1256 <p>
1257             The first step to giving your calculator expressions extra behaviors
1258             is to define a calculator domain. All expressions within the calculator
1259             domain will be imbued with calculator-ness, as we'll see.
1260           </p>
1261 <pre class="programlisting"><span class="comment">// A type to be used as a domain tag (to be defined below)</span>
1262 <span class="keyword">struct</span> <span class="identifier">calculator_domain</span><span class="special">;</span>
1263 </pre>
1264 <p>
1265             We use this domain type when extending the <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>
1266             type, which we do with the <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
1267             class template. Here is our expression wrapper, which imbues an expression
1268             with calculator-ness. It is described below.
1269           </p>
1270 <pre class="programlisting"><span class="comment">// The calculator&lt;&gt; expression wrapper makes expressions</span>
1271 <span class="comment">// function objects.</span>
1272 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
1273 <span class="keyword">struct</span> <span class="identifier">calculator</span>
1274   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">Expr</span> <span class="special">&gt;,</span> <span class="identifier">calculator_domain</span> <span class="special">&gt;</span>
1275 <span class="special">{</span>
1276     <span class="keyword">typedef</span>
1277         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">Expr</span> <span class="special">&gt;,</span> <span class="identifier">calculator_domain</span> <span class="special">&gt;</span>
1278     <span class="identifier">base_type</span><span class="special">;</span>
1279
1280     <span class="identifier">calculator</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">()</span> <span class="special">)</span>
1281       <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)</span>
1282     <span class="special">{}</span>
1283
1284     <span class="comment">// This is usually needed because by default, the compiler-</span>
1285     <span class="comment">// generated assignment operator hides extends&lt;&gt;::operator=</span>
1286     <span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">(</span><span class="identifier">calculator</span><span class="special">)</span>
1287
1288     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
1289
1290     <span class="comment">// Hide base_type::operator() by defining our own which</span>
1291     <span class="comment">// evaluates the calculator expression with a calculator context.</span>
1292     <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.0</span> <span class="special">)</span> <span class="keyword">const</span>
1293     <span class="special">{</span>
1294         <span class="comment">// As defined in the Hello Calculator section.</span>
1295         <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
1296
1297         <span class="comment">// ctx.args is a vector&lt;double&gt; that holds the values</span>
1298         <span class="comment">// with which we replace the placeholders (e.g., _1 and _2)</span>
1299         <span class="comment">// in the expression.</span>
1300         <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span> <span class="identifier">d1</span> <span class="special">);</span> <span class="comment">// _1 gets the value of d1</span>
1301         <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span> <span class="identifier">d2</span> <span class="special">);</span> <span class="comment">// _2 gets the value of d2</span>
1302
1303         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span> <span class="special">);</span> <span class="comment">// evaluate the expression</span>
1304     <span class="special">}</span>
1305 <span class="special">};</span>
1306 </pre>
1307 <p>
1308             We want calculator expressions to be function objects, so we have to
1309             define an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
1310             that takes and returns doubles. The <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> wrapper above does that with
1311             the help of the <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
1312             template. The first template to <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
1313             parameter is the expression type we are extending. The second is the
1314             type of the wrapped expression. The third parameter is the domain that
1315             this wrapper is associated with. A wrapper type like <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> that inherits from <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code> behaves just like
1316             the expression type it has extended, with any additional behaviors you
1317             choose to give it.
1318           </p>
1319 <div class="note"><table border="0" summary="Note">
1320 <tr>
1321 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
1322 <th align="left">Note</th>
1323 </tr>
1324 <tr><td align="left" valign="top">
1325 <p>
1326               <span class="bold"><strong>Why not just inherit from <code class="literal">proto::expr&lt;&gt;</code>?</strong></span>
1327             </p>
1328 <p>
1329               You might be thinking that this expression extension business is unnecessarily
1330               complicated. After all, isn't this why C++ supports inheritance? Why
1331               can't <code class="literal">calculator&lt;Expr&gt;</code> just inherit from
1332               <code class="literal">Expr</code> directly? The reason is because <code class="literal">Expr</code>,
1333               which presumably is an instantiation of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>,
1334               has expression template-building operator overloads that will be incorrect
1335               for derived types. They will store <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> by reference to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;&gt;</span></code>, effectively slicing off any
1336               derived parts. <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
1337               gives your derived types operator overloads that don't slice off your
1338               additional members.
1339             </p>
1340 </td></tr>
1341 </table></div>
1342 <p>
1343             Although not strictly necessary in this case, we bring <code class="computeroutput"><span class="identifier">extends</span><span class="special">&lt;&gt;::</span><span class="keyword">operator</span><span class="special">=</span></code>
1344             into scope with the <code class="computeroutput"><span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">()</span></code> macro. This is really only necessary
1345             if you want expressions like <code class="computeroutput"><span class="identifier">_1</span>
1346             <span class="special">=</span> <span class="number">3</span></code>
1347             to create a lazily evaluated assignment. <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
1348             defines the appropriate <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code> for you, but the compiler-generated
1349             <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;::</span><span class="keyword">operator</span><span class="special">=</span></code>
1350             will hide it unless you make it available with the macro.
1351           </p>
1352 <p>
1353             Note that in the implementation of <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;::</span><span class="keyword">operator</span><span class="special">()</span></code>, we evaluate the expression with the
1354             <code class="computeroutput"><span class="identifier">calculator_context</span></code> we
1355             defined earlier. As we saw before, the context is what gives the operators
1356             their meaning. In the case of the calculator, the context is also what
1357             defines the meaning of the placeholder terminals.
1358           </p>
1359 <p>
1360             Now that we have defined the <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> expression wrapper, we need to
1361             wrap the placeholders to imbue them with calculator-ness:
1362           </p>
1363 <pre class="programlisting"><span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span>
1364 <span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span>
1365 </pre>
1366 <h6>
1367 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.h0"></a>
1368             <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.retaining_pod_ness_with__literal_boost_proto_extends____literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.retaining_pod_ness_with__literal_boost_proto_extends____literal_">Retaining
1369             POD-ness with <code class="literal">BOOST_PROTO_EXTENDS()</code></a>
1370           </h6>
1371 <p>
1372             To use <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>, your extension type
1373             must derive from <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>.
1374             Unfortunately, that means that your extension type is no longer POD and
1375             its instances cannot be <span class="emphasis"><em>statically initialized</em></span>.
1376             (See the <a class="link" href="appendices.html#boost_proto.appendices.rationale.static_initialization" title="Static Initialization">Static
1377             Initialization</a> section in the <a class="link" href="appendices.html#boost_proto.appendices.rationale" title="Appendix C: Rationale">Rationale</a>
1378             appendix for why this matters.) In particular, as defined above, the
1379             global placeholder objects <code class="computeroutput"><span class="identifier">_1</span></code>
1380             and <code class="computeroutput"><span class="identifier">_2</span></code> will need to be
1381             initialized at runtime, which could lead to subtle order of initialization
1382             bugs.
1383           </p>
1384 <p>
1385             There is another way to make an expression extension that doesn't sacrifice
1386             POD-ness : the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>
1387             macro. You can use it much like you use <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>.
1388             We can use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>
1389             to keep <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code>
1390             a POD and our placeholders statically initialized.
1391           </p>
1392 <pre class="programlisting"><span class="comment">// The calculator&lt;&gt; expression wrapper makes expressions</span>
1393 <span class="comment">// function objects.</span>
1394 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
1395 <span class="keyword">struct</span> <span class="identifier">calculator</span>
1396 <span class="special">{</span>
1397     <span class="comment">// Use BOOST_PROTO_EXTENDS() instead of proto::extends&lt;&gt; to</span>
1398     <span class="comment">// make this type a Proto expression extension.</span>
1399     <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">)</span>
1400
1401     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
1402
1403     <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.0</span> <span class="special">)</span> <span class="keyword">const</span>
1404     <span class="special">{</span>
1405         <span class="comment">/* ... as before ... */</span>
1406     <span class="special">}</span>
1407 <span class="special">};</span>
1408 </pre>
1409 <p>
1410             With the new <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> type, we can redefine our placeholders
1411             to be statically initialized:
1412           </p>
1413 <pre class="programlisting"><span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{{}}};</span>
1414 <span class="identifier">calculator</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{{}}};</span>
1415 </pre>
1416 <p>
1417             We need to make one additional small change to accommodate the POD-ness
1418             of our expression extension, which we'll describe below in the section
1419             on expression generators.
1420           </p>
1421 <p>
1422             What does <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>
1423             do? It defines a data member of the expression type being extended; some
1424             nested typedefs that Proto requires; <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code> and <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> overloads for building expression templates;
1425             and a nested <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code>
1426             template for calculating the return type of <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>. In this case, however, the <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
1427             overloads and the <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code> template are not needed because
1428             we are defining our own <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> in the <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> type. Proto provides additional
1429             macros for finer control over which member functions are defined. We
1430             could improve our <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> type as follows:
1431           </p>
1432 <pre class="programlisting"><span class="comment">// The calculator&lt;&gt; expression wrapper makes expressions</span>
1433 <span class="comment">// function objects.</span>
1434 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
1435 <span class="keyword">struct</span> <span class="identifier">calculator</span>
1436 <span class="special">{</span>
1437     <span class="comment">// Use BOOST_PROTO_BASIC_EXTENDS() instead of proto::extends&lt;&gt; to</span>
1438     <span class="comment">// make this type a Proto expression extension:</span>
1439     <span class="identifier">BOOST_PROTO_BASIC_EXTENDS</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">)</span>
1440
1441     <span class="comment">// Define operator[] to build expression templates:</span>
1442     <span class="identifier">BOOST_PROTO_EXTENDS_SUBSCRIPT</span><span class="special">()</span>
1443
1444     <span class="comment">// Define operator= to build expression templates:</span>
1445     <span class="identifier">BOOST_PROTO_EXTENDS_ASSIGN</span><span class="special">()</span>
1446
1447     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
1448
1449     <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.0</span> <span class="special">)</span> <span class="keyword">const</span>
1450     <span class="special">{</span>
1451         <span class="comment">/* ... as before ... */</span>
1452     <span class="special">}</span>
1453 <span class="special">};</span>
1454 </pre>
1455 <p>
1456             Notice that we are now using <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code>
1457             instead of <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>.
1458             This just adds the data member and the nested typedefs but not any of
1459             the overloaded operators. Those are added separately with <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_ASSIGN">BOOST_PROTO_EXTENDS_ASSIGN</a></code>()</code>
1460             and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_10_7.html" title="Macro BOOST_PROTO_EXTENDS_SUBSCRIPT">BOOST_PROTO_EXTENDS_SUBSCRIPT</a></code>()</code>.
1461             We are leaving out the function call operator and the nested <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code>
1462             template that could have been defined with Proto's <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_10_6.html" title="Macro BOOST_PROTO_EXTENDS_FUNCTION">BOOST_PROTO_EXTENDS_FUNCTION</a></code>()</code>
1463             macro.
1464           </p>
1465 <p>
1466             In summary, here are the macros you can use to define expression extensions,
1467             and a brief description of each.
1468           </p>
1469 <div class="table">
1470 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.extends.t0"></a><p class="title"><b>Table&#160;33.2.&#160;Expression Extension Macros</b></p>
1471 <div class="table-contents"><table class="table" summary="Expression Extension Macros">
1472 <colgroup>
1473 <col>
1474 <col>
1475 </colgroup>
1476 <thead><tr>
1477 <th>
1478                     <p>
1479                       Macro
1480                     </p>
1481                   </th>
1482 <th>
1483                     <p>
1484                       Purpose
1485                     </p>
1486                   </th>
1487 </tr></thead>
1488 <tbody>
1489 <tr>
1490 <td>
1491                     <p>
1492 </p>
1493 <pre class="programlisting"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code><span class="special">(</span>
1494     <em class="replaceable"><code>expression</code></em>
1495   <span class="special">,</span> <em class="replaceable"><code>extension</code></em>
1496   <span class="special">,</span> <em class="replaceable"><code>domain</code></em>
1497 <span class="special">)</span></pre>
1498 <p>
1499                     </p>
1500                   </td>
1501 <td>
1502                     <p>
1503                       Defines a data member of type <code class="computeroutput"><em class="replaceable"><code>expression</code></em></code>
1504                       and some nested typedefs that Proto requires.
1505                     </p>
1506                   </td>
1507 </tr>
1508 <tr>
1509 <td>
1510                     <p>
1511                       <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_ASSIGN">BOOST_PROTO_EXTENDS_ASSIGN</a></code>()</code>
1512                     </p>
1513                   </td>
1514 <td>
1515                     <p>
1516                       Defines <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>. Only valid when preceded
1517                       by <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code>.
1518                     </p>
1519                   </td>
1520 </tr>
1521 <tr>
1522 <td>
1523                     <p>
1524                       <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_10_7.html" title="Macro BOOST_PROTO_EXTENDS_SUBSCRIPT">BOOST_PROTO_EXTENDS_SUBSCRIPT</a></code>()</code>
1525                     </p>
1526                   </td>
1527 <td>
1528                     <p>
1529                       Defines <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code>. Only valid when preceded
1530                       by <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code>.
1531                     </p>
1532                   </td>
1533 </tr>
1534 <tr>
1535 <td>
1536                     <p>
1537                       <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_10_6.html" title="Macro BOOST_PROTO_EXTENDS_FUNCTION">BOOST_PROTO_EXTENDS_FUNCTION</a></code>()</code>
1538                     </p>
1539                   </td>
1540 <td>
1541                     <p>
1542                       Defines <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> and a nested <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code>
1543                       template for return type calculation. Only valid when preceded
1544                       by <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code>()</code>.
1545                     </p>
1546                   </td>
1547 </tr>
1548 <tr>
1549 <td>
1550                     <p>
1551 </p>
1552 <pre class="programlisting"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code><span class="special">(</span>
1553     <em class="replaceable"><code>expression</code></em>
1554   <span class="special">,</span> <em class="replaceable"><code>extension</code></em>
1555   <span class="special">,</span> <em class="replaceable"><code>domain</code></em>
1556 <span class="special">)</span></pre>
1557 <p>
1558                     </p>
1559                   </td>
1560 <td>
1561                     <p>
1562                       Equivalent to:
1563 </p>
1564 <pre class="programlisting"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_BASIC_EXTENDS.html" title="Macro BOOST_PROTO_BASIC_EXTENDS">BOOST_PROTO_BASIC_EXTENDS</a></code><span class="special">(</span><em class="replaceable"><code>expression</code></em><span class="special">,</span> <em class="replaceable"><code>extension</code></em><span class="special">,</span> <em class="replaceable"><code>domain</code></em><span class="special">)</span>
1565
1566   <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS_ASSIGN.html" title="Macro BOOST_PROTO_EXTENDS_ASSIGN">BOOST_PROTO_EXTENDS_ASSIGN</a></code>()</code>
1567
1568   <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_10_7.html" title="Macro BOOST_PROTO_EXTENDS_SUBSCRIPT">BOOST_PROTO_EXTENDS_SUBSCRIPT</a></code>()</code>
1569
1570   <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_10_6.html" title="Macro BOOST_PROTO_EXTENDS_FUNCTION">BOOST_PROTO_EXTENDS_FUNCTION</a></code>()</code></pre>
1571 <p>
1572                     </p>
1573                   </td>
1574 </tr>
1575 </tbody>
1576 </table></div>
1577 </div>
1578 <br class="table-break"><div class="warning"><table border="0" summary="Warning">
1579 <tr>
1580 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td>
1581 <th align="left">Warning</th>
1582 </tr>
1583 <tr><td align="left" valign="top">
1584 <p>
1585               <span class="bold"><strong>Argument-Dependent Lookup and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code></strong></span>
1586             </p>
1587 <p>
1588               Proto's operator overloads are defined in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
1589               namespace and are found by argument-dependent lookup (ADL). This usually
1590               just works because expressions are made up of types that live in the
1591               <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> namespace. However, sometimes
1592               when you use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>
1593               that is not the case. Consider:
1594             </p>
1595 <p>
1596 </p>
1597 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
1598 <span class="keyword">struct</span> <span class="identifier">my_complex</span>
1599 <span class="special">{</span>
1600     <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span>
1601         <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
1602       <span class="special">,</span> <span class="identifier">my_complex</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
1603       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span>
1604     <span class="special">)</span>
1605 <span class="special">};</span>
1606
1607 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
1608 <span class="special">{</span>
1609     <span class="identifier">my_complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">c0</span><span class="special">,</span> <span class="identifier">c1</span><span class="special">;</span>
1610
1611     <span class="identifier">c0</span> <span class="special">+</span> <span class="identifier">c1</span><span class="special">;</span> <span class="comment">// ERROR: operator+ not found</span>
1612 <span class="special">}</span>
1613 </pre>
1614 <p>
1615             </p>
1616 <p>
1617               The problem has to do with how argument-dependent lookup works. The
1618               type <code class="computeroutput"><span class="identifier">my_complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
1619               is not associated in any way with the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
1620               namespace, so the operators defined there are not considered. (Had
1621               we inherited from <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
1622               instead of used <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>,
1623               we would have avoided the problem because inheriting from a type in
1624               <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> namespace is enough to get
1625               ADL to kick in.)
1626             </p>
1627 <p>
1628               So what can we do? By adding an extra dummy template parameter that
1629               defaults to a type in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code>
1630               namespace, we can trick ADL into finding the right operator overloads.
1631               The solution looks like this:
1632             </p>
1633 <p>
1634 </p>
1635 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Dummy</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_proto_expr</span><span class="special">&gt;</span>
1636 <span class="keyword">struct</span> <span class="identifier">my_complex</span>
1637 <span class="special">{</span>
1638     <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span>
1639         <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
1640       <span class="special">,</span> <span class="identifier">my_complex</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
1641       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span>
1642     <span class="special">)</span>
1643 <span class="special">};</span>
1644
1645 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
1646 <span class="special">{</span>
1647     <span class="identifier">my_complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">c0</span><span class="special">,</span> <span class="identifier">c1</span><span class="special">;</span>
1648
1649     <span class="identifier">c0</span> <span class="special">+</span> <span class="identifier">c1</span><span class="special">;</span> <span class="comment">// OK, operator+ found now!</span>
1650 <span class="special">}</span>
1651 </pre>
1652 <p>
1653             </p>
1654 <p>
1655               The type <code class="computeroutput"><a class="link" href="../boost/proto/is_proto_expr.html" title="Struct is_proto_expr">proto::is_proto_expr</a></code> is nothing
1656               but an empty struct, but by making it a template parameter we make
1657               <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span></code> an associated namespace of
1658               <code class="computeroutput"><span class="identifier">my_complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>.
1659               Now ADL can successfully find Proto's operator overloads.
1660             </p>
1661 </td></tr>
1662 </table></div>
1663 </div>
1664 <div class="section">
1665 <div class="titlepage"><div><div><h5 class="title">
1666 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.expression_generators"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.expression_generators" title="Expression Generators">Expression
1667           Generators</a>
1668 </h5></div></div></div>
1669 <p>
1670             The last thing that remains to be done is to tell Proto that it needs
1671             to wrap all of our calculator expressions in our <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> wrapper. We have already wrapped
1672             the placeholders, but we want <span class="emphasis"><em>all</em></span> expressions that
1673             involve the calculator placeholders to be calculators. We can do that
1674             by specifying an expression generator when we define our <code class="computeroutput"><span class="identifier">calculator_domain</span></code>, as follows:
1675           </p>
1676 <pre class="programlisting"><span class="comment">// Define the calculator_domain we forward-declared above.</span>
1677 <span class="comment">// Specify that all expression in this domain should be wrapped</span>
1678 <span class="comment">// in the calculator&lt;&gt; expression wrapper.</span>
1679 <span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
1680   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span> <span class="identifier">calculator</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
1681 <span class="special">{};</span>
1682 </pre>
1683 <p>
1684             The first template parameter to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;&gt;</span></code> is the generator. "Generator"
1685             is just a fancy name for a function object that accepts an expression
1686             and does something to it. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;&gt;</span></code> is a very simple one --- it wraps
1687             an expression in the wrapper you specify. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;&gt;</span></code> inherits from its generator parameter,
1688             so all domains are themselves function objects.
1689           </p>
1690 <p>
1691             If we used <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_EXTENDS.html" title="Macro BOOST_PROTO_EXTENDS">BOOST_PROTO_EXTENDS</a></code>()</code>
1692             to keep our expression extension type POD, then we need to use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special">&lt;&gt;</span></code>
1693             instead of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;&gt;</span></code>,
1694             as follows:
1695           </p>
1696 <pre class="programlisting"><span class="comment">// If calculator&lt;&gt; uses BOOST_PROTO_EXTENDS() instead of </span>
1697 <span class="comment">// use proto::extends&lt;&gt;, use proto::pod_generator&lt;&gt; instead</span>
1698 <span class="comment">// of proto::generator&lt;&gt;.</span>
1699 <span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
1700   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special">&lt;</span> <span class="identifier">calculator</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
1701 <span class="special">{};</span>
1702 </pre>
1703 <p>
1704             After Proto has calculated a new expression type, it checks the domains
1705             of the child expressions. They must match. Assuming they do, Proto creates
1706             the new expression and passes it to <code class="computeroutput"><em class="replaceable"><code>Domain</code></em><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> for any additional processing. If we
1707             don't specify a generator, the new expression gets passed through unchanged.
1708             But since we've specified a generator above, <code class="computeroutput"><span class="identifier">calculator_domain</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> returns <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> objects.
1709           </p>
1710 <p>
1711             Now we can use calculator expressions as function objects to STL algorithms,
1712             as follows:
1713           </p>
1714 <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">data</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">1.</span><span class="special">,</span> <span class="number">2.</span><span class="special">,</span> <span class="number">3.</span><span class="special">,</span> <span class="number">4.</span><span class="special">};</span>
1715
1716 <span class="comment">// Use the calculator EDSL to square each element ... WORKS! :-)</span>
1717 <span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">data</span> <span class="special">+</span> <span class="number">4</span><span class="special">,</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_1</span> <span class="special">);</span>
1718 </pre>
1719 </div>
1720 <div class="section">
1721 <div class="titlepage"><div><div><h5 class="title">
1722 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.inhibiting_overloads"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.inhibiting_overloads" title="Controlling Operator Overloads">Controlling
1723           Operator Overloads</a>
1724 </h5></div></div></div>
1725 <p>
1726             By default, Proto defines every possible operator overload for Protofied
1727             expressions. This makes it simple to bang together an EDSL. In some cases,
1728             however, the presence of Proto's promiscuous overloads can lead to confusion
1729             or worse. When that happens, you'll have to disable some of Proto's overloaded
1730             operators. That is done by defining the grammar for your domain and specifying
1731             it as the second parameter of the <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain&lt;&gt;</a></code>
1732             template.
1733           </p>
1734 <p>
1735             In the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello
1736             Calculator</a> section, we saw an example of a Proto grammar, which
1737             is repeated here:
1738           </p>
1739 <pre class="programlisting"><span class="comment">// Define the grammar of calculator expressions</span>
1740 <span class="keyword">struct</span> <span class="identifier">calculator_grammar</span>
1741   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
1742         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
1743       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
1744       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
1745       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span> <span class="identifier">calculator_grammar</span><span class="special">,</span> <span class="identifier">calculator_grammar</span> <span class="special">&gt;</span>
1746       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
1747     <span class="special">&gt;</span>
1748 <span class="special">{};</span>
1749 </pre>
1750 <p>
1751             We'll have much more to say about grammars in subsequent sections, but
1752             for now, we'll just say that the <code class="computeroutput"><span class="identifier">calculator_grammar</span></code>
1753             struct describes a subset of all expression types -- the subset that
1754             comprise valid calculator expressions. We would like to prohibit Proto
1755             from creating a calculator expression that does not conform to this grammar.
1756             We do that by changing the definition of the <code class="computeroutput"><span class="identifier">calculator_domain</span></code>
1757             struct.
1758           </p>
1759 <pre class="programlisting"><span class="comment">// Define the calculator_domain. Expressions in the calculator</span>
1760 <span class="comment">// domain are wrapped in the calculator&lt;&gt; wrapper, and they must</span>
1761 <span class="comment">// conform to the calculator_grammar:</span>
1762 <span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
1763   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span> <span class="identifier">calculator</span> <span class="special">&gt;,</span> <span class="bold"><strong>calculator_grammar</strong></span>  <span class="special">&gt;</span>
1764 <span class="special">{};</span>
1765 </pre>
1766 <p>
1767             The only new addition is <code class="computeroutput"><span class="identifier">calculator_grammar</span></code>
1768             as the second template parameter to the <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain&lt;&gt;</a></code>
1769             template. That has the effect of disabling any of Proto's operator overloads
1770             that would create an invalid calculator expression.
1771           </p>
1772 <p>
1773             Another common use for this feature would be to disable Proto's unary
1774             <code class="computeroutput"><span class="keyword">operator</span><span class="special">&amp;</span></code>
1775             overload. It may be surprising for users of your EDSL that they cannot
1776             take the address of their expressions! You can very easily disable Proto's
1777             unary <code class="computeroutput"><span class="keyword">operator</span><span class="special">&amp;</span></code>
1778             overload for your domain with a very simple grammar, as below:
1779           </p>
1780 <pre class="programlisting"><span class="comment">// For expressions in my_domain, disable Proto's</span>
1781 <span class="comment">// unary address-of operator.</span>
1782 <span class="keyword">struct</span> <span class="identifier">my_domain</span>
1783   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span>
1784         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span> <span class="identifier">my_wrapper</span> <span class="special">&gt;</span>
1785         <span class="comment">// A simple grammar that matches any expression that</span>
1786         <span class="comment">// is not a unary address-of expression.</span>
1787       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
1788     <span class="special">&gt;</span>
1789 <span class="special">{};</span>
1790 </pre>
1791 <p>
1792             The type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span>
1793             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special">&lt;</span>
1794             <span class="identifier">_</span> <span class="special">&gt;</span>
1795             <span class="special">&gt;</span></code> is a very simple grammar
1796             that matches all expressions except unary address-of expressions. In
1797             the section describing Proto's intermediate form, we'll have much more
1798             to say about grammars.
1799           </p>
1800 </div>
1801 <div class="section">
1802 <div class="titlepage"><div><div><h5 class="title">
1803 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child" title="Controlling How Child Expressions Are Captured">Controlling
1804           How Child Expressions Are Captured</a>
1805 </h5></div></div></div>
1806 <div class="note"><table border="0" summary="Note">
1807 <tr>
1808 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
1809 <th align="left">Note</th>
1810 </tr>
1811 <tr><td align="left" valign="top"><p>
1812               This is an advanced topic. Feel free to skip this if you're just getting
1813               started with Proto.
1814             </p></td></tr>
1815 </table></div>
1816 <p>
1817             Proto's operator overloads build expressions from sub-expressions. The
1818             sub-expressions become children of the new expression. By default, the
1819             children are stored in the parent by reference. This section describes
1820             how to change that default.
1821           </p>
1822 <h6>
1823 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h0"></a>
1824             <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.primer___literal_as_child__literal__vs___literal_as_expr__literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.primer___literal_as_child__literal__vs___literal_as_expr__literal_">Primer:
1825             <code class="literal">as_child</code> vs. <code class="literal">as_expr</code></a>
1826           </h6>
1827 <p>
1828             Proto lets you independently customize the behavior of <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code>.
1829             Both accept an object <code class="literal">x</code> and return a Proto expression
1830             by turning <code class="literal">x</code> it into a Proto terminal if necessary.
1831             Although similar, the two functions are used in different situations
1832             and have subtly different behavior by default. It's important to understand
1833             the difference so that you know which to customize to achieve the behavior
1834             you want.
1835           </p>
1836 <p>
1837             To wit: <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code> is typically used by
1838             <span class="emphasis"><em>you</em></span> to turn an object into a Proto expression that
1839             is to be held in a local variable, as so:
1840           </p>
1841 <pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// Turn x into a Proto expression, hold the result in a local</span>
1842 </pre>
1843 <p>
1844             The above works regardless of whether <code class="computeroutput"><span class="identifier">x</span></code>
1845             is already a Proto expression or not. The object <code class="computeroutput"><span class="identifier">l</span></code>
1846             is guaranteed to be a valid Proto expression. If <code class="computeroutput"><span class="identifier">x</span></code>
1847             is a non-Proto object, it is turned into a terminal expression that holds
1848             <code class="computeroutput"><span class="identifier">x</span></code> <span class="emphasis"><em>by value</em></span>.<a href="#ftn.boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0" class="footnote" name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0"><sup class="footnote">[34]</sup></a> If <code class="computeroutput"><span class="identifier">x</span></code> is a
1849             Proto object already, <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code>
1850             returns it <span class="emphasis"><em>by value</em></span> unmodified.
1851           </p>
1852 <p>
1853             In contrast, <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>
1854             is used internally by Proto to pre-process objects before making them
1855             children of another expression. Since it's internal to Proto, you don't
1856             see it explicitly, but it's there behind the scenes in expressions like
1857             this:
1858           </p>
1859 <pre class="programlisting"><span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// Consider that y is a Proto expression, but x may or may not be.</span>
1860 </pre>
1861 <p>
1862             In this case, Proto builds a plus node from the two children. Both are
1863             pre-processed by passing them to <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>
1864             before making them children of the new node. If <code class="computeroutput"><span class="identifier">x</span></code>
1865             is not a Proto expression, it becomes one by being wrapped in a Proto
1866             terminal that holds it <span class="emphasis"><em>by reference</em></span>. If <code class="computeroutput"><span class="identifier">x</span></code> is already a Proto expression, <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> returns it <span class="emphasis"><em>by
1867             reference</em></span> unmodified. Contrast this with the above description
1868             for <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code>.
1869           </p>
1870 <p>
1871             The table below summarizes the above description.
1872           </p>
1873 <div class="table">
1874 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.t0"></a><p class="title"><b>Table&#160;33.3.&#160;proto::as_expr() vs. proto::as_child()</b></p>
1875 <div class="table-contents"><table class="table" summary="proto::as_expr() vs. proto::as_child()">
1876 <colgroup>
1877 <col>
1878 <col>
1879 <col>
1880 </colgroup>
1881 <thead><tr>
1882 <th>
1883                     <p>
1884                       <span class="bold"><strong>Function</strong></span>
1885                     </p>
1886                   </th>
1887 <th>
1888                     <p>
1889                       <span class="bold"><strong>When <code class="literal">t</code> is not a Proto
1890                       expr...</strong></span>
1891                     </p>
1892                   </th>
1893 <th>
1894                     <p>
1895                       <span class="bold"><strong>When <code class="literal">t</code> is a Proto
1896                       expr...</strong></span>
1897                     </p>
1898                   </th>
1899 </tr></thead>
1900 <tbody>
1901 <tr>
1902 <td>
1903                     <p>
1904                       <code class="literal">proto::as_expr(t)</code>
1905                     </p>
1906                   </td>
1907 <td>
1908                     <p>
1909                       Return (by value) a new Proto terminal holding <code class="literal">t</code>
1910                       by value.
1911                     </p>
1912                   </td>
1913 <td>
1914                     <p>
1915                       Return <code class="literal">t</code> by value unmodified.
1916                     </p>
1917                   </td>
1918 </tr>
1919 <tr>
1920 <td>
1921                     <p>
1922                       <code class="literal">proto::as_child(t)</code>
1923                     </p>
1924                   </td>
1925 <td>
1926                     <p>
1927                       Return (by value) a new Proto terminal holding <code class="literal">t</code>
1928                       by reference.
1929                     </p>
1930                   </td>
1931 <td>
1932                     <p>
1933                       Return <code class="literal">t</code> by reference unmodified.
1934                     </p>
1935                   </td>
1936 </tr>
1937 </tbody>
1938 </table></div>
1939 </div>
1940 <br class="table-break"><div class="note"><table border="0" summary="Note">
1941 <tr>
1942 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
1943 <th align="left">Note</th>
1944 </tr>
1945 <tr><td align="left" valign="top"><p>
1946               There is one important place where Proto uses both <code class="computeroutput"><span class="identifier">as_expr</span></code>
1947               <span class="emphasis"><em>and</em></span> <code class="computeroutput"><span class="identifier">as_child</span></code>:
1948               <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code>. The <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code> function requires
1949               you to specify for each child whether it should be held by value or
1950               by reference. Proto uses <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code>
1951               to pre-process the children to be held by value, and <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> for the ones to be
1952               held by reference.
1953             </p></td></tr>
1954 </table></div>
1955 <p>
1956             Now that you know what <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>
1957             and <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code> are, where they are
1958             used, and what they do by default, you may decide that one or both of
1959             these functions should have different behavior for your domain. For instance,
1960             given the above description of <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>,
1961             the following code is always wrong:
1962           </p>
1963 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
1964 <span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span> <span class="comment">// This is WRONG! Don't do this.</span>
1965 </pre>
1966 <p>
1967             Why is this wrong? Because <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>
1968             will turn the integer literal 42 into a Proto terminal that holds a reference
1969             to a temporary integer initialized with 42. The lifetime of that temporary
1970             ends at the semicolon, guaranteeing that the local <code class="computeroutput"><span class="identifier">l</span></code>
1971             is left holding a dangling reference to a deceased integer. What to do?
1972             One answer is to use <code class="computeroutput"><a class="link" href="../boost/proto/deep_copy.html" title="Function template deep_copy">proto::deep_copy()</a></code>.
1973             Another is to customize the behavior of <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>
1974             for your domain. Read on for the details.
1975           </p>
1976 <h6>
1977 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h1"></a>
1978             <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_child__literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_child__literal_">Per-Domain
1979             <code class="literal">as_child</code></a>
1980           </h6>
1981 <p>
1982             To control how Proto builds expressions out of sub-expressions in your
1983             domain, define your domain as usual, and then define a nested <code class="computeroutput"><span class="identifier">as_child</span><span class="special">&lt;&gt;</span></code>
1984             class template within it, as follows:
1985           </p>
1986 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">my_domain</span>
1987   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">my_generator</span><span class="special">,</span> <span class="identifier">my_grammar</span> <span class="special">&gt;</span>
1988 <span class="special">{</span>
1989     <span class="comment">// Here is where you define how Proto should handle</span>
1990     <span class="comment">// sub-expressions that are about to be glommed into</span>
1991     <span class="comment">// a larger expression.</span>
1992     <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">&gt;</span>
1993     <span class="keyword">struct</span> <span class="identifier">as_child</span>
1994     <span class="special">{</span>
1995         <span class="keyword">typedef</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-type</code></em></span> <span class="identifier">result_type</span><span class="special">;</span>
1996
1997         <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="identifier">T</span> <span class="special">&amp;</span> <span class="identifier">t</span> <span class="special">)</span> <span class="keyword">const</span>
1998         <span class="special">{</span>
1999             <span class="keyword">return</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-object</code></em></span><span class="special">;</span>
2000         <span class="special">}</span>
2001     <span class="special">};</span>
2002 <span class="special">};</span>
2003 </pre>
2004 <p>
2005             There's one important thing to note: in the above code, the template
2006             parameter <code class="literal">T</code> may or may not be a Proto expression type,
2007             but the result <span class="emphasis"><em>must</em></span> be a Proto expression type,
2008             or a reference to one. That means that most user-defined <code class="literal">as_child&lt;&gt;</code>
2009             templates will need to check whether <code class="literal">T</code> is an expression
2010             or not (using <code class="computeroutput"><a class="link" href="../boost/proto/is_expr.html" title="Struct template is_expr">proto::is_expr&lt;&gt;</a></code>), and then turn non-expressions
2011             into Proto terminals by wrapping them as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="comment">/* ... */</span>
2012             <span class="special">&gt;::</span><span class="identifier">type</span></code>
2013             or equivalent.
2014           </p>
2015 <h6>
2016 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h2"></a>
2017             <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_expr__literal_"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.per_domain__literal_as_expr__literal_">Per-Domain
2018             <code class="literal">as_expr</code></a>
2019           </h6>
2020 <p>
2021             Although less common, Proto also lets you customize the behavior of
2022             <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code> on a per-domain basis.
2023             The technique is identical to that for <code class="literal">as_child</code>. See
2024             below:
2025           </p>
2026 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">my_domain</span>
2027   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">my_generator</span><span class="special">,</span> <span class="identifier">my_grammar</span> <span class="special">&gt;</span>
2028 <span class="special">{</span>
2029     <span class="comment">// Here is where you define how Proto should handle</span>
2030     <span class="comment">// objects that are to be turned into expressions</span>
2031     <span class="comment">// fit for storage in local variables.</span>
2032     <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">&gt;</span>
2033     <span class="keyword">struct</span> <span class="identifier">as_expr</span>
2034     <span class="special">{</span>
2035         <span class="keyword">typedef</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-type</code></em></span> <span class="identifier">result_type</span><span class="special">;</span>
2036
2037         <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="identifier">T</span> <span class="special">&amp;</span> <span class="identifier">t</span> <span class="special">)</span> <span class="keyword">const</span>
2038         <span class="special">{</span>
2039             <span class="keyword">return</span> <span class="emphasis"><em><code class="literal">unspecified-Proto-expr-object</code></em></span><span class="special">;</span>
2040         <span class="special">}</span>
2041     <span class="special">};</span>
2042 <span class="special">};</span>
2043 </pre>
2044 <h6>
2045 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.h3"></a>
2046             <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.making_proto_expressions__literal_auto__literal__safe"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.making_proto_expressions__literal_auto__literal__safe">Making
2047             Proto Expressions <code class="literal">auto</code>-safe</a>
2048           </h6>
2049 <p>
2050             Let's look again at the problem described above involving the C++11
2051             <code class="computeroutput"><span class="keyword">auto</span></code> keyword and the default
2052             behavior of <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code>.
2053           </p>
2054 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
2055 <span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span> <span class="comment">// This is WRONG! Don't do this.</span>
2056 </pre>
2057 <p>
2058             Recall that the problem is the lifetime of the temporary integer created
2059             to hold the value 42. The local <code class="computeroutput"><span class="identifier">l</span></code>
2060             will be left holding a dangling reference to it after its lifetime is
2061             over. What if we want Proto to make expressions safe to store this way
2062             in local variables? We can do so very easily by making <code class="computeroutput"><a class="link" href="../boost/proto/as_child.html" title="Function as_child">proto::as_child()</a></code> behave just like <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code>. The following code
2063             achieves this:
2064           </p>
2065 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">E</span> <span class="special">&gt;</span>
2066 <span class="keyword">struct</span> <span class="identifier">my_expr</span><span class="special">;</span>
2067
2068 <span class="keyword">struct</span> <span class="identifier">my_generator</span>
2069   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special">&lt;</span> <span class="identifier">my_expr</span> <span class="special">&gt;</span>
2070 <span class="special">{};</span>
2071
2072 <span class="keyword">struct</span> <span class="identifier">my_domain</span>
2073   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span> <span class="identifier">my_generator</span> <span class="special">&gt;</span>
2074 <span class="special">{</span>
2075      <span class="comment">// Make as_child() behave like as_expr() in my_domain.</span>
2076      <span class="comment">// (proto_base_domain is a typedef for proto::domain&lt; my_generator &gt;</span>
2077      <span class="comment">// that is defined in proto::domain&lt;&gt;.)</span>
2078      <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">&gt;</span>
2079      <span class="keyword">struct</span> <span class="identifier">as_child</span>
2080        <span class="special">:</span> <span class="identifier">proto_base_domain</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span> <span class="identifier">T</span> <span class="special">&gt;</span>
2081      <span class="special">{};</span>
2082 <span class="special">};</span>
2083
2084 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">E</span> <span class="special">&gt;</span>
2085 <span class="keyword">struct</span> <span class="identifier">my_expr</span>
2086 <span class="special">{</span>
2087     <span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">(</span> <span class="identifier">E</span><span class="special">,</span> <span class="identifier">my_expr</span><span class="special">&lt;</span> <span class="identifier">E</span> <span class="special">&gt;,</span> <span class="identifier">my_domain</span> <span class="special">)</span>
2088 <span class="special">};</span>
2089
2090 <span class="comment">/* ... */</span>
2091
2092 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">my_domain</span> <span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
2093 <span class="keyword">auto</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span> <span class="comment">// OK! Everything is stored by value here.</span>
2094 </pre>
2095 <p>
2096             Notice that <code class="computeroutput"><span class="identifier">my_domain</span><span class="special">::</span><span class="identifier">as_child</span><span class="special">&lt;&gt;</span></code> simply defers to the default
2097             implementation of <code class="computeroutput"><span class="identifier">as_expr</span><span class="special">&lt;&gt;</span></code> found in <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain&lt;&gt;</a></code>.
2098             By simply cross-wiring our domain's <code class="computeroutput"><span class="identifier">as_child</span><span class="special">&lt;&gt;</span></code> to <code class="computeroutput"><span class="identifier">as_expr</span><span class="special">&lt;&gt;</span></code>, we guarantee that all terminals
2099             that can be held by value are, and that all child expressions are also
2100             held by value. This increases copying and may incur a runtime performance
2101             cost, but it eliminates any spector of lifetime management issues.
2102           </p>
2103 <p>
2104             For another example, see the definition of <code class="computeroutput"><span class="identifier">lldomain</span></code>
2105             in <code class="literal">libs/proto/example/lambda.hpp</code>. That example is
2106             a complete reimplementation of the Boost Lambda Library (BLL) on top
2107             of Boost.Proto. The function objects the BLL generates are safe to be
2108             stored in local variables. To emulate this with Proto, the <code class="computeroutput"><span class="identifier">lldomain</span></code> cross-wires <code class="computeroutput"><span class="identifier">as_child</span><span class="special">&lt;&gt;</span></code>
2109             to <code class="computeroutput"><span class="identifier">as_expr</span><span class="special">&lt;&gt;</span></code>
2110             as above, but with one extra twist: objects with array type are also
2111             stored by reference. Check it out.
2112           </p>
2113 </div>
2114 <div class="section">
2115 <div class="titlepage"><div><div><h5 class="title">
2116 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains" title="EDSL Interoperatability: Sub-Domains">EDSL
2117           Interoperatability: Sub-Domains</a>
2118 </h5></div></div></div>
2119 <div class="note"><table border="0" summary="Note">
2120 <tr>
2121 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
2122 <th align="left">Note</th>
2123 </tr>
2124 <tr><td align="left" valign="top"><p>
2125               This is an advanced topic. Feel free to skip this if you're just getting
2126               started with Proto.
2127             </p></td></tr>
2128 </table></div>
2129 <p>
2130             The ability to <span class="emphasis"><em>compose</em></span> different EDSLs is one of
2131             their most exciting features. Consider how you build a parser using yacc.
2132             You write your grammar rules in yacc's domain-specific language. Then
2133             you embed semantic actions written in C within your grammar. Boost's
2134             Spirit parser generator gives you the same ability. You write grammar
2135             rules using Spirit.Qi and embed semantic actions using the Phoenix library.
2136             Phoenix and Spirit are both Proto-based domain-specific languages with
2137             their own distinct syntax and semantics. But you can freely embed Phoenix
2138             expressions within Spirit expressions. This section describes Proto's
2139             <span class="emphasis"><em>sub-domain</em></span> feature that lets you define families
2140             of interoperable domains.
2141           </p>
2142 <h6>
2143 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h0"></a>
2144             <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.dueling_domains"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.dueling_domains">Dueling
2145             Domains</a>
2146           </h6>
2147 <p>
2148             When you try to create an expression from two sub-expressions in different
2149             domains, what is the domain of the resulting expression? This is the
2150             fundamental problem that is addressed by sub-domains. Consider the following
2151             code:
2152           </p>
2153 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
2154 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
2155
2156 <span class="comment">// Forward-declare two expression wrappers</span>
2157 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">spirit_expr</span><span class="special">;</span>
2158 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">phoenix_expr</span><span class="special">;</span>
2159
2160 <span class="comment">// Define two domains</span>
2161 <span class="keyword">struct</span> <span class="identifier">spirit_domain</span>  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">spirit_expr</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">{};</span>
2162 <span class="keyword">struct</span> <span class="identifier">phoenix_domain</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">phoenix_expr</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">{};</span>
2163
2164 <span class="comment">// Implement the two expression wrappers</span>
2165 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span>
2166 <span class="keyword">struct</span> <span class="identifier">spirit_expr</span>
2167   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">spirit_expr</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">&gt;,</span> <span class="identifier">spirit_domain</span><span class="special">&gt;</span>
2168 <span class="special">{</span>
2169     <span class="identifier">spirit_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">spirit_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span>
2170 <span class="special">};</span>
2171
2172 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span>
2173 <span class="keyword">struct</span> <span class="identifier">phoenix_expr</span>
2174   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">phoenix_expr</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">&gt;,</span> <span class="identifier">phoenix_domain</span><span class="special">&gt;</span>
2175 <span class="special">{</span>
2176     <span class="identifier">phoenix_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">phoenix_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span>
2177 <span class="special">};</span>
2178
2179 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
2180 <span class="special">{</span>
2181     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">spirit_domain</span><span class="special">&gt;</span> <span class="identifier">sp</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
2182     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">phoenix_domain</span><span class="special">&gt;</span> <span class="identifier">phx</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
2183
2184     <span class="comment">// Whoops! What does it mean to add two expressions in different domains?</span>
2185     <span class="identifier">sp</span> <span class="special">+</span> <span class="identifier">phx</span><span class="special">;</span> <span class="comment">// ERROR</span>
2186 <span class="special">}</span>
2187 </pre>
2188 <p>
2189             Above, we define two domains called <code class="computeroutput"><span class="identifier">spirit_domain</span></code>
2190             and <code class="computeroutput"><span class="identifier">phoenix_domain</span></code> and
2191             declare two int literals in each. Then we try to compose them into a
2192             larger expression using Proto's binary plus operator, and it fails. Proto
2193             can't figure out whether the resulting expression should be in the Spirit
2194             domain or the Phoenix domain, and thus whether it should be an instance
2195             of <code class="computeroutput"><span class="identifier">spirit_expr</span><span class="special">&lt;&gt;</span></code>
2196             or <code class="computeroutput"><span class="identifier">phoenix_expr</span><span class="special">&lt;&gt;</span></code>.
2197             We have to tell Proto how to resolve the conflict. We can do that by
2198             declaring that Phoenix is a sub-domain of Spirit as in the following
2199             definition of <code class="computeroutput"><span class="identifier">phoenix_domain</span></code>:
2200           </p>
2201 <pre class="programlisting"><span class="comment">// Declare that phoenix_domain is a sub-domain of spirit_domain</span>
2202 <span class="keyword">struct</span> <span class="identifier">phoenix_domain</span>
2203   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">phoenix_expr</span><span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="bold"><strong>spirit_domain</strong></span><span class="special">&gt;</span>
2204 <span class="special">{};</span>
2205 </pre>
2206 <p>
2207             The third template parameter to <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain&lt;&gt;</a></code>
2208             is the super-domain. By defining <code class="computeroutput"><span class="identifier">phoenix_domain</span></code>
2209             as above, we are saying that Phoenix expressions can be combined with
2210             Spirit expressions, and that when that happens, the resulting expression
2211             should be a Spirit expression.
2212           </p>
2213 <div class="note"><table border="0" summary="Note">
2214 <tr>
2215 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
2216 <th align="left">Note</th>
2217 </tr>
2218 <tr><td align="left" valign="top"><p>
2219               If you are wondering what the purpose of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code>
2220               is in the definition of <code class="computeroutput"><span class="identifier">phoenix_domain</span></code>
2221               above, recall that the second template parameter to <code class="computeroutput"><a class="link" href="../boost/proto/domain.html" title="Struct template domain">proto::domain&lt;&gt;</a></code>
2222               is the domain's grammar. <span class="quote">&#8220;<span class="quote"><code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code></span>&#8221;</span>
2223               is the default and signifies that the domain places no restrictions
2224               on the expressions that are valid within it.
2225             </p></td></tr>
2226 </table></div>
2227 <h6>
2228 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h1"></a>
2229             <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.domain_resolution"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.domain_resolution">Domain
2230             Resolution</a>
2231           </h6>
2232 <p>
2233             When there are multiple domains in play within a given expression, Proto
2234             uses some rules to figure out which domain "wins". The rules
2235             are loosely modeled on the rules for C++ inheritance. <code class="computeroutput"><span class="identifier">Phoenix_domain</span></code>
2236             is a sub-domain of <code class="computeroutput"><span class="identifier">spirit_domain</span></code>.
2237             You can liken that to a derived/base relationship that gives Phoenix
2238             expressions a kind of implicit conversion to Spirit expressions. And
2239             since Phoenix expressions can be "converted" to Spirit expressions,
2240             they can be freely combined with Spirit expressions and the result is
2241             a Spirit expression.
2242           </p>
2243 <div class="note"><table border="0" summary="Note">
2244 <tr>
2245 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
2246 <th align="left">Note</th>
2247 </tr>
2248 <tr><td align="left" valign="top"><p>
2249               Super- and sub-domains are not actually implemented using inheritance.
2250               This is only a helpful mental model.
2251             </p></td></tr>
2252 </table></div>
2253 <p>
2254             The analogy with inheritance holds even in the case of three domains
2255             when two are sub-domains of the third. Imagine another domain called
2256             <code class="computeroutput"><span class="identifier">foobar_domain</span></code> that was
2257             also a sub-domain of <code class="computeroutput"><span class="identifier">spirit_domain</span></code>.
2258             Expressions in the <code class="computeroutput"><span class="identifier">foobar_domain</span></code>
2259             could be combined with expressions in the <code class="computeroutput"><span class="identifier">phoenix_domain</span></code>
2260             and the resulting expression would be in the <code class="computeroutput"><span class="identifier">spirit_domain</span></code>.
2261             That's because expressions in the two sub-domains both have "conversions"
2262             to the super-domain, so the operation is allowed and the super-domain
2263             wins.
2264           </p>
2265 <h6>
2266 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h2"></a>
2267             <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.the_default_domain"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.the_default_domain">The
2268             Default Domain</a>
2269           </h6>
2270 <p>
2271             When you don't assign a Proto expression to a particular domain, Proto
2272             considers it a member of the so-called default domain, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span></code>. Even non-Proto objects
2273             are treated as terminals in the default domain. Consider:
2274           </p>
2275 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
2276 <span class="special">{</span>
2277     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">spirit_domain</span><span class="special">&gt;</span> <span class="identifier">sp</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
2278
2279     <span class="comment">// Add 1 to a spirit expression. Result is a spirit expression.</span>
2280     <span class="identifier">sp</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span>
2281 <span class="special">}</span>
2282 </pre>
2283 <p>
2284             Expressions in the default domain (or non-expressions like <code class="literal">1</code>)
2285             have a kind of implicit conversion to expressions every other domain
2286             type. What's more, you can define your domain to be a sub-domain of the
2287             default domain. In so doing, you give expressions in your domain conversions
2288             to expressions in every other domain. This is like a <span class="quote">&#8220;<span class="quote">free love</span>&#8221;</span>
2289             domain, because it will freely mix with all other domains.
2290           </p>
2291 <p>
2292             Let's think again about the Phoenix EDSL. Since it provides generally
2293             useful lambda functionality, it's reasonable to assume that lots of other
2294             EDSLs besides Spirit might want the ability to embed Phoenix expressions.
2295             In other words, <code class="computeroutput"><span class="identifier">phoenix_domain</span></code>
2296             should be a sub-domain of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span></code>,
2297             not <code class="computeroutput"><span class="identifier">spirit_domain</span></code>:
2298           </p>
2299 <pre class="programlisting"><span class="comment">// Declare that phoenix_domain is a sub-domain of proto::default_domain</span>
2300 <span class="keyword">struct</span> <span class="identifier">phoenix_domain</span>
2301   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">phoenix_expr</span><span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span><span class="special">&gt;</span>
2302 <span class="special">{};</span>
2303 </pre>
2304 <p>
2305             That's much better. Phoenix expressions can now be put anywhere.
2306           </p>
2307 <h6>
2308 <a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.h3"></a>
2309             <span class="phrase"><a name="boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.sub_domain_summary"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.subdomains.sub_domain_summary">Sub-Domain
2310             Summary</a>
2311           </h6>
2312 <p>
2313             Use Proto sub-domains to make it possible to mix expressions from multiple
2314             domains. And when you want expressions in your domain to freely combine
2315             with <span class="emphasis"><em>all</em></span> expressions, make it a sub-domain of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span></code>.
2316           </p>
2317 </div>
2318 </div>
2319 <div class="section">
2320 <div class="titlepage"><div><div><h4 class="title">
2321 <a name="boost_proto.users_guide.front_end.define_operators"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.define_operators" title="Adapting Existing Types to Proto">Adapting
2322         Existing Types to Proto</a>
2323 </h4></div></div></div>
2324 <p>
2325           The preceding discussions of defining Proto front ends have all made a
2326           big assumption: that you have the luxury of defining everything from scratch.
2327           What happens if you have existing types, say a matrix type and a vector
2328           type, that you would like to treat as if they were Proto terminals? Proto
2329           usually trades only in its own expression types, but with <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_33_3.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code>,
2330           it can accomodate your custom terminal types, too.
2331         </p>
2332 <p>
2333           Let's say, for instance, that you have the following types and that you
2334           can't modify then to make them <span class="quote">&#8220;<span class="quote">native</span>&#8221;</span> Proto terminal types.
2335         </p>
2336 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">math</span>
2337 <span class="special">{</span>
2338     <span class="comment">// A matrix type ...</span>
2339     <span class="keyword">struct</span> <span class="identifier">matrix</span> <span class="special">{</span> <span class="comment">/*...*/</span> <span class="special">};</span>
2340
2341     <span class="comment">// A vector type ...</span>
2342     <span class="keyword">struct</span> <span class="identifier">vector</span> <span class="special">{</span> <span class="comment">/*...*/</span> <span class="special">};</span>
2343 <span class="special">}</span>
2344 </pre>
2345 <p>
2346           You can non-intrusively make objects of these types Proto terminals by
2347           defining the proper operator overloads using <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_33_3.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code>.
2348           The basic procedure is as follows:
2349         </p>
2350 <div class="orderedlist"><ol class="orderedlist" type="1">
2351 <li class="listitem">
2352               Define a trait that returns true for your types and false for all others.
2353             </li>
2354 <li class="listitem">
2355               Reopen the namespace of your types and use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_33_3.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code>
2356               to define a set of operator overloads, passing the name of the trait
2357               as the first macro parameter, and the name of a Proto domain (e.g.,
2358               <code class="computeroutput"><a class="link" href="../boost/proto/default_domain.html" title="Struct default_domain">proto::default_domain</a></code>)
2359               as the second.
2360             </li>
2361 </ol></div>
2362 <p>
2363           The following code demonstrates how it works.
2364         </p>
2365 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">math</span>
2366 <span class="special">{</span>
2367     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
2368     <span class="keyword">struct</span> <span class="identifier">is_terminal</span>
2369       <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span>
2370     <span class="special">{};</span>
2371
2372     <span class="comment">// OK, "matrix" is a custom terminal type</span>
2373     <span class="keyword">template</span><span class="special">&lt;&gt;</span>
2374     <span class="keyword">struct</span> <span class="identifier">is_terminal</span><span class="special">&lt;</span><span class="identifier">matrix</span><span class="special">&gt;</span>
2375       <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
2376     <span class="special">{};</span>
2377
2378     <span class="comment">// OK, "vector" is a custom terminal type</span>
2379     <span class="keyword">template</span><span class="special">&lt;&gt;</span>
2380     <span class="keyword">struct</span> <span class="identifier">is_terminal</span><span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
2381       <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
2382     <span class="special">{};</span>
2383
2384     <span class="comment">// Define all the operator overloads to construct Proto</span>
2385     <span class="comment">// expression templates, treating "matrix" and "vector"</span>
2386     <span class="comment">// objects as if they were Proto terminals.</span>
2387     <span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">(</span><span class="identifier">is_terminal</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span><span class="special">)</span>
2388 <span class="special">}</span>
2389 </pre>
2390 <p>
2391           The invocation of the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_33_3.html" title="Macro BOOST_PROTO_DEFINE_OPERATORS">BOOST_PROTO_DEFINE_OPERATORS</a></code>()</code>
2392           macro defines a complete set of operator overloads that treat <code class="computeroutput"><span class="identifier">matrix</span></code> and <code class="computeroutput"><span class="identifier">vector</span></code>
2393           objects as if they were Proto terminals. And since the operators are defined
2394           in the same namespace as the <code class="computeroutput"><span class="identifier">matrix</span></code>
2395           and <code class="computeroutput"><span class="identifier">vector</span></code> types, the operators
2396           will be found by argument-dependent lookup. With the code above, we can
2397           now construct expression templates with matrices and vectors, as shown
2398           below.
2399         </p>
2400 <pre class="programlisting"><span class="identifier">math</span><span class="special">::</span><span class="identifier">matrix</span> <span class="identifier">m1</span><span class="special">;</span>
2401 <span class="identifier">math</span><span class="special">::</span><span class="identifier">vector</span> <span class="identifier">v1</span><span class="special">;</span>
2402 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
2403
2404 <span class="identifier">m1</span> <span class="special">*</span> <span class="number">1</span><span class="special">;</span>  <span class="comment">// custom terminal and literals are OK</span>
2405 <span class="identifier">m1</span> <span class="special">*</span> <span class="identifier">i</span><span class="special">;</span>  <span class="comment">// custom terminal and Proto expressions are OK</span>
2406 <span class="identifier">m1</span> <span class="special">*</span> <span class="identifier">v1</span><span class="special">;</span> <span class="comment">// two custom terminals are OK, too.</span>
2407 </pre>
2408 </div>
2409 <div class="section">
2410 <div class="titlepage"><div><div><h4 class="title">
2411 <a name="boost_proto.users_guide.front_end.code_repetition"></a><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.code_repetition" title="Generating Repetitive Code with the Preprocessor">Generating
2412         Repetitive Code with the Preprocessor</a>
2413 </h4></div></div></div>
2414 <p>
2415           Sometimes as an EDSL designer, to make the lives of your users easy, you
2416           have to make your own life hard. Giving your users natural and flexible
2417           syntax often involves writing large numbers of repetitive function overloads.
2418           It can be enough to give you repetitive stress injury! Before you hurt
2419           yourself, check out the macros Proto provides for automating many repetitive
2420           code-generation chores.
2421         </p>
2422 <p>
2423           Imagine that we are writing a lambda EDSL, and we would like to enable
2424           syntax for constructing temporary objects of any type using the following
2425           syntax:
2426         </p>
2427 <pre class="programlisting"><span class="comment">// A lambda expression that takes two arguments and</span>
2428 <span class="comment">// uses them to construct a temporary std::complex&lt;&gt;</span>
2429 <span class="identifier">construct</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;(</span> <span class="identifier">_1</span><span class="special">,</span> <span class="identifier">_2</span> <span class="special">)</span>
2430 </pre>
2431 <p>
2432           For the sake of the discussion, imagine that we already have a function
2433           object template <code class="computeroutput"><span class="identifier">construct_impl</span><span class="special">&lt;&gt;</span></code> that accepts arguments and constructs
2434           new objects from them. We would want the above lambda expression to be
2435           equivalent to the following:
2436         </p>
2437 <pre class="programlisting"><span class="comment">// The above lambda expression should be roughly equivalent</span>
2438 <span class="comment">// to the following:</span>
2439 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">&gt;(</span>
2440     <span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;()</span> <span class="comment">// The function to invoke lazily</span>
2441   <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">_1</span><span class="special">)</span>                       <span class="comment">// The first argument to the function</span>
2442   <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">_2</span><span class="special">)</span>                       <span class="comment">// The second argument to the function</span>
2443 <span class="special">);</span>
2444 </pre>
2445 <p>
2446           We can define our <code class="computeroutput"><span class="identifier">construct</span><span class="special">()</span></code> function template as follows:
2447         </p>
2448 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">&gt;</span>
2449 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>
2450     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>
2451   <span class="special">,</span> <span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
2452   <span class="special">,</span> <span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;</span>
2453   <span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;</span>
2454 <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span>
2455 <span class="identifier">construct</span><span class="special">(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a1</span><span class="special">)</span>
2456 <span class="special">{</span>
2457     <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">&gt;(</span>
2458         <span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</span>
2459       <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">a0</span><span class="special">)</span>
2460       <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span>
2461     <span class="special">);</span>
2462 <span class="special">}</span>
2463 </pre>
2464 <p>
2465           This works for two arguments, but we would like it to work for any number
2466           of arguments, up to ( <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_MAX_ARITY.html" title="Macro BOOST_PROTO_MAX_ARITY">BOOST_PROTO_MAX_ARITY</a></code></code>
2467           - 1). (Why "- 1"? Because one child is taken up by the <code class="computeroutput"><span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</span></code>
2468           terminal leaving room for only ( <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_MAX_ARITY.html" title="Macro BOOST_PROTO_MAX_ARITY">BOOST_PROTO_MAX_ARITY</a></code></code>
2469           - 1) other children.)
2470         </p>
2471 <p>
2472           For cases like this, Proto provides the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT.html" title="Macro BOOST_PROTO_REPEAT">BOOST_PROTO_REPEAT</a></code>()</code>
2473           and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_FROM_TO.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO">BOOST_PROTO_REPEAT_FROM_TO</a></code>()</code>
2474           macros. To use it, we turn the function definition above into a macro as
2475           follows:
2476         </p>
2477 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">M0</span><span class="special">(</span><span class="identifier">N</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">,</span> <span class="identifier">A_const_ref_a</span><span class="special">,</span> <span class="identifier">ref_a</span><span class="special">)</span>  <span class="special">\</span>
2478 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">(</span><span class="identifier">N</span><span class="special">)&gt;</span>                           <span class="special">\</span>
2479 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>                         <span class="special">\</span>
2480     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>                                      <span class="special">\</span>
2481   <span class="special">,</span> <span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>                                         <span class="special">\</span>
2482   <span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span>                                            <span class="special">\</span>
2483 <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span>                                                 <span class="special">\</span>
2484 <span class="identifier">construct</span><span class="special">(</span><span class="identifier">A_const_ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">))</span>                                   <span class="special">\</span>
2485 <span class="special">{</span>                                                             <span class="special">\</span>
2486     <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">&gt;(</span>            <span class="special">\</span>
2487         <span class="identifier">construct_impl</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</span>                                   <span class="special">\</span>
2488       <span class="special">,</span> <span class="identifier">ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span>                                              <span class="special">\</span>
2489     <span class="special">);</span>                                                        <span class="special">\</span>
2490 <span class="special">}</span>
2491 </pre>
2492 <p>
2493           Notice that we turned the function into a macro that takes 5 arguments.
2494           The first is the current iteration number. The rest are the names of other
2495           macros that generate different sequences. For instance, Proto passes as
2496           the second parameter the name of a macro that will expand to <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">,</span> <span class="special">...</span></code>.
2497         </p>
2498 <p>
2499           Now that we have turned our function into a macro, we can pass the macro
2500           to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_FROM_TO.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO">BOOST_PROTO_REPEAT_FROM_TO</a></code>()</code>.
2501           Proto will invoke it iteratively, generating all the function overloads
2502           for us.
2503         </p>
2504 <pre class="programlisting"><span class="comment">// Generate overloads of construct() that accept from</span>
2505 <span class="comment">// 1 to BOOST_PROTO_MAX_ARITY-1 arguments:</span>
2506 <span class="identifier">BOOST_PROTO_REPEAT_FROM_TO</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="identifier">BOOST_PROTO_MAX_ARITY</span><span class="special">,</span> <span class="identifier">M0</span><span class="special">)</span>
2507 <span class="preprocessor">#undef</span> <span class="identifier">M0</span>
2508 </pre>
2509 <h6>
2510 <a name="boost_proto.users_guide.front_end.code_repetition.h0"></a>
2511           <span class="phrase"><a name="boost_proto.users_guide.front_end.code_repetition.non_default_sequences"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.front_end.code_repetition.non_default_sequences">Non-Default
2512           Sequences</a>
2513         </h6>
2514 <p>
2515           As mentioned above, Proto passes as the last 4 arguments to your macro
2516           the names of other macros that generate various sequences. The macros
2517           <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT.html" title="Macro BOOST_PROTO_REPEAT">BOOST_PROTO_REPEAT</a></code>()</code>
2518           and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_FROM_TO.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO">BOOST_PROTO_REPEAT_FROM_TO</a></code>()</code>
2519           select defaults for these parameters. If the defaults do not meet your
2520           needs, you can use <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_EX.html" title="Macro BOOST_PROTO_REPEAT_EX">BOOST_PROTO_REPEAT_EX</a></code>()</code>
2521           and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_37_5.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO_EX">BOOST_PROTO_REPEAT_FROM_TO_EX</a></code>()</code>
2522           and pass different macros that generate different sequences. Proto defines
2523           a number of such macros for use as parameters to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT_EX.html" title="Macro BOOST_PROTO_REPEAT_EX">BOOST_PROTO_REPEAT_EX</a></code>()</code>
2524           and <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO__1_3_34_5_37_5.html" title="Macro BOOST_PROTO_REPEAT_FROM_TO_EX">BOOST_PROTO_REPEAT_FROM_TO_EX</a></code>()</code>.
2525           Check the reference section for <code class="computeroutput"><a class="link" href="reference.html#header.boost.proto.repeat_hpp" title="Header &lt;boost/proto/repeat.hpp&gt;">boost/proto/repeat.hpp</a></code>
2526           for all the details.
2527         </p>
2528 <p>
2529           Also, check out <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_LOCAL_ITERATE.html" title="Macro BOOST_PROTO_LOCAL_ITERATE">BOOST_PROTO_LOCAL_ITERATE</a></code>()</code>.
2530           It works similarly to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_REPEAT.html" title="Macro BOOST_PROTO_REPEAT">BOOST_PROTO_REPEAT</a></code>()</code>
2531           and friends, but it can be easier to use when you want to change one macro
2532           argument and accept defaults for the others.
2533         </p>
2534 </div>
2535 </div>
2536 <div class="section">
2537 <div class="titlepage"><div><div><h3 class="title">
2538 <a name="boost_proto.users_guide.intermediate_form"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form" title="Intermediate Form: Understanding and Introspecting Expressions">Intermediate
2539       Form: Understanding and Introspecting Expressions</a>
2540 </h3></div></div></div>
2541 <div class="toc"><dl class="toc">
2542 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child">Accessing
2543         Parts of an Expression</a></span></dt>
2544 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.deep_copying_expressions">Deep-copying
2545         Expressions</a></span></dt>
2546 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.debugging_expressions">Debugging
2547         Expressions</a></span></dt>
2548 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.tags_and_metafunctions">Operator
2549         Tags and Metafunctions</a></span></dt>
2550 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences">Expressions
2551         as Fusion Sequences</a></span></dt>
2552 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection">Expression
2553         Introspection: Defining a Grammar</a></span></dt>
2554 </dl></div>
2555 <p>
2556         By now, you know a bit about how to build a front-end for your EDSL "compiler"
2557         -- you can define terminals and functions that generate expression templates.
2558         But we haven't said anything about the expression templates themselves. What
2559         do they look like? What can you do with them? In this section we'll see.
2560       </p>
2561 <h5>
2562 <a name="boost_proto.users_guide.intermediate_form.h0"></a>
2563         <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.the__literal_expr_lt__gt___literal__type"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.the__literal_expr_lt__gt___literal__type">The
2564         <code class="literal">expr&lt;&gt;</code> Type</a>
2565       </h5>
2566 <p>
2567         All Proto expressions are an instantiation of a template called <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> (or a wrapper around
2568         such an instantiation). When we define a terminal as below, we are really
2569         initializing an instance of the <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>
2570         template.
2571       </p>
2572 <pre class="programlisting"><span class="comment">// Define a placeholder type</span>
2573 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
2574 <span class="keyword">struct</span> <span class="identifier">placeholder</span>
2575 <span class="special">{};</span>
2576
2577 <span class="comment">// Define the Protofied placeholder terminal</span>
2578 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
2579 </pre>
2580 <p>
2581         The actual type of <code class="computeroutput"><span class="identifier">_1</span></code> looks
2582         like this:
2583       </p>
2584 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;,</span> <span class="number">0</span> <span class="special">&gt;</span>
2585 </pre>
2586 <p>
2587         The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> template is the most
2588         important type in Proto. Although you will rarely need to deal with it directly,
2589         it's always there behind the scenes holding your expression trees together.
2590         In fact, <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> <span class="emphasis"><em>is</em></span>
2591         the expression tree -- branches, leaves and all.
2592       </p>
2593 <p>
2594         The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> template makes up the
2595         nodes in expression trees. The first template parameter is the node type;
2596         in this case, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span></code>.
2597         That means that <code class="computeroutput"><span class="identifier">_1</span></code> is a leaf-node
2598         in the expression tree. The second template parameter is a list of child
2599         types, or in the case of terminals, the terminal's value type. Terminals
2600         will always have only one type in the type list. The last parameter is the
2601         arity of the expression. Terminals have arity 0, unary expressions have arity
2602         1, etc.
2603       </p>
2604 <p>
2605         The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> struct is defined as
2606         follows:
2607       </p>
2608 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Args</span><span class="special">,</span> <span class="keyword">long</span> <span class="identifier">Arity</span> <span class="special">=</span> <span class="identifier">Args</span><span class="special">::</span><span class="identifier">arity</span> <span class="special">&gt;</span>
2609 <span class="keyword">struct</span> <span class="identifier">expr</span><span class="special">;</span>
2610
2611 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Args</span> <span class="special">&gt;</span>
2612 <span class="keyword">struct</span> <span class="identifier">expr</span><span class="special">&lt;</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">,</span> <span class="number">1</span> <span class="special">&gt;</span>
2613 <span class="special">{</span>
2614     <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Args</span><span class="special">::</span><span class="identifier">child0</span> <span class="identifier">proto_child0</span><span class="special">;</span>
2615     <span class="identifier">proto_child0</span> <span class="identifier">child0</span><span class="special">;</span>
2616     <span class="comment">// ...</span>
2617 <span class="special">};</span>
2618 </pre>
2619 <p>
2620         The <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> struct does not define
2621         a constructor, or anything else that would prevent static initialization.
2622         All <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code> objects are initialized
2623         using <span class="emphasis"><em>aggregate initialization</em></span>, with curly braces. In
2624         our example, <code class="computeroutput"><span class="identifier">_1</span></code> is initialized
2625         with the initializer <code class="computeroutput"><span class="special">{{}}</span></code>. The
2626         outer braces are the initializer for the <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>
2627         struct, and the inner braces are for the member <code class="computeroutput"><span class="identifier">_1</span><span class="special">.</span><span class="identifier">child0</span></code>
2628         which is of type <code class="computeroutput"><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span></code>.
2629         Note that we use braces to initialize <code class="computeroutput"><span class="identifier">_1</span><span class="special">.</span><span class="identifier">child0</span></code>
2630         because <code class="computeroutput"><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span></code> is also
2631         an aggregate.
2632       </p>
2633 <h5>
2634 <a name="boost_proto.users_guide.intermediate_form.h1"></a>
2635         <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.building_expression_trees"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.building_expression_trees">Building
2636         Expression Trees</a>
2637       </h5>
2638 <p>
2639         The <code class="computeroutput"><span class="identifier">_1</span></code> node is an instantiation
2640         of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>, and expressions containing
2641         <code class="computeroutput"><span class="identifier">_1</span></code> are also instantiations
2642         of <code class="computeroutput"><a class="link" href="../boost/proto/expr.html" title="Struct template expr">proto::expr&lt;&gt;</a></code>. To use Proto effectively,
2643         you won't have to bother yourself with the actual types that Proto generates.
2644         These are details, but you're likely to encounter these types in compiler
2645         error messages, so it's helpful to be familiar with them. The types look
2646         like this:
2647       </p>
2648 <pre class="programlisting"><span class="comment">// The type of the expression -_1</span>
2649 <span class="keyword">typedef</span>
2650     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span>
2651         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">negate</span>
2652       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">list1</span><span class="special">&lt;</span>
2653             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span>
2654                 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span>
2655               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span>
2656               <span class="special">,</span> <span class="number">0</span>
2657             <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span>
2658         <span class="special">&gt;</span>
2659       <span class="special">,</span> <span class="number">1</span>
2660     <span class="special">&gt;</span>
2661 <span class="identifier">negate_placeholder_type</span><span class="special">;</span>
2662
2663 <span class="identifier">negate_placeholder_type</span> <span class="identifier">x</span> <span class="special">=</span> <span class="special">-</span><span class="identifier">_1</span><span class="special">;</span>
2664
2665 <span class="comment">// The type of the expression _1 + 42</span>
2666 <span class="keyword">typedef</span>
2667     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span>
2668         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span>
2669       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">list2</span><span class="special">&lt;</span>
2670             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span>
2671                 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span>
2672               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span>
2673               <span class="special">,</span> <span class="number">0</span>
2674             <span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span>
2675           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;</span>
2676                 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span>
2677               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">term</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="special">&gt;</span>
2678               <span class="special">,</span> <span class="number">0</span>
2679             <span class="special">&gt;</span>
2680         <span class="special">&gt;</span>
2681       <span class="special">,</span> <span class="number">2</span>
2682     <span class="special">&gt;</span>
2683 <span class="identifier">placeholder_plus_int_type</span><span class="special">;</span>
2684
2685 <span class="identifier">placeholder_plus_int_type</span> <span class="identifier">y</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">+</span> <span class="number">42</span><span class="special">;</span>
2686 </pre>
2687 <p>
2688         There are a few things to note about these types:
2689       </p>
2690 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
2691 <li class="listitem">
2692             Terminals have arity zero, unary expressions have arity one and binary
2693             expressions have arity two.
2694           </li>
2695 <li class="listitem">
2696             When one Proto expression is made a child node of another Proto expression,
2697             it is held by reference, <span class="emphasis"><em>even if it is a temporary object</em></span>.
2698             This last point becomes important later.
2699           </li>
2700 <li class="listitem">
2701             Non-Proto expressions, such as the integer literal, are turned into Proto
2702             expressions by wrapping them in new <code class="computeroutput"><span class="identifier">expr</span><span class="special">&lt;&gt;</span></code> terminal objects. These new wrappers
2703             are not themselves held by reference, but the object wrapped <span class="emphasis"><em>is</em></span>.
2704             Notice that the type of the Protofied <code class="computeroutput"><span class="number">42</span></code>
2705             literal is <code class="computeroutput"><span class="keyword">int</span> <span class="keyword">const</span>
2706             <span class="special">&amp;</span></code> -- held by reference.
2707           </li>
2708 </ul></div>
2709 <p>
2710         The types make it clear: everything in a Proto expression tree is held by
2711         reference. That means that building an expression tree is exceptionally cheap.
2712         It involves no copying at all.
2713       </p>
2714 <div class="note"><table border="0" summary="Note">
2715 <tr>
2716 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
2717 <th align="left">Note</th>
2718 </tr>
2719 <tr><td align="left" valign="top"><p>
2720           An astute reader will notice that the object <code class="computeroutput"><span class="identifier">y</span></code>
2721           defined above will be left holding a dangling reference to a temporary
2722           int. In the sorts of high-performance applications Proto addresses, it
2723           is typical to build and evaluate an expression tree before any temporary
2724           objects go out of scope, so this dangling reference situation often doesn't
2725           arise, but it is certainly something to be aware of. Proto provides utilities
2726           for deep-copying expression trees so they can be passed around as value
2727           types without concern for dangling references.
2728         </p></td></tr>
2729 </table></div>
2730 <div class="section">
2731 <div class="titlepage"><div><div><h4 class="title">
2732 <a name="boost_proto.users_guide.intermediate_form.left_right_child"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child" title="Accessing Parts of an Expression">Accessing
2733         Parts of an Expression</a>
2734 </h4></div></div></div>
2735 <p>
2736           After assembling an expression into a tree, you'll naturally want to be
2737           able to do the reverse, and access a node's children. You may even want
2738           to be able to iterate over the children with algorithms from the Boost.Fusion
2739           library. This section shows how.
2740         </p>
2741 <h6>
2742 <a name="boost_proto.users_guide.intermediate_form.left_right_child.h0"></a>
2743           <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.getting_expression_tags_and_arities"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.getting_expression_tags_and_arities">Getting
2744           Expression Tags and Arities</a>
2745         </h6>
2746 <p>
2747           Every node in an expression tree has both a <span class="emphasis"><em>tag</em></span> type
2748           that describes the node, and an <span class="emphasis"><em>arity</em></span> corresponding
2749           to the number of child nodes it has. You can use the <code class="computeroutput"><a class="link" href="../boost/proto/tag_of.html" title="Struct template tag_of">proto::tag_of&lt;&gt;</a></code>
2750           and <code class="computeroutput"><a class="link" href="../boost/proto/arity_of.html" title="Struct template arity_of">proto::arity_of&lt;&gt;</a></code> metafunctions to fetch
2751           them. Consider the following:
2752         </p>
2753 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
2754 <span class="keyword">void</span> <span class="identifier">check_plus_node</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;)</span>
2755 <span class="special">{</span>
2756     <span class="comment">// Assert that the tag type is proto::tag::plus</span>
2757     <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span>
2758         <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span>
2759             <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span>
2760           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span>
2761         <span class="special">&gt;::</span><span class="identifier">value</span>
2762     <span class="special">));</span>
2763
2764     <span class="comment">// Assert that the arity is 2</span>
2765     <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">arity_of</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">value</span> <span class="special">==</span> <span class="number">2</span> <span class="special">);</span>
2766 <span class="special">}</span>
2767
2768 <span class="comment">// Create a binary plus node and use check_plus_node()</span>
2769 <span class="comment">// to verify its tag type and arity:</span>
2770 <span class="identifier">check_plus_node</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span>
2771 </pre>
2772 <p>
2773           For a given type <code class="computeroutput"><span class="identifier">Expr</span></code>,
2774           you could access the tag and arity directly as <code class="computeroutput"><span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span></code>
2775           and <code class="computeroutput"><span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_arity</span></code>, where <code class="computeroutput"><span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_arity</span></code>
2776           is an MPL Integral Constant.
2777         </p>
2778 <h6>
2779 <a name="boost_proto.users_guide.intermediate_form.left_right_child.h1"></a>
2780           <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.getting_terminal_values"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.getting_terminal_values">Getting
2781           Terminal Values</a>
2782         </h6>
2783 <p>
2784           There is no simpler expression than a terminal, and no more basic operation
2785           than extracting its value. As we've already seen, that is what <code class="computeroutput"><a class="link" href="../boost/proto/value.html" title="Function value">proto::value()</a></code> is for.
2786         </p>
2787 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span>
2788
2789 <span class="comment">// Get the value of the cout_ terminal:</span>
2790 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="identifier">sout</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">);</span>
2791
2792 <span class="comment">// Assert that we got back what we put in:</span>
2793 <span class="identifier">assert</span><span class="special">(</span> <span class="special">&amp;</span><span class="identifier">sout</span> <span class="special">==</span> <span class="special">&amp;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">);</span>
2794 </pre>
2795 <p>
2796           To compute the return type of the <code class="computeroutput"><a class="link" href="../boost/proto/value.html" title="Function value">proto::value()</a></code>
2797           function, you can use <code class="computeroutput"><a class="link" href="../boost/proto/result_of/value.html" title="Struct template value">proto::result_of::value&lt;&gt;</a></code>.
2798           When the parameter to <code class="computeroutput"><a class="link" href="../boost/proto/result_of/value.html" title="Struct template value">proto::result_of::value&lt;&gt;</a></code>
2799           is a non-reference type, the result type of the metafunction is the type
2800           of the value as suitable for storage by value; that is, top-level reference
2801           and qualifiers are stripped from it. But when instantiated with a reference
2802           type, the result type has a reference <span class="emphasis"><em>added</em></span> to it,
2803           yielding a type suitable for storage by reference. If you want to know
2804           the actual type of the terminal's value including whether it is stored
2805           by value or reference, you can use <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="number">0</span><span class="special">&gt;::</span><span class="identifier">type</span></code>.
2806         </p>
2807 <p>
2808           The following table summarizes the above paragraph.
2809         </p>
2810 <div class="table">
2811 <a name="boost_proto.users_guide.intermediate_form.left_right_child.t0"></a><p class="title"><b>Table&#160;33.4.&#160;Accessing Value Types</b></p>
2812 <div class="table-contents"><table class="table" summary="Accessing Value Types">
2813 <colgroup>
2814 <col>
2815 <col>
2816 <col>
2817 </colgroup>
2818 <thead><tr>
2819 <th>
2820                   <p>
2821                     Metafunction Invocation
2822                   </p>
2823                 </th>
2824 <th>
2825                   <p>
2826                     When the Value Type Is ...
2827                   </p>
2828                 </th>
2829 <th>
2830                   <p>
2831                     The Result Is ...
2832                   </p>
2833                 </th>
2834 </tr></thead>
2835 <tbody>
2836 <tr>
2837 <td>
2838                   <p>
2839                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
2840                   </p>
2841                 </td>
2842 <td>
2843                   <p>
2844                     <code class="computeroutput"><span class="identifier">T</span></code>
2845                   </p>
2846                 </td>
2847 <td>
2848                   <p>
2849 </p>
2850 <pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_const</span><span class="special">&lt;</span>
2851     <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span>
2852 <span class="special">&gt;::</span><span class="identifier">type</span> <a href="#ftn.boost_proto.users_guide.intermediate_form.left_right_child.f0" class="footnote" name="boost_proto.users_guide.intermediate_form.left_right_child.f0"><sup class="footnote">[a]</sup></a></pre>
2853 <p>
2854                   </p>
2855                 </td>
2856 </tr>
2857 <tr>
2858 <td>
2859                   <p>
2860                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span></code>
2861                   </p>
2862                 </td>
2863 <td>
2864                   <p>
2865                     <code class="computeroutput"><span class="identifier">T</span></code>
2866                   </p>
2867                 </td>
2868 <td>
2869                   <p>
2870 </p>
2871 <pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span></pre>
2872 <p>
2873                   </p>
2874                 </td>
2875 </tr>
2876 <tr>
2877 <td>
2878                   <p>
2879                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="keyword">const</span>
2880                     <span class="special">&amp;&gt;::</span><span class="identifier">type</span></code>
2881                   </p>
2882                 </td>
2883 <td>
2884                   <p>
2885                     <code class="computeroutput"><span class="identifier">T</span></code>
2886                   </p>
2887                 </td>
2888 <td>
2889                   <p>
2890 </p>
2891 <pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special">&lt;</span>
2892     <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span>
2893 <span class="special">&gt;::</span><span class="identifier">type</span></pre>
2894 <p>
2895                   </p>
2896                 </td>
2897 </tr>
2898 <tr>
2899 <td>
2900                   <p>
2901                     <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span>
2902                     <span class="number">0</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
2903                   </p>
2904                 </td>
2905 <td>
2906                   <p>
2907                     <code class="computeroutput"><span class="identifier">T</span></code>
2908                   </p>
2909                 </td>
2910 <td>
2911                   <p>
2912                     <code class="computeroutput"><span class="identifier">T</span></code>
2913                   </p>
2914                 </td>
2915 </tr>
2916 </tbody>
2917 <tbody class="footnotes"><tr><td colspan="3"><div id="ftn.boost_proto.users_guide.intermediate_form.left_right_child.f0" class="footnote"><p><a href="#boost_proto.users_guide.intermediate_form.left_right_child.f0" class="para"><sup class="para">[a] </sup></a>If <code class="computeroutput"><span class="identifier">T</span></code> is a reference-to-function type, then the result type is simply <code class="computeroutput"><span class="identifier">T</span></code>.</p></div></td></tr></tbody>
2918 </table></div>
2919 </div>
2920 <br class="table-break"><h6>
2921 <a name="boost_proto.users_guide.intermediate_form.left_right_child.h2"></a>
2922           <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.getting_child_expressions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.getting_child_expressions">Getting
2923           Child Expressions</a>
2924         </h6>
2925 <p>
2926           Each non-terminal node in an expression tree corresponds to an operator
2927           in an expression, and the children correspond to the operands, or arguments
2928           of the operator. To access them, you can use the <code class="computeroutput"><a class="link" href="../boost/proto/child_c.html" title="Function child_c">proto::child_c()</a></code>
2929           function template, as demonstrated below:
2930         </p>
2931 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">};</span>
2932
2933 <span class="comment">// Get the 0-th operand of an addition operation:</span>
2934 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&amp;</span><span class="identifier">ri</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span>
2935
2936 <span class="comment">// Assert that we got back what we put in:</span>
2937 <span class="identifier">assert</span><span class="special">(</span> <span class="special">&amp;</span><span class="identifier">i</span> <span class="special">==</span> <span class="special">&amp;</span><span class="identifier">ri</span> <span class="special">);</span>
2938 </pre>
2939 <p>
2940           You can use the <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child_c.html" title="Struct template child_c">proto::result_of::child_c&lt;&gt;</a></code>
2941           metafunction to get the type of the Nth child of an expression node. Usually
2942           you don't care to know whether a child is stored by value or by reference,
2943           so when you ask for the type of the Nth child of an expression <code class="computeroutput"><span class="identifier">Expr</span></code> (where <code class="computeroutput"><span class="identifier">Expr</span></code>
2944           is not a reference type), you get the child's type after references and
2945           cv-qualifiers have been stripped from it.
2946         </p>
2947 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
2948 <span class="keyword">void</span> <span class="identifier">test_result_of_child_c</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
2949 <span class="special">{</span>
2950     <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="number">0</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
2951
2952     <span class="comment">// Since Expr is not a reference type,</span>
2953     <span class="comment">// result_of::child_c&lt;Expr, 0&gt;::type is a</span>
2954     <span class="comment">// non-cv qualified, non-reference type:</span>
2955     <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span>
2956         <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span> <span class="identifier">type</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span>
2957     <span class="special">));</span>
2958 <span class="special">}</span>
2959
2960 <span class="comment">// ...</span>
2961 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">};</span>
2962 <span class="identifier">test_result_of_child_c</span><span class="special">(</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span>
2963 </pre>
2964 <p>
2965           However, if you ask for the type of the Nth child of <code class="computeroutput"><span class="identifier">Expr</span>
2966           <span class="special">&amp;</span></code> or <code class="computeroutput"><span class="identifier">Expr</span>
2967           <span class="keyword">const</span> <span class="special">&amp;</span></code>
2968           (note the reference), the result type will be a reference, regardless of
2969           whether the child is actually stored by reference or not. If you need to
2970           know exactly how the child is stored in the node, whether by reference
2971           or by value, you can use <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span></code>. The following table summarizes
2972           the behavior of the <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child_c.html" title="Struct template child_c">proto::result_of::child_c&lt;&gt;</a></code>
2973           metafunction.
2974         </p>
2975 <div class="table">
2976 <a name="boost_proto.users_guide.intermediate_form.left_right_child.t1"></a><p class="title"><b>Table&#160;33.5.&#160;Accessing Child Types</b></p>
2977 <div class="table-contents"><table class="table" summary="Accessing Child Types">
2978 <colgroup>
2979 <col>
2980 <col>
2981 <col>
2982 </colgroup>
2983 <thead><tr>
2984 <th>
2985                   <p>
2986                     Metafunction Invocation
2987                   </p>
2988                 </th>
2989 <th>
2990                   <p>
2991                     When the Child Is ...
2992                   </p>
2993                 </th>
2994 <th>
2995                   <p>
2996                     The Result Is ...
2997                   </p>
2998                 </th>
2999 </tr></thead>
3000 <tbody>
3001 <tr>
3002 <td>
3003                   <p>
3004                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span>
3005                     <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
3006                   </p>
3007                 </td>
3008 <td>
3009                   <p>
3010                     <code class="computeroutput"><span class="identifier">T</span></code>
3011                   </p>
3012                 </td>
3013 <td>
3014                   <p>
3015 </p>
3016 <pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_const</span><span class="special">&lt;</span>
3017     <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span>
3018 <span class="special">&gt;::</span><span class="identifier">type</span></pre>
3019 <p>
3020                   </p>
3021                 </td>
3022 </tr>
3023 <tr>
3024 <td>
3025                   <p>
3026                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="special">&amp;,</span>
3027                     <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
3028                   </p>
3029                 </td>
3030 <td>
3031                   <p>
3032                     <code class="computeroutput"><span class="identifier">T</span></code>
3033                   </p>
3034                 </td>
3035 <td>
3036                   <p>
3037 </p>
3038 <pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span></pre>
3039 <p>
3040                   </p>
3041                 </td>
3042 </tr>
3043 <tr>
3044 <td>
3045                   <p>
3046                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="keyword">const</span>
3047                     <span class="special">&amp;,</span> <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
3048                   </p>
3049                 </td>
3050 <td>
3051                   <p>
3052                     <code class="computeroutput"><span class="identifier">T</span></code>
3053                   </p>
3054                 </td>
3055 <td>
3056                   <p>
3057 </p>
3058 <pre class="programlisting"><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special">&lt;</span>
3059     <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span>
3060 <span class="special">&gt;::</span><span class="identifier">type</span></pre>
3061 <p>
3062                   </p>
3063                 </td>
3064 </tr>
3065 <tr>
3066 <td>
3067                   <p>
3068                     <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span>
3069                     <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
3070                   </p>
3071                 </td>
3072 <td>
3073                   <p>
3074                     <code class="computeroutput"><span class="identifier">T</span></code>
3075                   </p>
3076                 </td>
3077 <td>
3078                   <p>
3079                     <code class="computeroutput"><span class="identifier">T</span></code>
3080                   </p>
3081                 </td>
3082 </tr>
3083 </tbody>
3084 </table></div>
3085 </div>
3086 <br class="table-break"><h6>
3087 <a name="boost_proto.users_guide.intermediate_form.left_right_child.h3"></a>
3088           <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.left_right_child.common_shortcuts"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.left_right_child.common_shortcuts">Common
3089           Shortcuts</a>
3090         </h6>
3091 <p>
3092           Most operators in C++ are unary or binary, so accessing the only operand,
3093           or the left and right operands, are very common operations. For this reason,
3094           Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/child.html" title="Function child">proto::child()</a></code>,
3095           <code class="computeroutput"><a class="link" href="../boost/proto/left.html" title="Function left">proto::left()</a></code>, and <code class="computeroutput"><a class="link" href="../boost/proto/right.html" title="Function right">proto::right()</a></code>
3096           functions. <code class="computeroutput"><a class="link" href="../boost/proto/child.html" title="Function child">proto::child()</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/left.html" title="Function left">proto::left()</a></code>
3097           are synonymous with <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;()</span></code>,
3098           and <code class="computeroutput"><a class="link" href="../boost/proto/right.html" title="Function right">proto::right()</a></code> is synonymous with <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;()</span></code>.
3099         </p>
3100 <p>
3101           There are also <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child.html" title="Struct template child">proto::result_of::child&lt;&gt;</a></code>,
3102           <code class="computeroutput"><a class="link" href="../boost/proto/result_of/left.html" title="Struct template left">proto::result_of::left&lt;&gt;</a></code>, and <code class="computeroutput"><a class="link" href="../boost/proto/result_of/right.html" title="Struct template right">proto::result_of::right&lt;&gt;</a></code>
3103           metafunctions that merely forward to their <code class="computeroutput"><a class="link" href="../boost/proto/result_of/child_c.html" title="Struct template child_c">proto::result_of::child_c&lt;&gt;</a></code>
3104           counterparts.
3105         </p>
3106 </div>
3107 <div class="section">
3108 <div class="titlepage"><div><div><h4 class="title">
3109 <a name="boost_proto.users_guide.intermediate_form.deep_copying_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.deep_copying_expressions" title="Deep-copying Expressions">Deep-copying
3110         Expressions</a>
3111 </h4></div></div></div>
3112 <p>
3113           When you build an expression template with Proto, all the intermediate
3114           child nodes are held <span class="emphasis"><em>by reference</em></span>. The avoids needless
3115           copies, which is crucial if you want your EDSL to perform well at runtime.
3116           Naturally, there is a danger if the temporary objects go out of scope before
3117           you try to evaluate your expression template. This is especially a problem
3118           in C++0x with the new <code class="computeroutput"><span class="keyword">decltype</span></code>
3119           and <code class="computeroutput"><span class="keyword">auto</span></code> keywords. Consider:
3120         </p>
3121 <pre class="programlisting"><span class="comment">// OOPS: "ex" is left holding dangling references</span>
3122 <span class="keyword">auto</span> <span class="identifier">ex</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span><span class="special">;</span>
3123 </pre>
3124 <p>
3125           The problem can happen in today's C++ also if you use <code class="computeroutput"><span class="identifier">BOOST_TYPEOF</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">BOOST_AUTO</span><span class="special">()</span></code>, or if you try to pass an expression
3126           template outside the scope of its constituents.
3127         </p>
3128 <p>
3129           In these cases, you want to deep-copy your expression template so that
3130           all intermediate nodes and the terminals are held <span class="emphasis"><em>by value</em></span>.
3131           That way, you can safely assign the expression template to a local variable
3132           or return it from a function without worrying about dangling references.
3133           You can do this with <code class="computeroutput"><a class="link" href="../boost/proto/deep_copy.html" title="Function template deep_copy">proto::deep_copy()</a></code>
3134           as fo llows:
3135         </p>
3136 <pre class="programlisting"><span class="comment">// OK, "ex" has no dangling references</span>
3137 <span class="keyword">auto</span> <span class="identifier">ex</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">deep_copy</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span>
3138 </pre>
3139 <p>
3140           If you are using <a href="../../../libs/typeof/index.html" target="_top">Boost.Typeof</a>,
3141           it would look like this:
3142         </p>
3143 <pre class="programlisting"><span class="comment">// OK, use BOOST_AUTO() and proto::deep_copy() to</span>
3144 <span class="comment">// store an expression template in a local variable </span>
3145 <span class="identifier">BOOST_AUTO</span><span class="special">(</span> <span class="identifier">ex</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">deep_copy</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">)</span> <span class="special">);</span>
3146 </pre>
3147 <p>
3148           For the above code to work, you must include the <code class="computeroutput"><a class="link" href="reference.html#header.boost.proto.proto_typeof_hpp" title="Header &lt;boost/proto/proto_typeof.hpp&gt;">boost/proto/proto_typeof.hpp</a></code>
3149           header, which also defines the <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_AUTO.html" title="Macro BOOST_PROTO_AUTO">BOOST_PROTO_AUTO</a></code>()</code>
3150           macro which automatically deep-copies its argument. With <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_AUTO.html" title="Macro BOOST_PROTO_AUTO">BOOST_PROTO_AUTO</a></code>()</code>, the above
3151           code can be writen as:
3152         </p>
3153 <pre class="programlisting"><span class="comment">// OK, BOOST_PROTO_AUTO() automatically deep-copies</span>
3154 <span class="comment">// its argument: </span>
3155 <span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span> <span class="identifier">ex</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="number">2</span> <span class="special">);</span>
3156 </pre>
3157 <p>
3158           When deep-copying an expression tree, all intermediate nodes and all terminals
3159           are stored by value. The only exception is terminals that are function
3160           references, which are left alone.
3161         </p>
3162 <div class="note"><table border="0" summary="Note">
3163 <tr>
3164 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
3165 <th align="left">Note</th>
3166 </tr>
3167 <tr><td align="left" valign="top"><p>
3168             <code class="computeroutput"><a class="link" href="../boost/proto/deep_copy.html" title="Function template deep_copy">proto::deep_copy()</a></code> makes no exception for
3169             arrays, which it stores by value. That can potentially cause a large
3170             amount of data to be copied.
3171           </p></td></tr>
3172 </table></div>
3173 </div>
3174 <div class="section">
3175 <div class="titlepage"><div><div><h4 class="title">
3176 <a name="boost_proto.users_guide.intermediate_form.debugging_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.debugging_expressions" title="Debugging Expressions">Debugging
3177         Expressions</a>
3178 </h4></div></div></div>
3179 <p>
3180           Proto provides a utility for pretty-printing expression trees that comes
3181           in very handy when you're trying to debug your EDSL. It's called <code class="computeroutput"><a class="link" href="../boost/proto/display_expr.html" title="Function display_expr">proto::display_expr()</a></code>, and you pass it the expression
3182           to print and optionally, an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>
3183           to which to send the output. Consider:
3184         </p>
3185 <pre class="programlisting"><span class="comment">// Use display_expr() to pretty-print an expression tree</span>
3186 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span>
3187     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lit</span><span class="special">(</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">+</span> <span class="number">42</span>
3188 <span class="special">);</span>
3189 </pre>
3190 <p>
3191           The above code writes this to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code>:
3192         </p>
3193 <pre class="programlisting">plus(
3194     terminal(hello)
3195   , terminal(42)
3196 )</pre>
3197 <p>
3198           In order to call <code class="computeroutput"><a class="link" href="../boost/proto/display_expr.html" title="Function display_expr">proto::display_expr()</a></code>,
3199           all the terminals in the expression must be Streamable (that is, they can
3200           be written to a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>). In addition, the tag types
3201           must all be Streamable as well. Here is an example that includes a custom
3202           terminal type and a custom tag:
3203         </p>
3204 <pre class="programlisting"><span class="comment">// A custom tag type that is Streamable</span>
3205 <span class="keyword">struct</span> <span class="identifier">MyTag</span>
3206 <span class="special">{</span>
3207     <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">s</span><span class="special">,</span> <span class="identifier">MyTag</span><span class="special">)</span>
3208     <span class="special">{</span>
3209         <span class="keyword">return</span> <span class="identifier">s</span> <span class="special">&lt;&lt;</span> <span class="string">"MyTag"</span><span class="special">;</span>
3210     <span class="special">}</span>
3211 <span class="special">};</span>
3212
3213 <span class="comment">// Some other Streamable type</span>
3214 <span class="keyword">struct</span> <span class="identifier">MyTerminal</span>
3215 <span class="special">{</span>
3216     <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">s</span><span class="special">,</span> <span class="identifier">MyTerminal</span><span class="special">)</span>
3217     <span class="special">{</span>
3218         <span class="keyword">return</span> <span class="identifier">s</span> <span class="special">&lt;&lt;</span> <span class="string">"MyTerminal"</span><span class="special">;</span>
3219     <span class="special">}</span>
3220 <span class="special">};</span>
3221
3222 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
3223 <span class="special">{</span>
3224     <span class="comment">// Display an expression tree that contains a custom</span>
3225     <span class="comment">// tag and a user-defined type in a terminal</span>
3226     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span>
3227         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">MyTag</span><span class="special">&gt;(</span><span class="identifier">MyTerminal</span><span class="special">())</span> <span class="special">+</span> <span class="number">42</span>
3228     <span class="special">);</span>
3229 <span class="special">}</span>
3230 </pre>
3231 <p>
3232           The above code prints the following:
3233         </p>
3234 <pre class="programlisting">plus(
3235     MyTag(
3236         terminal(MyTerminal)
3237     )
3238   , terminal(42)
3239 )</pre>
3240 </div>
3241 <div class="section">
3242 <div class="titlepage"><div><div><h4 class="title">
3243 <a name="boost_proto.users_guide.intermediate_form.tags_and_metafunctions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.tags_and_metafunctions" title="Operator Tags and Metafunctions">Operator
3244         Tags and Metafunctions</a>
3245 </h4></div></div></div>
3246 <p>
3247           The following table lists the overloadable C++ operators, the Proto tag
3248           types for each, and the name of the metafunctions for generating the corresponding
3249           Proto expression types. And as we'll see later, the metafunctions are also
3250           usable as grammars for matching such nodes, as well as pass-through transforms.
3251         </p>
3252 <div class="table">
3253 <a name="boost_proto.users_guide.intermediate_form.tags_and_metafunctions.t0"></a><p class="title"><b>Table&#160;33.6.&#160;Operators, Tags and Metafunctions</b></p>
3254 <div class="table-contents"><table class="table" summary="Operators, Tags and Metafunctions">
3255 <colgroup>
3256 <col>
3257 <col>
3258 <col>
3259 </colgroup>
3260 <thead><tr>
3261 <th>
3262                   <p>
3263                     Operator
3264                   </p>
3265                 </th>
3266 <th>
3267                   <p>
3268                     Proto Tag
3269                   </p>
3270                 </th>
3271 <th>
3272                   <p>
3273                     Proto Metafunction
3274                   </p>
3275                 </th>
3276 </tr></thead>
3277 <tbody>
3278 <tr>
3279 <td>
3280                   <p>
3281                     unary <code class="computeroutput"><span class="special">+</span></code>
3282                   </p>
3283                 </td>
3284 <td>
3285                   <p>
3286                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">unary_plus</span></code>
3287                   </p>
3288                 </td>
3289 <td>
3290                   <p>
3291                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&lt;&gt;</span></code>
3292                   </p>
3293                 </td>
3294 </tr>
3295 <tr>
3296 <td>
3297                   <p>
3298                     unary <code class="computeroutput"><span class="special">-</span></code>
3299                   </p>
3300                 </td>
3301 <td>
3302                   <p>
3303                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">negate</span></code>
3304                   </p>
3305                 </td>
3306 <td>
3307                   <p>
3308                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special">&lt;&gt;</span></code>
3309                   </p>
3310                 </td>
3311 </tr>
3312 <tr>
3313 <td>
3314                   <p>
3315                     unary <code class="computeroutput"><span class="special">*</span></code>
3316                   </p>
3317                 </td>
3318 <td>
3319                   <p>
3320                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">dereference</span></code>
3321                   </p>
3322                 </td>
3323 <td>
3324                   <p>
3325                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">dereference</span><span class="special">&lt;&gt;</span></code>
3326                   </p>
3327                 </td>
3328 </tr>
3329 <tr>
3330 <td>
3331                   <p>
3332                     unary <code class="computeroutput"><span class="special">~</span></code>
3333                   </p>
3334                 </td>
3335 <td>
3336                   <p>
3337                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">complement</span></code>
3338                   </p>
3339                 </td>
3340 <td>
3341                   <p>
3342                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special">&lt;&gt;</span></code>
3343                   </p>
3344                 </td>
3345 </tr>
3346 <tr>
3347 <td>
3348                   <p>
3349                     unary <code class="computeroutput"><span class="special">&amp;</span></code>
3350                   </p>
3351                 </td>
3352 <td>
3353                   <p>
3354                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">address_of</span></code>
3355                   </p>
3356                 </td>
3357 <td>
3358                   <p>
3359                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special">&lt;&gt;</span></code>
3360                   </p>
3361                 </td>
3362 </tr>
3363 <tr>
3364 <td>
3365                   <p>
3366                     unary <code class="computeroutput"><span class="special">!</span></code>
3367                   </p>
3368                 </td>
3369 <td>
3370                   <p>
3371                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">logical_not</span></code>
3372                   </p>
3373                 </td>
3374 <td>
3375                   <p>
3376                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_not</span><span class="special">&lt;&gt;</span></code>
3377                   </p>
3378                 </td>
3379 </tr>
3380 <tr>
3381 <td>
3382                   <p>
3383                     unary prefix <code class="computeroutput"><span class="special">++</span></code>
3384                   </p>
3385                 </td>
3386 <td>
3387                   <p>
3388                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">pre_inc</span></code>
3389                   </p>
3390                 </td>
3391 <td>
3392                   <p>
3393                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_inc</span><span class="special">&lt;&gt;</span></code>
3394                   </p>
3395                 </td>
3396 </tr>
3397 <tr>
3398 <td>
3399                   <p>
3400                     unary prefix <code class="computeroutput"><span class="special">--</span></code>
3401                   </p>
3402                 </td>
3403 <td>
3404                   <p>
3405                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">pre_dec</span></code>
3406                   </p>
3407                 </td>
3408 <td>
3409                   <p>
3410                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_dec</span><span class="special">&lt;&gt;</span></code>
3411                   </p>
3412                 </td>
3413 </tr>
3414 <tr>
3415 <td>
3416                   <p>
3417                     unary postfix <code class="computeroutput"><span class="special">++</span></code>
3418                   </p>
3419                 </td>
3420 <td>
3421                   <p>
3422                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">post_inc</span></code>
3423                   </p>
3424                 </td>
3425 <td>
3426                   <p>
3427                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_inc</span><span class="special">&lt;&gt;</span></code>
3428                   </p>
3429                 </td>
3430 </tr>
3431 <tr>
3432 <td>
3433                   <p>
3434                     unary postfix <code class="computeroutput"><span class="special">--</span></code>
3435                   </p>
3436                 </td>
3437 <td>
3438                   <p>
3439                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">post_dec</span></code>
3440                   </p>
3441                 </td>
3442 <td>
3443                   <p>
3444                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_dec</span><span class="special">&lt;&gt;</span></code>
3445                   </p>
3446                 </td>
3447 </tr>
3448 <tr>
3449 <td>
3450                   <p>
3451                     binary <code class="computeroutput"><span class="special">&lt;&lt;</span></code>
3452                   </p>
3453                 </td>
3454 <td>
3455                   <p>
3456                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left</span></code>
3457                   </p>
3458                 </td>
3459 <td>
3460                   <p>
3461                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;&gt;</span></code>
3462                   </p>
3463                 </td>
3464 </tr>
3465 <tr>
3466 <td>
3467                   <p>
3468                     binary <code class="computeroutput"><span class="special">&gt;&gt;</span></code>
3469                   </p>
3470                 </td>
3471 <td>
3472                   <p>
3473                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right</span></code>
3474                   </p>
3475                 </td>
3476 <td>
3477                   <p>
3478                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;&gt;</span></code>
3479                   </p>
3480                 </td>
3481 </tr>
3482 <tr>
3483 <td>
3484                   <p>
3485                     binary <code class="computeroutput"><span class="special">*</span></code>
3486                   </p>
3487                 </td>
3488 <td>
3489                   <p>
3490                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies</span></code>
3491                   </p>
3492                 </td>
3493 <td>
3494                   <p>
3495                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;&gt;</span></code>
3496                   </p>
3497                 </td>
3498 </tr>
3499 <tr>
3500 <td>
3501                   <p>
3502                     binary <code class="computeroutput"><span class="special">/</span></code>
3503                   </p>
3504                 </td>
3505 <td>
3506                   <p>
3507                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides</span></code>
3508                   </p>
3509                 </td>
3510 <td>
3511                   <p>
3512                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;&gt;</span></code>
3513                   </p>
3514                 </td>
3515 </tr>
3516 <tr>
3517 <td>
3518                   <p>
3519                     binary <code class="computeroutput"><span class="special">%</span></code>
3520                   </p>
3521                 </td>
3522 <td>
3523                   <p>
3524                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus</span></code>
3525                   </p>
3526                 </td>
3527 <td>
3528                   <p>
3529                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special">&lt;&gt;</span></code>
3530                   </p>
3531                 </td>
3532 </tr>
3533 <tr>
3534 <td>
3535                   <p>
3536                     binary <code class="computeroutput"><span class="special">+</span></code>
3537                   </p>
3538                 </td>
3539 <td>
3540                   <p>
3541                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code>
3542                   </p>
3543                 </td>
3544 <td>
3545                   <p>
3546                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;&gt;</span></code>
3547                   </p>
3548                 </td>
3549 </tr>
3550 <tr>
3551 <td>
3552                   <p>
3553                     binary <code class="computeroutput"><span class="special">-</span></code>
3554                   </p>
3555                 </td>
3556 <td>
3557                   <p>
3558                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span></code>
3559                   </p>
3560                 </td>
3561 <td>
3562                   <p>
3563                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;&gt;</span></code>
3564                   </p>
3565                 </td>
3566 </tr>
3567 <tr>
3568 <td>
3569                   <p>
3570                     binary <code class="computeroutput"><span class="special">&lt;</span></code>
3571                   </p>
3572                 </td>
3573 <td>
3574                   <p>
3575                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">less</span></code>
3576                   </p>
3577                 </td>
3578 <td>
3579                   <p>
3580                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less</span><span class="special">&lt;&gt;</span></code>
3581                   </p>
3582                 </td>
3583 </tr>
3584 <tr>
3585 <td>
3586                   <p>
3587                     binary <code class="computeroutput"><span class="special">&gt;</span></code>
3588                   </p>
3589                 </td>
3590 <td>
3591                   <p>
3592                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">greater</span></code>
3593                   </p>
3594                 </td>
3595 <td>
3596                   <p>
3597                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater</span><span class="special">&lt;&gt;</span></code>
3598                   </p>
3599                 </td>
3600 </tr>
3601 <tr>
3602 <td>
3603                   <p>
3604                     binary <code class="computeroutput"><span class="special">&lt;=</span></code>
3605                   </p>
3606                 </td>
3607 <td>
3608                   <p>
3609                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">less_equal</span></code>
3610                   </p>
3611                 </td>
3612 <td>
3613                   <p>
3614                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less_equal</span><span class="special">&lt;&gt;</span></code>
3615                   </p>
3616                 </td>
3617 </tr>
3618 <tr>
3619 <td>
3620                   <p>
3621                     binary <code class="computeroutput"><span class="special">&gt;=</span></code>
3622                   </p>
3623                 </td>
3624 <td>
3625                   <p>
3626                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">greater_equal</span></code>
3627                   </p>
3628                 </td>
3629 <td>
3630                   <p>
3631                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater_equal</span><span class="special">&lt;&gt;</span></code>
3632                   </p>
3633                 </td>
3634 </tr>
3635 <tr>
3636 <td>
3637                   <p>
3638                     binary <code class="computeroutput"><span class="special">==</span></code>
3639                   </p>
3640                 </td>
3641 <td>
3642                   <p>
3643                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">equal_to</span></code>
3644                   </p>
3645                 </td>
3646 <td>
3647                   <p>
3648                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special">&lt;&gt;</span></code>
3649                   </p>
3650                 </td>
3651 </tr>
3652 <tr>
3653 <td>
3654                   <p>
3655                     binary <code class="computeroutput"><span class="special">!=</span></code>
3656                   </p>
3657                 </td>
3658 <td>
3659                   <p>
3660                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">not_equal_to</span></code>
3661                   </p>
3662                 </td>
3663 <td>
3664                   <p>
3665                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_equal_to</span><span class="special">&lt;&gt;</span></code>
3666                   </p>
3667                 </td>
3668 </tr>
3669 <tr>
3670 <td>
3671                   <p>
3672                     binary <code class="computeroutput"><span class="special">||</span></code>
3673                   </p>
3674                 </td>
3675 <td>
3676                   <p>
3677                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">logical_or</span></code>
3678                   </p>
3679                 </td>
3680 <td>
3681                   <p>
3682                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_or</span><span class="special">&lt;&gt;</span></code>
3683                   </p>
3684                 </td>
3685 </tr>
3686 <tr>
3687 <td>
3688                   <p>
3689                     binary <code class="computeroutput"><span class="special">&amp;&amp;</span></code>
3690                   </p>
3691                 </td>
3692 <td>
3693                   <p>
3694                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">logical_and</span></code>
3695                   </p>
3696                 </td>
3697 <td>
3698                   <p>
3699                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_and</span><span class="special">&lt;&gt;</span></code>
3700                   </p>
3701                 </td>
3702 </tr>
3703 <tr>
3704 <td>
3705                   <p>
3706                     binary <code class="computeroutput"><span class="special">&amp;</span></code>
3707                   </p>
3708                 </td>
3709 <td>
3710                   <p>
3711                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and</span></code>
3712                   </p>
3713                 </td>
3714 <td>
3715                   <p>
3716                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and</span><span class="special">&lt;&gt;</span></code>
3717                   </p>
3718                 </td>
3719 </tr>
3720 <tr>
3721 <td>
3722                   <p>
3723                     binary <code class="computeroutput"><span class="special">|</span></code>
3724                   </p>
3725                 </td>
3726 <td>
3727                   <p>
3728                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or</span></code>
3729                   </p>
3730                 </td>
3731 <td>
3732                   <p>
3733                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or</span><span class="special">&lt;&gt;</span></code>
3734                   </p>
3735                 </td>
3736 </tr>
3737 <tr>
3738 <td>
3739                   <p>
3740                     binary <code class="computeroutput"><span class="special">^</span></code>
3741                   </p>
3742                 </td>
3743 <td>
3744                   <p>
3745                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor</span></code>
3746                   </p>
3747                 </td>
3748 <td>
3749                   <p>
3750                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor</span><span class="special">&lt;&gt;</span></code>
3751                   </p>
3752                 </td>
3753 </tr>
3754 <tr>
3755 <td>
3756                   <p>
3757                     binary <code class="computeroutput"><span class="special">,</span></code>
3758                   </p>
3759                 </td>
3760 <td>
3761                   <p>
3762                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">comma</span></code>
3763                   </p>
3764                 </td>
3765 <td>
3766                   <p>
3767                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">comma</span><span class="special">&lt;&gt;</span></code>
3768                   </p>
3769                 </td>
3770 </tr>
3771 <tr>
3772 <td>
3773                   <p>
3774                     binary <code class="computeroutput"><span class="special">-&gt;*</span></code>
3775                   </p>
3776                 </td>
3777 <td>
3778                   <p>
3779                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">mem_ptr</span></code>
3780                   </p>
3781                 </td>
3782 <td>
3783                   <p>
3784                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">mem_ptr</span><span class="special">&lt;&gt;</span></code>
3785                   </p>
3786                 </td>
3787 </tr>
3788 <tr>
3789 <td>
3790                   <p>
3791                     binary <code class="computeroutput"><span class="special">=</span></code>
3792                   </p>
3793                 </td>
3794 <td>
3795                   <p>
3796                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">assign</span></code>
3797                   </p>
3798                 </td>
3799 <td>
3800                   <p>
3801                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">assign</span><span class="special">&lt;&gt;</span></code>
3802                   </p>
3803                 </td>
3804 </tr>
3805 <tr>
3806 <td>
3807                   <p>
3808                     binary <code class="computeroutput"><span class="special">&lt;&lt;=</span></code>
3809                   </p>
3810                 </td>
3811 <td>
3812                   <p>
3813                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left_assign</span></code>
3814                   </p>
3815                 </td>
3816 <td>
3817                   <p>
3818                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special">&lt;&gt;</span></code>
3819                   </p>
3820                 </td>
3821 </tr>
3822 <tr>
3823 <td>
3824                   <p>
3825                     binary <code class="computeroutput"><span class="special">&gt;&gt;=</span></code>
3826                   </p>
3827                 </td>
3828 <td>
3829                   <p>
3830                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right_assign</span></code>
3831                   </p>
3832                 </td>
3833 <td>
3834                   <p>
3835                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special">&lt;&gt;</span></code>
3836                   </p>
3837                 </td>
3838 </tr>
3839 <tr>
3840 <td>
3841                   <p>
3842                     binary <code class="computeroutput"><span class="special">*=</span></code>
3843                   </p>
3844                 </td>
3845 <td>
3846                   <p>
3847                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies_assign</span></code>
3848                   </p>
3849                 </td>
3850 <td>
3851                   <p>
3852                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special">&lt;&gt;</span></code>
3853                   </p>
3854                 </td>
3855 </tr>
3856 <tr>
3857 <td>
3858                   <p>
3859                     binary <code class="computeroutput"><span class="special">/=</span></code>
3860                   </p>
3861                 </td>
3862 <td>
3863                   <p>
3864                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides_assign</span></code>
3865                   </p>
3866                 </td>
3867 <td>
3868                   <p>
3869                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special">&lt;&gt;</span></code>
3870                   </p>
3871                 </td>
3872 </tr>
3873 <tr>
3874 <td>
3875                   <p>
3876                     binary <code class="computeroutput"><span class="special">%=</span></code>
3877                   </p>
3878                 </td>
3879 <td>
3880                   <p>
3881                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus_assign</span></code>
3882                   </p>
3883                 </td>
3884 <td>
3885                   <p>
3886                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special">&lt;&gt;</span></code>
3887                   </p>
3888                 </td>
3889 </tr>
3890 <tr>
3891 <td>
3892                   <p>
3893                     binary <code class="computeroutput"><span class="special">+=</span></code>
3894                   </p>
3895                 </td>
3896 <td>
3897                   <p>
3898                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus_assign</span></code>
3899                   </p>
3900                 </td>
3901 <td>
3902                   <p>
3903                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special">&lt;&gt;</span></code>
3904                   </p>
3905                 </td>
3906 </tr>
3907 <tr>
3908 <td>
3909                   <p>
3910                     binary <code class="computeroutput"><span class="special">-=</span></code>
3911                   </p>
3912                 </td>
3913 <td>
3914                   <p>
3915                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus_assign</span></code>
3916                   </p>
3917                 </td>
3918 <td>
3919                   <p>
3920                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special">&lt;&gt;</span></code>
3921                   </p>
3922                 </td>
3923 </tr>
3924 <tr>
3925 <td>
3926                   <p>
3927                     binary <code class="computeroutput"><span class="special">&amp;=</span></code>
3928                   </p>
3929                 </td>
3930 <td>
3931                   <p>
3932                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span></code>
3933                   </p>
3934                 </td>
3935 <td>
3936                   <p>
3937                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special">&lt;&gt;</span></code>
3938                   </p>
3939                 </td>
3940 </tr>
3941 <tr>
3942 <td>
3943                   <p>
3944                     binary <code class="computeroutput"><span class="special">|=</span></code>
3945                   </p>
3946                 </td>
3947 <td>
3948                   <p>
3949                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span></code>
3950                   </p>
3951                 </td>
3952 <td>
3953                   <p>
3954                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special">&lt;&gt;</span></code>
3955                   </p>
3956                 </td>
3957 </tr>
3958 <tr>
3959 <td>
3960                   <p>
3961                     binary <code class="computeroutput"><span class="special">^=</span></code>
3962                   </p>
3963                 </td>
3964 <td>
3965                   <p>
3966                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span></code>
3967                   </p>
3968                 </td>
3969 <td>
3970                   <p>
3971                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special">&lt;&gt;</span></code>
3972                   </p>
3973                 </td>
3974 </tr>
3975 <tr>
3976 <td>
3977                   <p>
3978                     binary subscript
3979                   </p>
3980                 </td>
3981 <td>
3982                   <p>
3983                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">subscript</span></code>
3984                   </p>
3985                 </td>
3986 <td>
3987                   <p>
3988                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">subscript</span><span class="special">&lt;&gt;</span></code>
3989                   </p>
3990                 </td>
3991 </tr>
3992 <tr>
3993 <td>
3994                   <p>
3995                     ternary <code class="computeroutput"><span class="special">?:</span></code>
3996                   </p>
3997                 </td>
3998 <td>
3999                   <p>
4000                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">if_else_</span></code>
4001                   </p>
4002                 </td>
4003 <td>
4004                   <p>
4005                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">if_else_</span><span class="special">&lt;&gt;</span></code>
4006                   </p>
4007                 </td>
4008 </tr>
4009 <tr>
4010 <td>
4011                   <p>
4012                     n-ary function call
4013                   </p>
4014                 </td>
4015 <td>
4016                   <p>
4017                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span></code>
4018                   </p>
4019                 </td>
4020 <td>
4021                   <p>
4022                     <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;&gt;</span></code>
4023                   </p>
4024                 </td>
4025 </tr>
4026 </tbody>
4027 </table></div>
4028 </div>
4029 <br class="table-break">
4030 </div>
4031 <div class="section">
4032 <div class="titlepage"><div><div><h4 class="title">
4033 <a name="boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences" title="Expressions as Fusion Sequences">Expressions
4034         as Fusion Sequences</a>
4035 </h4></div></div></div>
4036 <p>
4037           Boost.Fusion is a library of iterators, algorithms, containers and adaptors
4038           for manipulating heterogeneous sequences. In essence, a Proto expression
4039           is just a heterogeneous sequence of its child expressions, and so Proto
4040           expressions are valid Fusion random-access sequences. That means you can
4041           apply Fusion algorithms to them, transform them, apply Fusion filters and
4042           views to them, and access their elements using <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">at</span><span class="special">()</span></code>. The things Fusion can do to heterogeneous
4043           sequences are beyond the scope of this users' guide, but below is a simple
4044           example. It takes a lazy function invocation like <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">)</span></code>
4045           and uses Fusion to print the function arguments in order.
4046         </p>
4047 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">display</span>
4048 <span class="special">{</span>
4049     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
4050     <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">)</span> <span class="keyword">const</span>
4051     <span class="special">{</span>
4052         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">t</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
4053     <span class="special">}</span>
4054 <span class="special">};</span>
4055
4056 <span class="keyword">struct</span> <span class="identifier">fun_t</span> <span class="special">{};</span>
4057 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">fun_t</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">fun</span> <span class="special">=</span> <span class="special">{{}};</span>
4058
4059 <span class="comment">// ...</span>
4060 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span>
4061     <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span>
4062         <span class="comment">// pop_front() removes the "fun" child</span>
4063         <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">pop_front</span><span class="special">(</span><span class="identifier">fun</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">))</span>
4064         <span class="comment">// Extract the ints from the terminal nodes</span>
4065       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span>
4066     <span class="special">)</span>
4067   <span class="special">,</span> <span class="identifier">display</span><span class="special">()</span>
4068 <span class="special">);</span>
4069 </pre>
4070 <p>
4071           Recall from the Introduction that types in the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span></code>
4072           namespace define function objects that correspond to Proto's free functions.
4073           So <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code>
4074           creates a function object that is equivalent to the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> function. The above invocation of <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">()</span></code>
4075           displays the following:
4076         </p>
4077 <pre class="programlisting">1
4078 2
4079 3
4080 4
4081 </pre>
4082 <p>
4083           Terminals are also valid Fusion sequences. They contain exactly one element:
4084           their value.
4085         </p>
4086 <h6>
4087 <a name="boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences.h0"></a>
4088           <span class="phrase"><a name="boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences.flattening_proto_expression_tress"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expressions_as_fusion_sequences.flattening_proto_expression_tress">Flattening
4089           Proto Expression Tress</a>
4090         </h6>
4091 <p>
4092           Imagine a slight variation of the above example where, instead of iterating
4093           over the arguments of a lazy function invocation, we would like to iterate
4094           over the terminals in an addition expression:
4095         </p>
4096 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{</span><span class="number">1</span><span class="special">};</span>
4097
4098 <span class="comment">// ERROR: this doesn't work! Why?</span>
4099 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span>
4100     <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span>
4101         <span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span> <span class="special">+</span> <span class="number">3</span> <span class="special">+</span> <span class="number">4</span>
4102       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span>
4103     <span class="special">)</span>
4104   <span class="special">,</span> <span class="identifier">display</span><span class="special">()</span>
4105 <span class="special">);</span>
4106 </pre>
4107 <p>
4108           The reason this doesn't work is because the expression <code class="computeroutput"><span class="identifier">_1</span>
4109           <span class="special">+</span> <span class="number">2</span> <span class="special">+</span> <span class="number">3</span> <span class="special">+</span>
4110           <span class="number">4</span></code> does not describe a flat sequence
4111           of terminals --- it describes a binary tree. We can treat it as a flat
4112           sequence of terminals, however, using Proto's <code class="computeroutput"><a class="link" href="../boost/proto/flatten.html" title="Function flatten">proto::flatten()</a></code>
4113           function. <code class="computeroutput"><a class="link" href="../boost/proto/flatten.html" title="Function flatten">proto::flatten()</a></code> returns a view which makes
4114           a tree appear as a flat Fusion sequence. If the top-most node has a tag
4115           type <code class="computeroutput"><span class="identifier">T</span></code>, then the elements
4116           of the flattened sequence are the child nodes that do <span class="emphasis"><em>not</em></span>
4117           have tag type <code class="computeroutput"><span class="identifier">T</span></code>. This process
4118           is evaluated recursively. So the above can correctly be written as:
4119         </p>
4120 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{</span><span class="number">1</span><span class="special">};</span>
4121
4122 <span class="comment">// OK, iterate over a flattened view</span>
4123 <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span>
4124     <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span>
4125         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">flatten</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span> <span class="special">+</span> <span class="number">3</span> <span class="special">+</span> <span class="number">4</span><span class="special">)</span>
4126       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span>
4127     <span class="special">)</span>
4128   <span class="special">,</span> <span class="identifier">display</span><span class="special">()</span>
4129 <span class="special">);</span>
4130 </pre>
4131 <p>
4132           The above invocation of <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">()</span></code> displays the following:
4133         </p>
4134 <pre class="programlisting">1
4135 2
4136 3
4137 4
4138 </pre>
4139 </div>
4140 <div class="section">
4141 <div class="titlepage"><div><div><h4 class="title">
4142 <a name="boost_proto.users_guide.intermediate_form.expression_introspection"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection" title="Expression Introspection: Defining a Grammar">Expression
4143         Introspection: Defining a Grammar</a>
4144 </h4></div></div></div>
4145 <div class="toc"><dl class="toc">
4146 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.patterns">Finding
4147           Patterns in Expressions</a></span></dt>
4148 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals">Fuzzy
4149           and Exact Matches of Terminals</a></span></dt>
4150 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.if_and_not"><code class="literal">if_&lt;&gt;</code>,
4151           <code class="literal">and_&lt;&gt;</code>, and <code class="literal">not_&lt;&gt;</code></a></span></dt>
4152 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.switch">Improving
4153           Compile Times With <code class="literal">switch_&lt;&gt;</code></a></span></dt>
4154 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.matching_vararg_expressions">Matching
4155           Vararg Expressions</a></span></dt>
4156 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.defining_edsl_grammars">Defining
4157           EDSL Grammars</a></span></dt>
4158 </dl></div>
4159 <p>
4160           Expression trees can have a very rich and complicated structure. Often,
4161           you need to know some things about an expression's structure before you
4162           can process it. This section describes the tools Proto provides for peering
4163           inside an expression tree and discovering its structure. And as you'll
4164           see in later sections, all the really interesting things you can do with
4165           Proto begin right here.
4166         </p>
4167 <div class="section">
4168 <div class="titlepage"><div><div><h5 class="title">
4169 <a name="boost_proto.users_guide.intermediate_form.expression_introspection.patterns"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.patterns" title="Finding Patterns in Expressions">Finding
4170           Patterns in Expressions</a>
4171 </h5></div></div></div>
4172 <p>
4173             Imagine your EDSL is a miniature I/O facility, with iostream operations
4174             that execute lazily. You might want expressions representing input operations
4175             to be processed by one function, and output operations to be processed
4176             by a different function. How would you do that?
4177           </p>
4178 <p>
4179             The answer is to write patterns (a.k.a, <span class="emphasis"><em>grammars</em></span>)
4180             that match the structure of input and output expressions. Proto provides
4181             utilities for defining the grammars, and the <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code>
4182             template for checking whether a given expression type matches the grammar.
4183           </p>
4184 <p>
4185             First, let's define some terminals we can use in our lazy I/O expressions:
4186           </p>
4187 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cin_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cin</span> <span class="special">};</span>
4188 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span>
4189 </pre>
4190 <p>
4191             Now, we can use <code class="computeroutput"><span class="identifier">cout_</span></code>
4192             instead of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code>, and get I/O expression trees
4193             that we can execute later. To define grammars that match input and output
4194             expressions of the form <code class="computeroutput"><span class="identifier">cin_</span>
4195             <span class="special">&gt;&gt;</span> <span class="identifier">i</span></code>
4196             and <code class="computeroutput"><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span>
4197             <span class="number">1</span></code> we do this:
4198           </p>
4199 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Input</span>
4200   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span> <span class="special">&amp;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
4201 <span class="special">{};</span>
4202
4203 <span class="keyword">struct</span> <span class="identifier">Output</span>
4204   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
4205 <span class="special">{};</span>
4206 </pre>
4207 <p>
4208             We've seen the template <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;&gt;</span></code> before, but here we're using
4209             it without accessing the nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>. When used like this, it is a
4210             very simple grammar, as are <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;&gt;</span></code> and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;&gt;</span></code>. The newcomer here is <code class="computeroutput"><span class="identifier">_</span></code> in the <code class="computeroutput"><span class="identifier">proto</span></code>
4211             namespace. It is a wildcard that matches anything. The <code class="computeroutput"><span class="identifier">Input</span></code> struct is a grammar that matches
4212             any right-shift expression that has a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span></code>
4213             terminal as its left operand.
4214           </p>
4215 <p>
4216             We can use these grammars together with the <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code>
4217             template to query at compile time whether a given I/O expression type
4218             is an input or output operation. Consider the following:
4219           </p>
4220 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
4221 <span class="keyword">void</span> <span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
4222 <span class="special">{</span>
4223     <span class="keyword">if</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Input</span> <span class="special">&gt;::</span><span class="identifier">value</span> <span class="special">)</span>
4224     <span class="special">{</span>
4225         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Input!\n"</span><span class="special">;</span>
4226     <span class="special">}</span>
4227
4228     <span class="keyword">if</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Output</span> <span class="special">&gt;::</span><span class="identifier">value</span> <span class="special">)</span>
4229     <span class="special">{</span>
4230         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Output!\n"</span><span class="special">;</span>
4231     <span class="special">}</span>
4232 <span class="special">}</span>
4233
4234 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
4235 <span class="special">{</span>
4236     <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
4237     <span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="number">1</span> <span class="special">);</span>
4238     <span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">cin_</span> <span class="special">&gt;&gt;</span> <span class="identifier">i</span> <span class="special">);</span>
4239
4240     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
4241 <span class="special">}</span>
4242 </pre>
4243 <p>
4244             This program prints the following:
4245           </p>
4246 <pre class="programlisting">Output!
4247 Input!
4248 </pre>
4249 <p>
4250             If we wanted to break the <code class="computeroutput"><span class="identifier">input_output</span><span class="special">()</span></code> function into two functions, one that
4251             handles input expressions and one for output expressions, we can use
4252             <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;&gt;</span></code>,
4253             as follows:
4254           </p>
4255 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
4256 <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Input</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
4257 <span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
4258 <span class="special">{</span>
4259     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Input!\n"</span><span class="special">;</span>
4260 <span class="special">}</span>
4261
4262 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
4263 <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Output</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
4264 <span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
4265 <span class="special">{</span>
4266     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Output!\n"</span><span class="special">;</span>
4267 <span class="special">}</span>
4268 </pre>
4269 <p>
4270             This works as the previous version did. However, the following does not
4271             compile at all:
4272           </p>
4273 <pre class="programlisting"><span class="identifier">input_output</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="number">1</span> <span class="special">&lt;&lt;</span> <span class="number">2</span> <span class="special">);</span> <span class="comment">// oops!</span>
4274 </pre>
4275 <p>
4276             What's wrong? The problem is that this expression does not match our
4277             grammar. The expression groups as if it were written like <code class="computeroutput"><span class="special">(</span><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="number">1</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="number">2</span></code>. It will not match the <code class="computeroutput"><span class="identifier">Output</span></code> grammar, which expects the left
4278             operand to be a terminal, not another left-shift operation. We need to
4279             fix the grammar.
4280           </p>
4281 <p>
4282             We notice that in order to verify an expression as input or output, we'll
4283             need to recurse down to the bottom-left-most leaf and check that it is
4284             a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span></code> or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>.
4285             When we get to the terminal, we must stop recursing. We can express this
4286             in our grammar using <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>.
4287             Here are the correct <code class="computeroutput"><span class="identifier">Input</span></code>
4288             and <code class="computeroutput"><span class="identifier">Output</span></code> grammars:
4289           </p>
4290 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Input</span>
4291   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
4292         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span> <span class="special">&amp;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
4293       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;</span> <span class="identifier">Input</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
4294     <span class="special">&gt;</span>
4295 <span class="special">{};</span>
4296
4297 <span class="keyword">struct</span> <span class="identifier">Output</span>
4298   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
4299         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
4300       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span> <span class="identifier">Output</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
4301     <span class="special">&gt;</span>
4302 <span class="special">{};</span>
4303 </pre>
4304 <p>
4305             This may look a little odd at first. We seem to be defining the <code class="computeroutput"><span class="identifier">Input</span></code> and <code class="computeroutput"><span class="identifier">Output</span></code>
4306             types in terms of themselves. This is perfectly OK, actually. At the
4307             point in the grammar that the <code class="computeroutput"><span class="identifier">Input</span></code>
4308             and <code class="computeroutput"><span class="identifier">Output</span></code> types are
4309             being used, they are <span class="emphasis"><em>incomplete</em></span>, but by the time
4310             we actually evaluate the grammar with <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code>,
4311             the types will be complete. These are recursive grammars, and rightly
4312             so because they must match a recursive data structure!
4313           </p>
4314 <p>
4315             Matching an expression such as <code class="computeroutput"><span class="identifier">cout_</span>
4316             <span class="special">&lt;&lt;</span> <span class="number">1</span>
4317             <span class="special">&lt;&lt;</span> <span class="number">2</span></code>
4318             against the <code class="computeroutput"><span class="identifier">Output</span></code> grammar
4319             procedes as follows:
4320           </p>
4321 <div class="orderedlist"><ol class="orderedlist" type="1">
4322 <li class="listitem">
4323                 The first alternate of the <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>
4324                 is tried first. It will fail, because the expression <code class="computeroutput"><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span>
4325                 <span class="number">1</span> <span class="special">&lt;&lt;</span>
4326                 <span class="number">2</span></code> does not match the grammar
4327                 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span>
4328                 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span>
4329                 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span>
4330                 <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span></code>.
4331               </li>
4332 <li class="listitem">
4333                 Then the second alternate is tried next. We match the expression
4334                 against <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span>
4335                 <span class="identifier">Output</span><span class="special">,</span>
4336                 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span></code>.
4337                 The expression is a left-shift, so we next try to match the operands.
4338               </li>
4339 <li class="listitem">
4340                 The right operand <code class="computeroutput"><span class="number">2</span></code> matches
4341                 <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code> trivially.
4342               </li>
4343 <li class="listitem">
4344                 To see if the left operand <code class="computeroutput"><span class="identifier">cout_</span>
4345                 <span class="special">&lt;&lt;</span> <span class="number">1</span></code>
4346                 matches <code class="computeroutput"><span class="identifier">Output</span></code>, we
4347                 must recursively evaluate the <code class="computeroutput"><span class="identifier">Output</span></code>
4348                 grammar. This time we succeed, because <code class="computeroutput"><span class="identifier">cout_</span>
4349                 <span class="special">&lt;&lt;</span> <span class="number">1</span></code>
4350                 will match the first alternate of the <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>.
4351               </li>
4352 </ol></div>
4353 <p>
4354             We're done -- the grammar matches successfully.
4355           </p>
4356 </div>
4357 <div class="section">
4358 <div class="titlepage"><div><div><h5 class="title">
4359 <a name="boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals" title="Fuzzy and Exact Matches of Terminals">Fuzzy
4360           and Exact Matches of Terminals</a>
4361 </h5></div></div></div>
4362 <p>
4363             The terminals in an expression tree could be const or non-const references,
4364             or they might not be references at all. When writing grammars, you usually
4365             don't have to worry about it because <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code>
4366             gives you a little wiggle room when matching terminals. A grammar such
4367             as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
4368             will match a terminal of type <code class="computeroutput"><span class="keyword">int</span></code>,
4369             <code class="computeroutput"><span class="keyword">int</span> <span class="special">&amp;</span></code>,
4370             or <code class="computeroutput"><span class="keyword">int</span> <span class="keyword">const</span>
4371             <span class="special">&amp;</span></code>.
4372           </p>
4373 <p>
4374             You can explicitly specify that you want to match a reference type. If
4375             you do, the type must match exactly. For instance, a grammar such as
4376             <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="special">&amp;&gt;</span></code>
4377             will only match an <code class="computeroutput"><span class="keyword">int</span> <span class="special">&amp;</span></code>. It will not match an <code class="computeroutput"><span class="keyword">int</span></code> or an <code class="computeroutput"><span class="keyword">int</span>
4378             <span class="keyword">const</span> <span class="special">&amp;</span></code>.
4379           </p>
4380 <p>
4381             The table below shows how Proto matches terminals. The simple rule is:
4382             if you want to match only reference types, you must specify the reference
4383             in your grammar. Otherwise, leave it off and Proto will ignore const
4384             and references.
4385           </p>
4386 <div class="table">
4387 <a name="boost_proto.users_guide.intermediate_form.expression_introspection.fuzzy_and_exact_matches_of_terminals.t0"></a><p class="title"><b>Table&#160;33.7.&#160;proto::matches&lt;&gt; and Reference / CV-Qualification of Terminals</b></p>
4388 <div class="table-contents"><table class="table" summary="proto::matches&lt;&gt; and Reference / CV-Qualification of Terminals">
4389 <colgroup>
4390 <col>
4391 <col>
4392 <col>
4393 </colgroup>
4394 <thead><tr>
4395 <th>
4396                     <p>
4397                       Terminal
4398                     </p>
4399                   </th>
4400 <th>
4401                     <p>
4402                       Grammar
4403                     </p>
4404                   </th>
4405 <th>
4406                     <p>
4407                       Matches?
4408                     </p>
4409                   </th>
4410 </tr></thead>
4411 <tbody>
4412 <tr>
4413 <td>
4414                     <p>
4415                       T
4416                     </p>
4417                   </td>
4418 <td>
4419                     <p>
4420                       T
4421                     </p>
4422                   </td>
4423 <td>
4424                     <p>
4425                       yes
4426                     </p>
4427                   </td>
4428 </tr>
4429 <tr>
4430 <td>
4431                     <p>
4432                       T &amp;
4433                     </p>
4434                   </td>
4435 <td>
4436                     <p>
4437                       T
4438                     </p>
4439                   </td>
4440 <td>
4441                     <p>
4442                       yes
4443                     </p>
4444                   </td>
4445 </tr>
4446 <tr>
4447 <td>
4448                     <p>
4449                       T const &amp;
4450                     </p>
4451                   </td>
4452 <td>
4453                     <p>
4454                       T
4455                     </p>
4456                   </td>
4457 <td>
4458                     <p>
4459                       yes
4460                     </p>
4461                   </td>
4462 </tr>
4463 <tr>
4464 <td>
4465                     <p>
4466                       T
4467                     </p>
4468                   </td>
4469 <td>
4470                     <p>
4471                       T &amp;
4472                     </p>
4473                   </td>
4474 <td>
4475                     <p>
4476                       no
4477                     </p>
4478                   </td>
4479 </tr>
4480 <tr>
4481 <td>
4482                     <p>
4483                       T &amp;
4484                     </p>
4485                   </td>
4486 <td>
4487                     <p>
4488                       T &amp;
4489                     </p>
4490                   </td>
4491 <td>
4492                     <p>
4493                       yes
4494                     </p>
4495                   </td>
4496 </tr>
4497 <tr>
4498 <td>
4499                     <p>
4500                       T const &amp;
4501                     </p>
4502                   </td>
4503 <td>
4504                     <p>
4505                       T &amp;
4506                     </p>
4507                   </td>
4508 <td>
4509                     <p>
4510                       no
4511                     </p>
4512                   </td>
4513 </tr>
4514 <tr>
4515 <td>
4516                     <p>
4517                       T
4518                     </p>
4519                   </td>
4520 <td>
4521                     <p>
4522                       T const &amp;
4523                     </p>
4524                   </td>
4525 <td>
4526                     <p>
4527                       no
4528                     </p>
4529                   </td>
4530 </tr>
4531 <tr>
4532 <td>
4533                     <p>
4534                       T &amp;
4535                     </p>
4536                   </td>
4537 <td>
4538                     <p>
4539                       T const &amp;
4540                     </p>
4541                   </td>
4542 <td>
4543                     <p>
4544                       no
4545                     </p>
4546                   </td>
4547 </tr>
4548 <tr>
4549 <td>
4550                     <p>
4551                       T const &amp;
4552                     </p>
4553                   </td>
4554 <td>
4555                     <p>
4556                       T const &amp;
4557                     </p>
4558                   </td>
4559 <td>
4560                     <p>
4561                       yes
4562                     </p>
4563                   </td>
4564 </tr>
4565 </tbody>
4566 </table></div>
4567 </div>
4568 <br class="table-break"><p>
4569             This begs the question: What if you want to match an <code class="computeroutput"><span class="keyword">int</span></code>,
4570             but not an <code class="computeroutput"><span class="keyword">int</span> <span class="special">&amp;</span></code>
4571             or an <code class="computeroutput"><span class="keyword">int</span> <span class="keyword">const</span>
4572             <span class="special">&amp;</span></code>? For forcing exact matches,
4573             Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact&lt;&gt;</a></code>
4574             template. For instance, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">exact</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;</span></code>
4575             would only match an <code class="computeroutput"><span class="keyword">int</span></code>
4576             held by value.
4577           </p>
4578 <p>
4579             Proto gives you extra wiggle room when matching array types. Array types
4580             match themselves or the pointer types they decay to. This is especially
4581             useful with character arrays. The type returned by <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="string">"hello"</span><span class="special">)</span></code> is <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]&gt;::</span><span class="identifier">type</span></code>. That's a terminal containing
4582             a 6-element character array. Naturally, you can match this terminal with
4583             the grammar <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]&gt;</span></code>,
4584             but the grammar <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*&gt;</span></code>
4585             will match it as well, as the following code fragment illustrates.
4586           </p>
4587 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CharString</span>
4588   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">&gt;</span>
4589 <span class="special">{};</span>
4590
4591 <span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">char_array</span><span class="special">;</span>
4592
4593 <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">char_array</span><span class="special">,</span> <span class="identifier">CharString</span> <span class="special">&gt;</span> <span class="special">));</span>
4594 </pre>
4595 <p>
4596             What if we only wanted <code class="computeroutput"><span class="identifier">CharString</span></code>
4597             to match terminals of exactly the type <code class="computeroutput"><span class="keyword">char</span>
4598             <span class="keyword">const</span> <span class="special">*</span></code>?
4599             You can use <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact&lt;&gt;</a></code> here to turn off
4600             the fuzzy matching of terminals, as follows:
4601           </p>
4602 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CharString</span>
4603   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">exact</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
4604 <span class="special">{};</span>
4605
4606 <span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span><span class="number">6</span><span class="special">]&gt;::</span><span class="identifier">type</span> <span class="identifier">char_array</span><span class="special">;</span>
4607 <span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*&gt;::</span><span class="identifier">type</span>  <span class="identifier">char_string</span><span class="special">;</span>
4608
4609 <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">char_string</span><span class="special">,</span> <span class="identifier">CharString</span> <span class="special">&gt;</span> <span class="special">));</span>
4610 <span class="identifier">BOOST_MPL_ASSERT_NOT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">char_array</span><span class="special">,</span> <span class="identifier">CharString</span> <span class="special">&gt;</span> <span class="special">));</span>
4611 </pre>
4612 <p>
4613             Now, <code class="computeroutput"><span class="identifier">CharString</span></code> does
4614             not match array types, only character string pointers.
4615           </p>
4616 <p>
4617             The inverse problem is a little trickier: what if you wanted to match
4618             all character arrays, but not character pointers? As mentioned above,
4619             the expression <code class="computeroutput"><span class="identifier">as_expr</span><span class="special">(</span><span class="string">"hello"</span><span class="special">)</span></code> has the type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span> <span class="number">6</span> <span class="special">]</span> <span class="special">&gt;::</span><span class="identifier">type</span></code>. If you wanted to match character
4620             arrays of arbitrary size, you could use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">N</span></code>,
4621             which is an array-size wildcard. The following grammar would match any
4622             string literal: <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">[</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">N</span> <span class="special">]</span> <span class="special">&gt;</span></code>.
4623           </p>
4624 <p>
4625             Sometimes you need even more wiggle room when matching terminals. For
4626             example, maybe you're building a calculator EDSL and you want to allow
4627             any terminals that are convertible to <code class="computeroutput"><span class="keyword">double</span></code>.
4628             For that, Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/convertible_to.html" title="Struct template convertible_to">proto::convertible_to&lt;&gt;</a></code>
4629             template. You can use it as: <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">convertible_to</span><span class="special">&lt;</span> <span class="keyword">double</span>
4630             <span class="special">&gt;</span> <span class="special">&gt;</span></code>.
4631           </p>
4632 <p>
4633             There is one more way you can perform a fuzzy match on terminals. Consider
4634             the problem of trying to match a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;&gt;</span></code> terminal. You can easily match
4635             a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">float</span><span class="special">&gt;</span></code>
4636             or a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span></code>,
4637             but how would you match any instantiation of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;&gt;</span></code>? You can use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code>
4638             here to solve this problem. Here is the grammar to match any <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;&gt;</span></code>
4639             instantiation:
4640           </p>
4641 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">StdComplex</span>
4642   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
4643 <span class="special">{};</span>
4644 </pre>
4645 <p>
4646             When given a grammar like this, Proto will deconstruct the grammar and
4647             the terminal it is being matched against and see if it can match all
4648             the constituents.
4649           </p>
4650 </div>
4651 <div class="section">
4652 <div class="titlepage"><div><div><h5 class="title">
4653 <a name="boost_proto.users_guide.intermediate_form.expression_introspection.if_and_not"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.if_and_not" title="if_&lt;&gt;, and_&lt;&gt;, and not_&lt;&gt;"><code class="literal">if_&lt;&gt;</code>,
4654           <code class="literal">and_&lt;&gt;</code>, and <code class="literal">not_&lt;&gt;</code></a>
4655 </h5></div></div></div>
4656 <p>
4657             We've already seen how to use expression generators like <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;&gt;</span></code>
4658             and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;&gt;</span></code>
4659             as grammars. We've also seen <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>,
4660             which we can use to express a set of alternate grammars. There are a
4661             few others of interest; in particular, <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_&lt;&gt;</a></code>,
4662             <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/not_.html" title="Struct template not_">proto::not_&lt;&gt;</a></code>.
4663           </p>
4664 <p>
4665             The <code class="computeroutput"><a class="link" href="../boost/proto/not_.html" title="Struct template not_">proto::not_&lt;&gt;</a></code> template is the simplest.
4666             It takes a grammar as a template parameter and logically negates it;
4667             <code class="computeroutput"><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">Grammar</span><span class="special">&gt;</span></code>
4668             will match any expression that <code class="computeroutput"><span class="identifier">Grammar</span></code>
4669             does <span class="emphasis"><em>not</em></span> match.
4670           </p>
4671 <p>
4672             The <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_&lt;&gt;</a></code> template is used
4673             together with a Proto transform that is evaluated against expression
4674             types to find matches. (Proto transforms will be described later.)
4675           </p>
4676 <p>
4677             The <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code> template is like
4678             <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>, except that each
4679             argument of the <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code> must match in order
4680             for the <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code> to match. As an example,
4681             consider the definition of <code class="computeroutput"><span class="identifier">CharString</span></code>
4682             above that uses <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact&lt;&gt;</a></code>. It could have been
4683             written without <code class="computeroutput"><a class="link" href="../boost/proto/exact.html" title="Struct template exact">proto::exact&lt;&gt;</a></code> as follows:
4684           </p>
4685 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CharString</span>
4686   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special">&lt;</span>
4687         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
4688       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">if_</span><span class="special">&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">&gt;()</span> <span class="special">&gt;</span>
4689     <span class="special">&gt;</span>
4690 <span class="special">{};</span>
4691 </pre>
4692 <p>
4693             This says that a <code class="computeroutput"><span class="identifier">CharString</span></code>
4694             must be a terminal, <span class="emphasis"><em>and</em></span> its value type must be the
4695             same as <code class="computeroutput"><span class="keyword">char</span> <span class="keyword">const</span>
4696             <span class="special">*</span></code>. Notice the template argument
4697             of <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_&lt;&gt;</a></code>: <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="special">&gt;()</span></code>. This is Proto transform that compares
4698             the value type of a terminal to <code class="computeroutput"><span class="keyword">char</span>
4699             <span class="keyword">const</span> <span class="special">*</span></code>.
4700           </p>
4701 <p>
4702             The <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_&lt;&gt;</a></code> template has a couple
4703             of variants. In addition to <code class="computeroutput"><span class="identifier">if_</span><span class="special">&lt;</span><span class="identifier">Condition</span><span class="special">&gt;</span></code> you can also say <code class="computeroutput"><span class="identifier">if_</span><span class="special">&lt;</span><span class="identifier">Condition</span><span class="special">,</span> <span class="identifier">ThenGrammar</span><span class="special">&gt;</span></code> and <code class="computeroutput"><span class="identifier">if_</span><span class="special">&lt;</span><span class="identifier">Condition</span><span class="special">,</span> <span class="identifier">ThenGrammar</span><span class="special">,</span> <span class="identifier">ElseGrammar</span><span class="special">&gt;</span></code>. These let you select one sub-grammar
4704             or another based on the <code class="computeroutput"><span class="identifier">Condition</span></code>.
4705           </p>
4706 </div>
4707 <div class="section">
4708 <div class="titlepage"><div><div><h5 class="title">
4709 <a name="boost_proto.users_guide.intermediate_form.expression_introspection.switch"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.switch" title="Improving Compile Times With switch_&lt;&gt;">Improving
4710           Compile Times With <code class="literal">switch_&lt;&gt;</code></a>
4711 </h5></div></div></div>
4712 <p>
4713             When your Proto grammar gets large, you'll start to run into some scalability
4714             problems with <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>, the construct you
4715             use to specify alternate sub-grammars. First, due to limitations in C++,
4716             <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code> can only accept up
4717             to a certain number of sub-grammars, controlled by the <code class="computeroutput"><span class="identifier">BOOST_PROTO_MAX_LOGICAL_ARITY</span></code> macro.
4718             This macro defaults to eight, and you can set it higher, but doing so
4719             will aggravate another scalability problem: long compile times. With
4720             <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>, alternate sub-grammars
4721             are tried in order -- like a series of cascading <code class="computeroutput"><span class="keyword">if</span></code>'s
4722             -- leading to lots of unnecessary template instantiations. What you would
4723             prefer instead is something like <code class="computeroutput"><span class="keyword">switch</span></code>
4724             that avoids the expense of cascading <code class="computeroutput"><span class="keyword">if</span></code>'s.
4725             That's the purpose of <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>;
4726             although less convenient than <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>,
4727             it improves compile times for larger grammars and does not have an arbitrary
4728             fixed limit on the number of sub-grammars.
4729           </p>
4730 <p>
4731             Let's illustrate how to use <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>
4732             by first writing a big grammar with <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>
4733             and then translating it to an equivalent grammar using <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>:
4734           </p>
4735 <pre class="programlisting"><span class="comment">// Here is a big, inefficient grammar</span>
4736 <span class="keyword">struct</span> <span class="identifier">ABigGrammar</span>
4737   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
4738         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
4739       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span>
4740       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4741       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4742       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4743       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4744       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4745       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
4746             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4747           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4748           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4749         <span class="special">&gt;</span>
4750     <span class="special">&gt;</span>
4751 <span class="special">{};</span>
4752 </pre>
4753 <p>
4754             The above might be the grammar to a more elaborate calculator EDSL. Notice
4755             that since there are more than eight sub-grammars, we had to chain the
4756             sub-grammars with a nested <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>
4757             -- not very nice.
4758           </p>
4759 <p>
4760             The idea behind <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>
4761             is to dispatch based on an expression's tag type to a sub-grammar that
4762             handles expressions of that type. To use <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>,
4763             you define a struct with a nested <code class="computeroutput"><span class="identifier">case_</span><span class="special">&lt;&gt;</span></code> template, specialized on tag
4764             types. The above grammar can be expressed using <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code>
4765             as follows. It is described below.
4766           </p>
4767 <pre class="programlisting"><span class="comment">// Redefine ABigGrammar more efficiently using proto::switch_&lt;&gt;</span>
4768 <span class="keyword">struct</span> <span class="identifier">ABigGrammar</span><span class="special">;</span>
4769
4770 <span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span>
4771 <span class="special">{</span>
4772     <span class="comment">// The primary template matches nothing:</span>
4773     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">&gt;</span>
4774     <span class="keyword">struct</span> <span class="identifier">case_</span>
4775       <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
4776     <span class="special">{};</span>
4777 <span class="special">};</span>
4778
4779 <span class="comment">// Terminal expressions are handled here</span>
4780 <span class="keyword">template</span><span class="special">&lt;&gt;</span>
4781 <span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&gt;</span>
4782   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
4783         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
4784       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span>
4785     <span class="special">&gt;</span>
4786 <span class="special">{};</span>
4787
4788 <span class="comment">// Non-terminals are handled similarly</span>
4789 <span class="keyword">template</span><span class="special">&lt;&gt;</span>
4790 <span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&gt;</span>
4791   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4792 <span class="special">{};</span>
4793
4794 <span class="keyword">template</span><span class="special">&lt;&gt;</span>
4795 <span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">negate</span><span class="special">&gt;</span>
4796   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4797 <span class="special">{};</span>
4798
4799 <span class="keyword">template</span><span class="special">&lt;&gt;</span>
4800 <span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">complement</span><span class="special">&gt;</span>
4801   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4802 <span class="special">{};</span>
4803
4804 <span class="keyword">template</span><span class="special">&lt;&gt;</span>
4805 <span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&gt;</span>
4806   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4807 <span class="special">{};</span>
4808
4809 <span class="keyword">template</span><span class="special">&lt;&gt;</span>
4810 <span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&gt;</span>
4811   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4812 <span class="special">{};</span>
4813
4814 <span class="keyword">template</span><span class="special">&lt;&gt;</span>
4815 <span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&gt;</span>
4816   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4817 <span class="special">{};</span>
4818
4819 <span class="keyword">template</span><span class="special">&lt;&gt;</span>
4820 <span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&gt;</span>
4821   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4822 <span class="special">{};</span>
4823
4824 <span class="keyword">template</span><span class="special">&lt;&gt;</span>
4825 <span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus</span><span class="special">&gt;</span>
4826   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special">&lt;</span><span class="identifier">ABigGrammar</span><span class="special">,</span> <span class="identifier">ABigGrammar</span><span class="special">&gt;</span>
4827 <span class="special">{};</span>
4828
4829 <span class="comment">// Define ABigGrammar in terms of ABigGrammarCases</span>
4830 <span class="comment">// using proto::switch_&lt;&gt;</span>
4831 <span class="keyword">struct</span> <span class="identifier">ABigGrammar</span>
4832   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special">&lt;</span><span class="identifier">ABigGrammarCases</span><span class="special">&gt;</span>
4833 <span class="special">{};</span>
4834 </pre>
4835 <p>
4836             Matching an expression type <code class="computeroutput"><span class="identifier">E</span></code>
4837             against <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special">&lt;</span><span class="identifier">C</span><span class="special">&gt;</span></code>
4838             is equivalent to matching it against <code class="computeroutput"><span class="identifier">C</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">&gt;</span></code>. By dispatching on the expression's
4839             tag type, we can jump to the sub-grammar that handles expressions of
4840             that type, skipping over all the other sub-grammars that couldn't possibly
4841             match. If there is no specialization of <code class="computeroutput"><span class="identifier">case_</span><span class="special">&lt;&gt;</span></code> for a particular tag type, we
4842             select the primary template. In this case, the primary template inherits
4843             from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span></code>
4844             which matches no expressions.
4845           </p>
4846 <p>
4847             Notice the specialization that handles terminals:
4848           </p>
4849 <pre class="programlisting"><span class="comment">// Terminal expressions are handled here</span>
4850 <span class="keyword">template</span><span class="special">&lt;&gt;</span>
4851 <span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&gt;</span>
4852   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
4853         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
4854       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span>
4855     <span class="special">&gt;</span>
4856 <span class="special">{};</span>
4857 </pre>
4858 <p>
4859             The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span></code> type by itself isn't enough
4860             to select an appropriate sub-grammar, so we use <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>
4861             to list the alternate sub-grammars that match terminals.
4862           </p>
4863 <div class="note"><table border="0" summary="Note">
4864 <tr>
4865 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
4866 <th align="left">Note</th>
4867 </tr>
4868 <tr><td align="left" valign="top">
4869 <p>
4870               You might be tempted to define your <code class="computeroutput"><span class="identifier">case_</span><span class="special">&lt;&gt;</span></code> specializations <span class="emphasis"><em>in
4871               situ</em></span> as follows:
4872             </p>
4873 <p>
4874 </p>
4875 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span>
4876 <span class="special">{</span>
4877     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">&gt;</span>
4878     <span class="keyword">struct</span> <span class="identifier">case_</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">{};</span>
4879
4880     <span class="comment">// ERROR: not legal C++</span>
4881     <span class="keyword">template</span><span class="special">&lt;&gt;</span>
4882     <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&gt;</span>
4883       <span class="comment">/* ... */</span>
4884 <span class="special">};</span>
4885 </pre>
4886 <p>
4887             </p>
4888 <p>
4889               Unfortunately, for arcane reasons, it is not legal to define an explicit
4890               nested specialization <span class="emphasis"><em>in situ</em></span> like this. It is,
4891               however, perfectly legal to define <span class="emphasis"><em>partial</em></span> specializations
4892               <span class="emphasis"><em>in situ</em></span>, so you can add a extra dummy template
4893               parameter that has a default, as follows:
4894             </p>
4895 <p>
4896 </p>
4897 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">ABigGrammarCases</span>
4898 <span class="special">{</span>
4899     <span class="comment">// Note extra "Dummy" template parameter here:</span>
4900     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">Dummy</span> <span class="special">=</span> <span class="number">0</span><span class="special">&gt;</span>
4901     <span class="keyword">struct</span> <span class="identifier">case_</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">{};</span>
4902
4903     <span class="comment">// OK: "Dummy" makes this a partial specialization</span>
4904     <span class="comment">// instead of an explicit specialization.</span>
4905     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">Dummy</span><span class="special">&gt;</span>
4906     <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">Dummy</span><span class="special">&gt;</span>
4907       <span class="comment">/* ... */</span>
4908 <span class="special">};</span>
4909 </pre>
4910 <p>
4911             </p>
4912 <p>
4913               You might find this cleaner than defining explicit <code class="computeroutput"><span class="identifier">case_</span><span class="special">&lt;&gt;</span></code> specializations outside of
4914               their enclosing struct.
4915             </p>
4916 </td></tr>
4917 </table></div>
4918 </div>
4919 <div class="section">
4920 <div class="titlepage"><div><div><h5 class="title">
4921 <a name="boost_proto.users_guide.intermediate_form.expression_introspection.matching_vararg_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.matching_vararg_expressions" title="Matching Vararg Expressions">Matching
4922           Vararg Expressions</a>
4923 </h5></div></div></div>
4924 <p>
4925             Not all of C++'s overloadable operators are unary or binary. There is
4926             the oddball <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
4927             -- the function call operator -- which can have any number of arguments.
4928             Likewise, with Proto you may define your own "operators" that
4929             could also take more that two arguments. As a result, there may be nodes
4930             in your Proto expression tree that have an arbitrary number of children
4931             (up to <code class="literal"><code class="computeroutput"><a class="link" href="../BOOST_PROTO_MAX_ARITY.html" title="Macro BOOST_PROTO_MAX_ARITY">BOOST_PROTO_MAX_ARITY</a></code></code>,
4932             which is configurable). How do you write a grammar to match such a node?
4933           </p>
4934 <p>
4935             For such cases, Proto provides the <code class="computeroutput"><a class="link" href="../boost/proto/vararg.html" title="Struct template vararg">proto::vararg&lt;&gt;</a></code>
4936             class template. Its template argument is a grammar, and the <code class="computeroutput"><a class="link" href="../boost/proto/vararg.html" title="Struct template vararg">proto::vararg&lt;&gt;</a></code> will match the grammar
4937             zero or more times. Consider a Proto lazy function called <code class="computeroutput"><span class="identifier">fun</span><span class="special">()</span></code>
4938             that can take zero or more characters as arguments, as follows:
4939           </p>
4940 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">fun_tag</span> <span class="special">{};</span>
4941 <span class="keyword">struct</span> <span class="identifier">FunTag</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">fun_tag</span> <span class="special">&gt;</span> <span class="special">{};</span>
4942 <span class="identifier">FunTag</span><span class="special">::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">fun</span> <span class="special">=</span> <span class="special">{{}};</span>
4943
4944 <span class="comment">// example usage:</span>
4945 <span class="identifier">fun</span><span class="special">();</span>
4946 <span class="identifier">fun</span><span class="special">(</span><span class="char">'a'</span><span class="special">);</span>
4947 <span class="identifier">fun</span><span class="special">(</span><span class="char">'a'</span><span class="special">,</span> <span class="char">'b'</span><span class="special">);</span>
4948 <span class="special">...</span>
4949 </pre>
4950 <p>
4951             Below is the grammar that matches all the allowable invocations of <code class="computeroutput"><span class="identifier">fun</span><span class="special">()</span></code>:
4952           </p>
4953 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">FunCall</span>
4954   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span> <span class="identifier">FunTag</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">char</span> <span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
4955 <span class="special">{};</span>
4956 </pre>
4957 <p>
4958             The <code class="computeroutput"><span class="identifier">FunCall</span></code> grammar uses
4959             <code class="computeroutput"><a class="link" href="../boost/proto/vararg.html" title="Struct template vararg">proto::vararg&lt;&gt;</a></code> to match zero or
4960             more character literals as arguments of the <code class="computeroutput"><span class="identifier">fun</span><span class="special">()</span></code> function.
4961           </p>
4962 <p>
4963             As another example, can you guess what the following grammar matches?
4964           </p>
4965 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Foo</span>
4966   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
4967         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span>
4968       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span> <span class="identifier">Foo</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
4969     <span class="special">&gt;</span>
4970 <span class="special">{};</span>
4971 </pre>
4972 <p>
4973             Here's a hint: the first template parameter to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;&gt;</span></code> represents the node type, and
4974             any additional template parameters represent child nodes. The answer
4975             is that this is a degenerate grammar that matches every possible expression
4976             tree, from root to leaves.
4977           </p>
4978 </div>
4979 <div class="section">
4980 <div class="titlepage"><div><div><h5 class="title">
4981 <a name="boost_proto.users_guide.intermediate_form.expression_introspection.defining_edsl_grammars"></a><a class="link" href="users_guide.html#boost_proto.users_guide.intermediate_form.expression_introspection.defining_edsl_grammars" title="Defining EDSL Grammars">Defining
4982           EDSL Grammars</a>
4983 </h5></div></div></div>
4984 <p>
4985             In this section we'll see how to use Proto to define a grammar for your
4986             EDSL and use it to validate expression templates, giving short, readable
4987             compile-time errors for invalid expressions.
4988           </p>
4989 <div class="tip"><table border="0" summary="Tip">
4990 <tr>
4991 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td>
4992 <th align="left">Tip</th>
4993 </tr>
4994 <tr><td align="left" valign="top">
4995 <p>
4996               You might think that this is a backwards way of doing things. <span class="quote">&#8220;<span class="quote">If
4997               Proto let me select which operators to overload, my users wouldn't
4998               be able to create invalid expressions in the first place, and I wouldn't
4999               need a grammar at all!</span>&#8221;</span> That may be true, but there are reasons
5000               for preferring to do things this way.
5001             </p>
5002 <p>
5003               First, it lets you develop your EDSL rapidly -- all the operators are
5004               there for you already! -- and worry about invalid syntax later.
5005             </p>
5006 <p>
5007               Second, it might be the case that some operators are only allowed in
5008               certain contexts within your EDSL. This is easy to express with a grammar,
5009               and hard to do with straight operator overloading.
5010             </p>
5011 <p>
5012               Third, using an EDSL grammar to flag invalid expressions can often
5013               yield better errors than manually selecting the overloaded operators.
5014             </p>
5015 <p>
5016               Fourth, the grammar can be used for more than just validation. You
5017               can use your grammar to define <span class="emphasis"><em>tree transformations</em></span>
5018               that convert expression templates into other more useful objects.
5019             </p>
5020 <p>
5021               If none of the above convinces you, you actually <span class="emphasis"><em>can</em></span>
5022               use Proto to control which operators are overloaded within your domain.
5023               And to do it, you need to define a grammar!
5024             </p>
5025 </td></tr>
5026 </table></div>
5027 <p>
5028             In a previous section, we used Proto to define an EDSL for a lazily evaluated
5029             calculator that allowed any combination of placeholders, floating-point
5030             literals, addition, subtraction, multiplication, division and grouping.
5031             If we were to write the grammar for this EDSL in <a href="http://en.wikipedia.org/wiki/Extended_Backus_Naur_Form" target="_top">EBNF</a>,
5032             it might look like this:
5033           </p>
5034 <pre class="programlisting">group       ::= '(' expression ')'
5035 factor      ::= double | '_1' | '_2' | group
5036 term        ::= factor (('*' factor) | ('/' factor))*
5037 expression  ::= term (('+' term) | ('-' term))*
5038 </pre>
5039 <p>
5040             This captures the syntax, associativity and precedence rules of a calculator.
5041             Writing the grammar for our calculator EDSL using Proto is <span class="emphasis"><em>even
5042             simpler</em></span>. Since we are using C++ as the host language, we are
5043             bound to the associativity and precedence rules for the C++ operators.
5044             Our grammar can assume them. Also, in C++ grouping is already handled
5045             for us with the use of parenthesis, so we don't have to code that into
5046             our grammar.
5047           </p>
5048 <p>
5049             Let's begin our grammar for forward-declaring it:
5050           </p>
5051 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CalculatorGrammar</span><span class="special">;</span>
5052 </pre>
5053 <p>
5054             It's an incomplete type at this point, but we'll still be able to use
5055             it to define the rules of our grammar. Let's define grammar rules for
5056             the terminals:
5057           </p>
5058 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Double</span>
5059   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">convertible_to</span><span class="special">&lt;</span> <span class="keyword">double</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
5060 <span class="special">{};</span>
5061
5062 <span class="keyword">struct</span> <span class="identifier">Placeholder1</span>
5063   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span>
5064 <span class="special">{};</span>
5065
5066 <span class="keyword">struct</span> <span class="identifier">Placeholder2</span>
5067   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span>
5068 <span class="special">{};</span>
5069
5070 <span class="keyword">struct</span> <span class="identifier">Terminal</span>
5071   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span> <span class="identifier">Double</span><span class="special">,</span> <span class="identifier">Placeholder1</span><span class="special">,</span> <span class="identifier">Placeholder2</span> <span class="special">&gt;</span>
5072 <span class="special">{};</span>
5073 </pre>
5074 <p>
5075             Now let's define the rules for addition, subtraction, multiplication
5076             and division. Here, we can ignore issues of associativity and precedence
5077             -- the C++ compiler will enforce that for us. We only must enforce that
5078             the arguments to the operators must themselves conform to the <code class="computeroutput"><span class="identifier">CalculatorGrammar</span></code> that we forward-declared
5079             above.
5080           </p>
5081 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Plus</span>
5082   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">&gt;</span>
5083 <span class="special">{};</span>
5084
5085 <span class="keyword">struct</span> <span class="identifier">Minus</span>
5086   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">&gt;</span>
5087 <span class="special">{};</span>
5088
5089 <span class="keyword">struct</span> <span class="identifier">Multiplies</span>
5090   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">&gt;</span>
5091 <span class="special">{};</span>
5092
5093 <span class="keyword">struct</span> <span class="identifier">Divides</span>
5094   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span> <span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">&gt;</span>
5095 <span class="special">{};</span>
5096 </pre>
5097 <p>
5098             Now that we've defined all the parts of the grammar, we can define <code class="computeroutput"><span class="identifier">CalculatorGrammar</span></code>:
5099           </p>
5100 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CalculatorGrammar</span>
5101   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
5102         <span class="identifier">Terminal</span>
5103       <span class="special">,</span> <span class="identifier">Plus</span>
5104       <span class="special">,</span> <span class="identifier">Minus</span>
5105       <span class="special">,</span> <span class="identifier">Multiplies</span>
5106       <span class="special">,</span> <span class="identifier">Divides</span>
5107     <span class="special">&gt;</span>
5108 <span class="special">{};</span>
5109 </pre>
5110 <p>
5111             That's it! Now we can use <code class="computeroutput"><span class="identifier">CalculatorGrammar</span></code>
5112             to enforce that an expression template conforms to our grammar. We can
5113             use <code class="computeroutput"><a class="link" href="../boost/proto/matches.html" title="Struct template matches">proto::matches&lt;&gt;</a></code> and <code class="computeroutput"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">()</span></code>
5114             to issue readable compile-time errors for invalid expressions, as below:
5115           </p>
5116 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
5117 <span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
5118 <span class="special">{</span>
5119     <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">CalculatorGrammar</span> <span class="special">&gt;</span> <span class="special">));</span>
5120     <span class="comment">// ...</span>
5121 <span class="special">}</span>
5122 </pre>
5123 </div>
5124 </div>
5125 </div>
5126 <div class="section">
5127 <div class="titlepage"><div><div><h3 class="title">
5128 <a name="boost_proto.users_guide.back_end"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end" title="Back Ends: Making Expression Templates Do Useful Work">Back Ends: Making Expression
5129       Templates Do Useful Work</a>
5130 </h3></div></div></div>
5131 <div class="toc"><dl class="toc">
5132 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation">Expression
5133         Evaluation: Imparting Behaviors with a Context</a></span></dt>
5134 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation">Expression
5135         Transformation: Semantic Actions</a></span></dt>
5136 </dl></div>
5137 <p>
5138         Now that you've written the front end for your EDSL compiler, and you've
5139         learned a bit about the intermediate form it produces, it's time to think
5140         about what to <span class="emphasis"><em>do</em></span> with the intermediate form. This is
5141         where you put your domain-specific algorithms and optimizations. Proto gives
5142         you two ways to evaluate and manipulate expression templates: contexts and
5143         transforms.
5144       </p>
5145 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
5146 <li class="listitem">
5147             A <span class="emphasis"><em>context</em></span> is like a function object that you pass
5148             along with an expression to the <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>
5149             function. It associates behaviors with node types. <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>
5150             walks the expression and invokes your context at each node.
5151           </li>
5152 <li class="listitem">
5153             A <span class="emphasis"><em>transform</em></span> is a way to associate behaviors, not
5154             with node types in an expression, but with rules in a Proto grammar.
5155             In this way, they are like semantic actions in other compiler-construction
5156             toolkits.
5157           </li>
5158 </ul></div>
5159 <p>
5160         Two ways to evaluate expressions! How to choose? Since contexts are largely
5161         procedural, they are a bit simpler to understand and debug so they are a
5162         good place to start. But although transforms are more advanced, they are
5163         also more powerful; since they are associated with rules in your grammar,
5164         you can select the proper transform based on the entire <span class="emphasis"><em>structure</em></span>
5165         of a sub-expression rather than simply on the type of its top-most node.
5166       </p>
5167 <p>
5168         Also, transforms have a concise and declarative syntax that can be confusing
5169         at first, but highly expressive and fungible once you become accustomed to
5170         it. And -- this is admittedly very subjective -- the author finds programming
5171         with Proto transforms to be an inordinate amount of <span class="emphasis"><em>fun!</em></span>
5172         Your mileage may vary.
5173       </p>
5174 <div class="section">
5175 <div class="titlepage"><div><div><h4 class="title">
5176 <a name="boost_proto.users_guide.back_end.expression_evaluation"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation" title="Expression Evaluation: Imparting Behaviors with a Context">Expression
5177         Evaluation: Imparting Behaviors with a Context</a>
5178 </h4></div></div></div>
5179 <div class="toc"><dl class="toc">
5180 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.proto_eval">Evaluating
5181           an Expression with <code class="literal">proto::eval()</code></a></span></dt>
5182 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.contexts">Defining
5183           an Evaluation Context</a></span></dt>
5184 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts">Proto's
5185           Built-In Contexts</a></span></dt>
5186 </dl></div>
5187 <p>
5188           Once you have constructed a Proto expression tree, either by using Proto's
5189           operator overloads or with <code class="computeroutput"><a class="link" href="../boost/proto/make_expr.html" title="Function make_expr">proto::make_expr()</a></code>
5190           and friends, you probably want to actually <span class="emphasis"><em>do</em></span> something
5191           with it. The simplest option is to use <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>, a generic expression evaluator. To use
5192           <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>, you'll need to define
5193           a <span class="emphasis"><em>context</em></span> that tells <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>
5194           how each node should be evaluated. This section goes through the nuts and
5195           bolts of using <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>, defining evaluation contexts,
5196           and using the contexts that Proto provides.
5197         </p>
5198 <div class="note"><table border="0" summary="Note">
5199 <tr>
5200 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
5201 <th align="left">Note</th>
5202 </tr>
5203 <tr><td align="left" valign="top"><p>
5204             <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>
5205             is a less powerful but easier-to-use evaluation technique than Proto
5206             transforms, which are covered later. Although very powerful, transforms
5207             have a steep learning curve and can be more difficult to debug. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>
5208             is a rather weak tree traversal algorithm. Dan Marsden has been working
5209             on a more general and powerful tree traversal library. When it is ready,
5210             I anticipate that it will eliminate the need for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>.
5211           </p></td></tr>
5212 </table></div>
5213 <div class="section">
5214 <div class="titlepage"><div><div><h5 class="title">
5215 <a name="boost_proto.users_guide.back_end.expression_evaluation.proto_eval"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.proto_eval" title="Evaluating an Expression with proto::eval()">Evaluating
5216           an Expression with <code class="literal">proto::eval()</code></a>
5217 </h5></div></div></div>
5218 <div class="blockquote"><blockquote class="blockquote"><p>
5219               <span class="bold"><strong>Synopsis:</strong></span>
5220             </p></blockquote></div>
5221 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">proto</span>
5222 <span class="special">{</span>
5223     <span class="keyword">namespace</span> <span class="identifier">result_of</span>
5224     <span class="special">{</span>
5225         <span class="comment">// A metafunction for calculating the return</span>
5226         <span class="comment">// type of proto::eval() given certain Expr</span>
5227         <span class="comment">// and Context types.</span>
5228         <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
5229         <span class="keyword">struct</span> <span class="identifier">eval</span>
5230         <span class="special">{</span>
5231             <span class="keyword">typedef</span>
5232                 <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">result_type</span>
5233             <span class="identifier">type</span><span class="special">;</span>
5234         <span class="special">};</span>
5235     <span class="special">}</span>
5236
5237     <span class="keyword">namespace</span> <span class="identifier">functional</span>
5238     <span class="special">{</span>
5239         <span class="comment">// A callable function object type for evaluating</span>
5240         <span class="comment">// a Proto expression with a certain context.</span>
5241         <span class="keyword">struct</span> <span class="identifier">eval</span> <span class="special">:</span> <span class="identifier">callable</span>
5242         <span class="special">{</span>
5243             <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
5244             <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>
5245
5246             <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
5247             <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&gt;::</span><span class="identifier">type</span>
5248             <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span><span class="identifier">context</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
5249
5250             <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
5251             <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&gt;::</span><span class="identifier">type</span>
5252             <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">context</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
5253         <span class="special">};</span>
5254     <span class="special">}</span>
5255
5256     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
5257     <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&gt;::</span><span class="identifier">type</span>
5258     <span class="identifier">eval</span><span class="special">(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span><span class="identifier">context</span><span class="special">);</span>
5259
5260     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
5261     <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&gt;::</span><span class="identifier">type</span>
5262     <span class="identifier">eval</span><span class="special">(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">context</span><span class="special">);</span>
5263 <span class="special">}</span>
5264 </pre>
5265 <p>
5266             Given an expression and an evaluation context, using <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>
5267             is quite simple. Simply pass the expression and the context to <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> and it does the rest
5268             and returns the result. You can use the <code class="computeroutput"><span class="identifier">eval</span><span class="special">&lt;&gt;</span></code> metafunction in the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span></code> namespace to compute the
5269             return type of <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>. The following demonstrates
5270             a use of <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>:
5271           </p>
5272 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
5273 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">MyContext</span><span class="special">&gt;::</span><span class="identifier">type</span>
5274 <span class="identifier">MyEvaluate</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
5275 <span class="special">{</span>
5276     <span class="comment">// Some user-defined context type</span>
5277     <span class="identifier">MyContext</span> <span class="identifier">ctx</span><span class="special">;</span>
5278
5279     <span class="comment">// Evaluate an expression with the context</span>
5280     <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
5281 <span class="special">}</span>
5282 </pre>
5283 <p>
5284             What <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> does is also very simple.
5285             It defers most of the work to the context itself. Here essentially is
5286             the implementation of <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code>:
5287           </p>
5288 <pre class="programlisting"><span class="comment">// eval() dispatches to a nested "eval&lt;&gt;" function</span>
5289 <span class="comment">// object within the Context:</span>
5290 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
5291 <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">result_type</span>
5292 <span class="identifier">eval</span><span class="special">(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span>
5293 <span class="special">{</span>
5294     <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;</span> <span class="identifier">eval_fun</span><span class="special">;</span>
5295     <span class="keyword">return</span> <span class="identifier">eval_fun</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
5296 <span class="special">}</span>
5297 </pre>
5298 <p>
5299             Really, <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> is nothing more than
5300             a thin wrapper that dispatches to the appropriate handler within the
5301             context class. In the next section, we'll see how to implement a context
5302             class from scratch.
5303           </p>
5304 </div>
5305 <div class="section">
5306 <div class="titlepage"><div><div><h5 class="title">
5307 <a name="boost_proto.users_guide.back_end.expression_evaluation.contexts"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.contexts" title="Defining an Evaluation Context">Defining
5308           an Evaluation Context</a>
5309 </h5></div></div></div>
5310 <p>
5311             As we saw in the previous section, there is really not much to the <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> function. Rather, all
5312             the interesting expression evaluation goes on within a context class.
5313             This section shows how to implement one from scratch.
5314           </p>
5315 <p>
5316             All context classes have roughly the following form:
5317           </p>
5318 <pre class="programlisting"><span class="comment">// A prototypical user-defined context.</span>
5319 <span class="keyword">struct</span> <span class="identifier">MyContext</span>
5320 <span class="special">{</span>
5321     <span class="comment">// A nested eval&lt;&gt; class template</span>
5322     <span class="keyword">template</span><span class="special">&lt;</span>
5323         <span class="keyword">typename</span> <span class="identifier">Expr</span>
5324       <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Tag</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span>
5325     <span class="special">&gt;</span>
5326     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">;</span>
5327
5328     <span class="comment">// Handle terminal nodes here...</span>
5329     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
5330     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&gt;</span>
5331     <span class="special">{</span>
5332         <span class="comment">// Must have a nested result_type typedef.</span>
5333         <span class="keyword">typedef</span> <span class="special">...</span> <span class="identifier">result_type</span><span class="special">;</span>
5334
5335         <span class="comment">// Must have a function call operator that takes</span>
5336         <span class="comment">// an expression and the context.</span>
5337         <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">MyContext</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
5338         <span class="special">{</span>
5339             <span class="keyword">return</span> <span class="special">...;</span>
5340         <span class="special">}</span>
5341     <span class="special">};</span>
5342
5343     <span class="comment">// ... other specializations of struct eval&lt;&gt; ...</span>
5344 <span class="special">};</span>
5345 </pre>
5346 <p>
5347             Context classes are nothing more than a collection of specializations
5348             of a nested <code class="computeroutput"><span class="identifier">eval</span><span class="special">&lt;&gt;</span></code>
5349             class template. Each specialization handles a different expression type.
5350           </p>
5351 <p>
5352             In the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello
5353             Calculator</a> section, we saw an example of a user-defined context
5354             class for evaluating calculator expressions. That context class was implemented
5355             with the help of Proto's <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>.
5356             If we were to implement it from scratch, it would look something like
5357             this:
5358           </p>
5359 <pre class="programlisting"><span class="comment">// The calculator_context from the "Hello Calculator" section,</span>
5360 <span class="comment">// implemented from scratch.</span>
5361 <span class="keyword">struct</span> <span class="identifier">calculator_context</span>
5362 <span class="special">{</span>
5363     <span class="comment">// The values with which we'll replace the placeholders</span>
5364     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">args</span><span class="special">;</span>
5365
5366     <span class="keyword">template</span><span class="special">&lt;</span>
5367         <span class="keyword">typename</span> <span class="identifier">Expr</span>
5368         <span class="comment">// defaulted template parameters, so we can</span>
5369         <span class="comment">// specialize on the expressions that need</span>
5370         <span class="comment">// special handling.</span>
5371       <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Tag</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span>
5372       <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg0</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="number">0</span><span class="special">&gt;::</span><span class="identifier">type</span>
5373     <span class="special">&gt;</span>
5374     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">;</span>
5375
5376     <span class="comment">// Handle placeholder terminals here...</span>
5377     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
5378     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;</span> <span class="special">&gt;</span>
5379     <span class="special">{</span>
5380         <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
5381
5382         <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;,</span> <span class="identifier">MyContext</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
5383         <span class="special">{</span>
5384             <span class="keyword">return</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">[</span><span class="identifier">I</span><span class="special">];</span>
5385         <span class="special">}</span>
5386     <span class="special">};</span>
5387
5388     <span class="comment">// Handle other terminals here...</span>
5389     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg0</span><span class="special">&gt;</span>
5390     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">Arg0</span><span class="special">&gt;</span>
5391     <span class="special">{</span>
5392         <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
5393
5394         <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">MyContext</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
5395         <span class="special">{</span>
5396             <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child</span><span class="special">(</span><span class="identifier">expr</span><span class="special">);</span>
5397         <span class="special">}</span>
5398     <span class="special">};</span>
5399
5400     <span class="comment">// Handle addition here...</span>
5401     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg0</span><span class="special">&gt;</span>
5402     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">,</span> <span class="identifier">Arg0</span><span class="special">&gt;</span>
5403     <span class="special">{</span>
5404         <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
5405
5406         <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">MyContext</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
5407         <span class="special">{</span>
5408             <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">left</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">)</span>
5409                  <span class="special">+</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">right</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span>
5410         <span class="special">}</span>
5411     <span class="special">};</span>
5412
5413     <span class="comment">// ... other eval&lt;&gt; specializations for other node types ...</span>
5414 <span class="special">};</span>
5415 </pre>
5416 <p>
5417             Now we can use <code class="computeroutput"><a class="link" href="../boost/proto/eval.html" title="Function eval">proto::eval()</a></code> with the context class
5418             above to evaluate calculator expressions as follows:
5419           </p>
5420 <pre class="programlisting"><span class="comment">// Evaluate an expression with a calculator_context</span>
5421 <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
5422 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">5</span><span class="special">);</span>
5423 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">6</span><span class="special">);</span>
5424 <span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
5425 <span class="identifier">assert</span><span class="special">(</span><span class="number">11</span> <span class="special">==</span> <span class="identifier">d</span><span class="special">);</span>
5426 </pre>
5427 <p>
5428             Defining a context from scratch this way is tedious and verbose, but
5429             it gives you complete control over how the expression is evaluated. The
5430             context class in the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">Hello
5431             Calculator</a> example was much simpler. In the next section we'll
5432             see the helper class Proto provides to ease the job of implementing context
5433             classes.
5434           </p>
5435 </div>
5436 <div class="section">
5437 <div class="titlepage"><div><div><h5 class="title">
5438 <a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts" title="Proto's Built-In Contexts">Proto's
5439           Built-In Contexts</a>
5440 </h5></div></div></div>
5441 <p>
5442             Proto provides some ready-made context classes that you can use as-is,
5443             or that you can use to help while implementing your own contexts. They
5444             are:
5445           </p>
5446 <div class="variablelist">
5447 <p class="title"><b></b></p>
5448 <dl class="variablelist">
5449 <dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.default_context" title="default_context"><code class="literal">default_context</code></a></span></dt>
5450 <dd><p>
5451                   An evaluation context that assigns the usual C++ meanings to all
5452                   the operators. For example, addition nodes are handled by evaluating
5453                   the left and right children and then adding the results. The <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code>
5454                   uses Boost.Typeof to deduce the types of the expressions it evaluates.
5455                 </p></dd>
5456 <dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context" title="null_context"><code class="literal">null_context</code></a></span></dt>
5457 <dd><p>
5458                   A simple context that recursively evaluates children but does not
5459                   combine the results in any way and returns void.
5460                 </p></dd>
5461 <dt><span class="term"><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.callable_context" title="callable_context&lt;&gt;"><code class="literal">callable_context&lt;&gt;</code></a></span></dt>
5462 <dd><p>
5463                   A helper that simplifies the job of writing context classes. Rather
5464                   than writing template specializations, with <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
5465                   you write a function object with an overloaded function call operator.
5466                   Any expressions not handled by an overload are automatically dispatched
5467                   to a default evaluation context that you can specify.
5468                 </p></dd>
5469 </dl>
5470 </div>
5471 <div class="section">
5472 <div class="titlepage"><div><div><h6 class="title">
5473 <a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.default_context"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.default_context" title="default_context"><code class="literal">default_context</code></a>
5474 </h6></div></div></div>
5475 <p>
5476               The <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code> is an
5477               evaluation context that assigns the usual C++ meanings to all the operators.
5478               For example, addition nodes are handled by evaluating the left and
5479               right children and then adding the results. The <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code> uses
5480               Boost.Typeof to deduce the types of the expressions it evaluates.
5481             </p>
5482 <p>
5483               For example, consider the following "Hello World" example:
5484             </p>
5485 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
5486 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
5487 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
5488 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
5489 <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">;</span>
5490
5491 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span>
5492
5493 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
5494 <span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
5495 <span class="special">{</span>
5496     <span class="comment">// Evaluate the expression with default_context,</span>
5497     <span class="comment">// to give the operators their C++ meanings:</span>
5498     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span>
5499     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
5500 <span class="special">}</span>
5501
5502 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
5503 <span class="special">{</span>
5504     <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="string">"hello"</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="string">" world"</span> <span class="special">);</span>
5505     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
5506 <span class="special">}</span>
5507 </pre>
5508 <p>
5509               This program outputs the following:
5510             </p>
5511 <pre class="programlisting">hello, world
5512 </pre>
5513 <p>
5514               <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code> is trivially
5515               defined in terms of a <code class="computeroutput"><span class="identifier">default_eval</span><span class="special">&lt;&gt;</span></code> template, as follows:
5516             </p>
5517 <pre class="programlisting"><span class="comment">// Definition of default_context</span>
5518 <span class="keyword">struct</span> <span class="identifier">default_context</span>
5519 <span class="special">{</span>
5520     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
5521     <span class="keyword">struct</span> <span class="identifier">eval</span>
5522       <span class="special">:</span> <span class="identifier">default_eval</span><span class="special">&lt;</span>
5523             <span class="identifier">Expr</span>
5524           <span class="special">,</span> <span class="identifier">default_context</span> <span class="keyword">const</span>
5525           <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span>
5526         <span class="special">&gt;</span>
5527     <span class="special">{};</span>
5528 <span class="special">};</span>
5529 </pre>
5530 <p>
5531               There are a bunch of <code class="computeroutput"><span class="identifier">default_eval</span><span class="special">&lt;&gt;</span></code> specializations, each of which
5532               handles a different C++ operator. Here, for instance, is the specialization
5533               for binary addition:
5534             </p>
5535 <pre class="programlisting"><span class="comment">// A default expression evaluator for binary addition</span>
5536 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
5537 <span class="keyword">struct</span> <span class="identifier">default_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&gt;</span>
5538 <span class="special">{</span>
5539 <span class="keyword">private</span><span class="special">:</span>
5540     <span class="keyword">static</span> <span class="identifier">Expr</span>    <span class="special">&amp;</span> <span class="identifier">s_expr</span><span class="special">;</span>
5541     <span class="keyword">static</span> <span class="identifier">Context</span> <span class="special">&amp;</span> <span class="identifier">s_ctx</span><span class="special">;</span>
5542
5543 <span class="keyword">public</span><span class="special">:</span>
5544     <span class="keyword">typedef</span>
5545         <span class="keyword">decltype</span><span class="special">(</span>
5546             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span><span class="identifier">s_expr</span><span class="special">),</span> <span class="identifier">s_ctx</span><span class="special">)</span>
5547           <span class="special">+</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;(</span><span class="identifier">s_expr</span><span class="special">),</span> <span class="identifier">s_ctx</span><span class="special">)</span>
5548         <span class="special">)</span>
5549     <span class="identifier">result_type</span><span class="special">;</span>
5550
5551     <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
5552     <span class="special">{</span>
5553         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">)</span>
5554              <span class="special">+</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span>
5555     <span class="special">}</span>
5556 <span class="special">};</span>
5557 </pre>
5558 <p>
5559               The above code uses <code class="computeroutput"><span class="keyword">decltype</span></code>
5560               to calculate the return type of the function call operator. <code class="computeroutput"><span class="keyword">decltype</span></code> is a new keyword in the next
5561               version of C++ that gets the type of any expression. Most compilers
5562               do not yet support <code class="computeroutput"><span class="keyword">decltype</span></code>
5563               directly, so <code class="computeroutput"><span class="identifier">default_eval</span><span class="special">&lt;&gt;</span></code> uses the Boost.Typeof library
5564               to emulate it. On some compilers, that may mean that <code class="computeroutput"><span class="identifier">default_context</span></code> either doesn't work
5565               or that it requires you to register your types with the Boost.Typeof
5566               library. Check the documentation for Boost.Typeof to see.
5567             </p>
5568 </div>
5569 <div class="section">
5570 <div class="titlepage"><div><div><h6 class="title">
5571 <a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context" title="null_context"><code class="literal">null_context</code></a>
5572 </h6></div></div></div>
5573 <p>
5574               The <code class="computeroutput"><a class="link" href="../boost/proto/context/null_context.html" title="Struct null_context">proto::null_context&lt;&gt;</a></code>
5575               is a simple context that recursively evaluates children but does not
5576               combine the results in any way and returns void. It is useful in conjunction
5577               with <code class="computeroutput"><span class="identifier">callable_context</span><span class="special">&lt;&gt;</span></code>, or when defining your own
5578               contexts which mutate an expression tree in-place rather than accumulate
5579               a result, as we'll see below.
5580             </p>
5581 <p>
5582               <code class="computeroutput"><a class="link" href="../boost/proto/context/null_context.html" title="Struct null_context">proto::null_context&lt;&gt;</a></code>
5583               is trivially implemented in terms of <code class="computeroutput"><span class="identifier">null_eval</span><span class="special">&lt;&gt;</span></code> as follows:
5584             </p>
5585 <pre class="programlisting"><span class="comment">// Definition of null_context</span>
5586 <span class="keyword">struct</span> <span class="identifier">null_context</span>
5587 <span class="special">{</span>
5588     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
5589     <span class="keyword">struct</span> <span class="identifier">eval</span>
5590       <span class="special">:</span> <span class="identifier">null_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">null_context</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_arity</span><span class="special">::</span><span class="identifier">value</span><span class="special">&gt;</span>
5591     <span class="special">{};</span>
5592 <span class="special">};</span>
5593 </pre>
5594 <p>
5595               And <code class="computeroutput"><span class="identifier">null_eval</span><span class="special">&lt;&gt;</span></code>
5596               is also trivially implemented. Here, for instance is a binary <code class="computeroutput"><span class="identifier">null_eval</span><span class="special">&lt;&gt;</span></code>:
5597             </p>
5598 <pre class="programlisting"><span class="comment">// Binary null_eval&lt;&gt;</span>
5599 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
5600 <span class="keyword">struct</span> <span class="identifier">null_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">,</span> <span class="number">2</span><span class="special">&gt;</span>
5601 <span class="special">{</span>
5602     <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>
5603
5604     <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
5605     <span class="special">{</span>
5606         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span>
5607         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">ctx</span><span class="special">);</span>
5608     <span class="special">}</span>
5609 <span class="special">};</span>
5610 </pre>
5611 <p>
5612               When would such classes be useful? Imagine you have an expression tree
5613               with integer terminals, and you would like to increment each integer
5614               in-place. You might define an evaluation context as follows:
5615             </p>
5616 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">increment_ints</span>
5617 <span class="special">{</span>
5618     <span class="comment">// By default, just evaluate all children by delegating</span>
5619     <span class="comment">// to the null_eval&lt;&gt;</span>
5620     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span>
5621     <span class="keyword">struct</span> <span class="identifier">eval</span>
5622       <span class="special">:</span> <span class="identifier">null_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">increment_ints</span> <span class="keyword">const</span><span class="special">&gt;</span>
5623     <span class="special">{};</span>
5624
5625     <span class="comment">// Increment integer terminals</span>
5626     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
5627     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span>
5628     <span class="special">{</span>
5629         <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>
5630
5631         <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">increment_ints</span> <span class="keyword">const</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
5632         <span class="special">{</span>
5633             <span class="special">++</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">child</span><span class="special">(</span><span class="identifier">expr</span><span class="special">);</span>
5634         <span class="special">}</span>
5635     <span class="special">};</span>
5636 <span class="special">};</span>
5637 </pre>
5638 <p>
5639               In the next section on <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>,
5640               we'll see an even simpler way to achieve the same thing.
5641             </p>
5642 </div>
5643 <div class="section">
5644 <div class="titlepage"><div><div><h6 class="title">
5645 <a name="boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.callable_context"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.callable_context" title="callable_context&lt;&gt;"><code class="literal">callable_context&lt;&gt;</code></a>
5646 </h6></div></div></div>
5647 <p>
5648               The <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
5649               is a helper that simplifies the job of writing context classes. Rather
5650               than writing template specializations, with <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
5651               you write a function object with an overloaded function call operator.
5652               Any expressions not handled by an overload are automatically dispatched
5653               to a default evaluation context that you can specify.
5654             </p>
5655 <p>
5656               Rather than an evaluation context in its own right, <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
5657               is more properly thought of as a context adaptor. To use it, you must
5658               define your own context that inherits from <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>.
5659             </p>
5660 <p>
5661               In the <a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_evaluation.canned_contexts.null_context" title="null_context"><code class="literal">null_context</code></a>
5662               section, we saw how to implement an evaluation context that increments
5663               all the integers within an expression tree. Here is how to do the same
5664               thing with the <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>:
5665             </p>
5666 <pre class="programlisting"><span class="comment">// An evaluation context that increments all</span>
5667 <span class="comment">// integer terminals in-place.</span>
5668 <span class="keyword">struct</span> <span class="identifier">increment_ints</span>
5669   <span class="special">:</span> <span class="identifier">callable_context</span><span class="special">&lt;</span>
5670         <span class="identifier">increment_ints</span> <span class="keyword">const</span> <span class="comment">// derived context</span>
5671       <span class="special">,</span> <span class="identifier">null_context</span> <span class="keyword">const</span>  <span class="comment">// fall-back context</span>
5672     <span class="special">&gt;</span>
5673 <span class="special">{</span>
5674     <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>
5675
5676     <span class="comment">// Handle int terminals here:</span>
5677     <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="special">&amp;</span><span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
5678     <span class="special">{</span>
5679         <span class="special">++</span><span class="identifier">i</span><span class="special">;</span>
5680     <span class="special">}</span>
5681 <span class="special">};</span>
5682 </pre>
5683 <p>
5684               With such a context, we can do the following:
5685             </p>
5686 <pre class="programlisting"><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">j</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
5687 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="identifier">i</span> <span class="special">-</span> <span class="identifier">j</span> <span class="special">*</span> <span class="number">3.14</span><span class="special">,</span> <span class="identifier">increment_ints</span><span class="special">()</span> <span class="special">);</span>
5688
5689 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"i = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
5690 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"j = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">j</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
5691 </pre>
5692 <p>
5693               This program outputs the following, which shows that the integers
5694               <code class="computeroutput"><span class="identifier">i</span></code> and <code class="computeroutput"><span class="identifier">j</span></code> have been incremented by <code class="computeroutput"><span class="number">1</span></code>:
5695             </p>
5696 <pre class="programlisting">i = 1
5697 j = 11
5698 </pre>
5699 <p>
5700               In the <code class="computeroutput"><span class="identifier">increment_ints</span></code>
5701               context, we didn't have to define any nested <code class="computeroutput"><span class="identifier">eval</span><span class="special">&lt;&gt;</span></code> templates. That's because
5702               <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
5703               implements them for us. <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
5704               takes two template parameters: the derived context and a fall-back
5705               context. For each node in the expression tree being evaluated, <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code> checks to see if
5706               there is an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> in the derived context that accepts
5707               it. Given some expression <code class="computeroutput"><span class="identifier">expr</span></code>
5708               of type <code class="computeroutput"><span class="identifier">Expr</span></code>, and a
5709               context <code class="computeroutput"><span class="identifier">ctx</span></code>, it attempts
5710               to call:
5711             </p>
5712 <pre class="programlisting"><span class="identifier">ctx</span><span class="special">(</span>
5713     <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">()</span>
5714   <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">)</span>
5715   <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">)</span>
5716     <span class="special">...</span>
5717 <span class="special">);</span>
5718 </pre>
5719 <p>
5720               Using function overloading and metaprogramming tricks, <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
5721               can detect at compile-time whether such a function exists or not. If
5722               so, that function is called. If not, the current expression is passed
5723               to the fall-back evaluation context to be processed.
5724             </p>
5725 <p>
5726               We saw another example of the <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
5727               when we looked at the simple calculator expression evaluator. There,
5728               we wanted to customize the evaluation of placeholder terminals, and
5729               delegate the handling of all other nodes to the <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code>. We did
5730               that as follows:
5731             </p>
5732 <pre class="programlisting"><span class="comment">// An evaluation context for calculator expressions that</span>
5733 <span class="comment">// explicitly handles placeholder terminals, but defers the</span>
5734 <span class="comment">// processing of all other nodes to the default_context.</span>
5735 <span class="keyword">struct</span> <span class="identifier">calculator_context</span>
5736   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">&gt;</span>
5737 <span class="special">{</span>
5738     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">args</span><span class="special">;</span>
5739
5740     <span class="comment">// Define the result type of the calculator.</span>
5741     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
5742
5743     <span class="comment">// Handle the placeholders:</span>
5744     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
5745     <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;)</span> <span class="keyword">const</span>
5746     <span class="special">{</span>
5747         <span class="keyword">return</span> <span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">args</span><span class="special">[</span><span class="identifier">I</span><span class="special">];</span>
5748     <span class="special">}</span>
5749 <span class="special">};</span>
5750 </pre>
5751 <p>
5752               In this case, we didn't specify a fall-back context. In that case,
5753               <code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context&lt;&gt;</a></code>
5754               uses the <code class="computeroutput"><a class="link" href="../boost/proto/context/default_context.html" title="Struct default_context">proto::default_context</a></code>. With
5755               the above <code class="computeroutput"><span class="identifier">calculator_context</span></code>
5756               and a couple of appropriately defined placeholder terminals, we can
5757               evaluate calculator expressions, as demonstrated below:
5758             </p>
5759 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
5760 <span class="keyword">struct</span> <span class="identifier">placeholder</span>
5761 <span class="special">{};</span>
5762
5763 <span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
5764 <span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span>
5765 <span class="comment">// ...</span>
5766
5767 <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
5768 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">4</span><span class="special">);</span>
5769 <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">5</span><span class="special">);</span>
5770
5771 <span class="keyword">double</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">,</span> <span class="identifier">ctx</span> <span class="special">);</span>
5772 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"j = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">j</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
5773 </pre>
5774 <p>
5775               The above code displays the following:
5776             </p>
5777 <pre class="programlisting">j = 20
5778 </pre>
5779 </div>
5780 </div>
5781 </div>
5782 <div class="section">
5783 <div class="titlepage"><div><div><h4 class="title">
5784 <a name="boost_proto.users_guide.back_end.expression_transformation"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation" title="Expression Transformation: Semantic Actions">Expression
5785         Transformation: Semantic Actions</a>
5786 </h4></div></div></div>
5787 <div class="toc"><dl class="toc">
5788 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.__activating__your_grammars"><span class="quote">&#8220;<span class="quote">Activating</span>&#8221;</span>
5789           Your Grammars</a></span></dt>
5790 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.handling_alternation_and_recursion">Handling
5791           Alternation and Recursion</a></span></dt>
5792 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.callable_transforms">Callable
5793           Transforms</a></span></dt>
5794 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.object_transforms">Object
5795           Transforms</a></span></dt>
5796 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity">Example:
5797           Calculator Arity</a></span></dt>
5798 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.state">Transforms
5799           With State Accumulation</a></span></dt>
5800 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.data">Passing
5801           Auxiliary Data to Transforms</a></span></dt>
5802 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.implicit_params">Implicit
5803           Parameters to Primitive Transforms</a></span></dt>
5804 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions">Unpacking
5805           Expressions</a></span></dt>
5806 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.external_transforms">Separating
5807           Grammars And Transforms</a></span></dt>
5808 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms">Proto's
5809           Built-In Transforms</a></span></dt>
5810 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.primitives">Building
5811           Custom Primitive Transforms</a></span></dt>
5812 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.is_callable">Making
5813           Your Transform Callable</a></span></dt>
5814 </dl></div>
5815 <p>
5816           If you have ever built a parser with the help of a tool like Antlr, yacc
5817           or Boost.Spirit, you might be familiar with <span class="emphasis"><em>semantic actions</em></span>.
5818           In addition to allowing you to define the grammar of the language recognized
5819           by the parser, these tools let you embed code within your grammar that
5820           executes when parts of the grammar participate in a parse. Proto has the
5821           equivalent of semantic actions. They are called <span class="emphasis"><em>transforms</em></span>.
5822           This section describes how to embed transforms within your Proto grammars,
5823           turning your grammars into function objects that can manipulate or evaluate
5824           expressions in powerful ways.
5825         </p>
5826 <p>
5827           Proto transforms are an advanced topic. We'll take it slow, using examples
5828           to illustrate the key concepts, starting simple.
5829         </p>
5830 <div class="section">
5831 <div class="titlepage"><div><div><h5 class="title">
5832 <a name="boost_proto.users_guide.back_end.expression_transformation.__activating__your_grammars"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.__activating__your_grammars" title="&#8220;Activating&#8221; Your Grammars"><span class="quote">&#8220;<span class="quote">Activating</span>&#8221;</span>
5833           Your Grammars</a>
5834 </h5></div></div></div>
5835 <p>
5836             The Proto grammars we've seen so far are static. You can check at compile-time
5837             to see if an expression type matches a grammar, but that's it. Things
5838             get more interesting when you give them runtime behaviors. A grammar
5839             with embedded transforms is more than just a static grammar. It is a
5840             function object that accepts expressions that match the grammar and does
5841             <span class="emphasis"><em>something</em></span> with them.
5842           </p>
5843 <p>
5844             Below is a very simple grammar. It matches terminal expressions.
5845           </p>
5846 <pre class="programlisting"><span class="comment">// A simple Proto grammar that matches all terminals</span>
5847 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span>
5848 </pre>
5849 <p>
5850             Here is the same grammar with a transform that extracts the value from
5851             the terminal:
5852           </p>
5853 <pre class="programlisting"><span class="comment">// A simple Proto grammar that matches all terminals</span>
5854 <span class="comment">// *and* a function object that extracts the value from</span>
5855 <span class="comment">// the terminal</span>
5856 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
5857     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span>
5858   <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>          <span class="comment">// &lt;-- Look, a transform!</span>
5859 <span class="special">&gt;</span>
5860 </pre>
5861 <p>
5862             You can read this as follows: when you match a terminal expression, extract
5863             the value. The type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code>
5864             is a so-called transform. Later we'll see what makes it a transform,
5865             but for now just think of it as a kind of function object. Note the use
5866             of <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code>: the first template
5867             parameter is the grammar to match and the second is the transform to
5868             execute. The result is both a grammar that matches terminal expressions
5869             and a function object that accepts terminal expressions and extracts
5870             their values.
5871           </p>
5872 <p>
5873             As with ordinary grammars, we can define an empty struct that inherits
5874             from a grammar+transform to give us an easy way to refer back to the
5875             thing we're defining, as follows:
5876           </p>
5877 <pre class="programlisting"><span class="comment">// A grammar and a function object, as before</span>
5878 <span class="keyword">struct</span> <span class="identifier">Value</span>
5879   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
5880         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span>
5881       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>
5882     <span class="special">&gt;</span>
5883 <span class="special">{};</span>
5884
5885 <span class="comment">// "Value" is a grammar that matches terminal expressions</span>
5886 <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">Value</span> <span class="special">&gt;</span> <span class="special">));</span>
5887
5888 <span class="comment">// "Value" also defines a function object that accepts terminals</span>
5889 <span class="comment">// and extracts their value.</span>
5890 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">answer</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">};</span>
5891 <span class="identifier">Value</span> <span class="identifier">get_value</span><span class="special">;</span>
5892 <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">get_value</span><span class="special">(</span> <span class="identifier">answer</span> <span class="special">);</span>
5893 </pre>
5894 <p>
5895             As already mentioned, <code class="computeroutput"><span class="identifier">Value</span></code>
5896             is a grammar that matches terminal expressions and a function object
5897             that operates on terminal expressions. It would be an error to pass a
5898             non-terminal expression to the <code class="computeroutput"><span class="identifier">Value</span></code>
5899             function object. This is a general property of grammars with transforms;
5900             when using them as function objects, expressions passed to them must
5901             match the grammar.
5902           </p>
5903 <p>
5904             Proto grammars are valid TR1-style function objects. That means you can
5905             use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;&gt;</span></code>
5906             to ask a grammar what its return type will be, given a particular expression
5907             type. For instance, we can access the <code class="computeroutput"><span class="identifier">Value</span></code>
5908             grammar's return type as follows:
5909           </p>
5910 <pre class="programlisting"><span class="comment">// We can use boost::result_of&lt;&gt; to get the return type</span>
5911 <span class="comment">// of a Proto grammar.</span>
5912 <span class="keyword">typedef</span>
5913     <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">Value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">)&gt;::</span><span class="identifier">type</span>
5914 <span class="identifier">result_type</span><span class="special">;</span>
5915
5916 <span class="comment">// Check that we got the type we expected</span>
5917 <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span><span class="identifier">result_type</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span> <span class="special">));</span>
5918 </pre>
5919 <div class="note"><table border="0" summary="Note">
5920 <tr>
5921 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
5922 <th align="left">Note</th>
5923 </tr>
5924 <tr><td align="left" valign="top"><p>
5925               A grammar with embedded transforms is both a grammar and a function
5926               object. Calling these things "grammars with transforms" would
5927               get tedious. We could call them something like "active grammars",
5928               but as we'll see <span class="emphasis"><em>every</em></span> grammar that you can define
5929               with Proto is "active"; that is, every grammar has some behavior
5930               when used as a function object. So we'll continue calling these things
5931               plain "grammars". The term "transform" is reserved
5932               for the thing that is used as the second parameter to the <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code> template.
5933             </p></td></tr>
5934 </table></div>
5935 </div>
5936 <div class="section">
5937 <div class="titlepage"><div><div><h5 class="title">
5938 <a name="boost_proto.users_guide.back_end.expression_transformation.handling_alternation_and_recursion"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.handling_alternation_and_recursion" title="Handling Alternation and Recursion">Handling
5939           Alternation and Recursion</a>
5940 </h5></div></div></div>
5941 <p>
5942             Most grammars are a little more complicated than the one in the preceding
5943             section. For the sake of illustration, let's define a rather nonsensical
5944             grammar that matches any expression and recurses to the leftmost terminal
5945             and returns its value. It will demonstrate how two key concepts of Proto
5946             grammars -- alternation and recursion -- interact with transforms. The
5947             grammar is described below.
5948           </p>
5949 <pre class="programlisting"><span class="comment">// A grammar that matches any expression, and a function object</span>
5950 <span class="comment">// that returns the value of the leftmost terminal.</span>
5951 <span class="keyword">struct</span> <span class="identifier">LeftmostLeaf</span>
5952   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
5953         <span class="comment">// If the expression is a terminal, return its value</span>
5954         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
5955             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span>
5956           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>
5957         <span class="special">&gt;</span>
5958         <span class="comment">// Otherwise, it is a non-terminal. Return the result</span>
5959         <span class="comment">// of invoking LeftmostLeaf on the 0th (leftmost) child.</span>
5960       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
5961             <span class="identifier">_</span>
5962           <span class="special">,</span> <span class="identifier">LeftmostLeaf</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span> <span class="special">)</span>
5963         <span class="special">&gt;</span>
5964     <span class="special">&gt;</span>
5965 <span class="special">{};</span>
5966
5967 <span class="comment">// A Proto terminal wrapping std::cout</span>
5968 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">};</span>
5969
5970 <span class="comment">// Create an expression and use LeftmostLeaf to extract the</span>
5971 <span class="comment">// value of the leftmost terminal, which will be std::cout.</span>
5972 <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="identifier">sout</span> <span class="special">=</span> <span class="identifier">LeftmostLeaf</span><span class="special">()(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="string">"the answer: "</span> <span class="special">&lt;&lt;</span> <span class="number">42</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span> <span class="special">);</span>
5973 </pre>
5974 <p>
5975             We've seen <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;&gt;</span></code>
5976             before. Here it is serving two roles. First, it is a grammar that matches
5977             any of its alternate sub-grammars; in this case, either a terminal or
5978             a non-terminal. Second, it is also a function object that accepts an
5979             expression, finds the alternate sub-grammar that matches the expression,
5980             and applies its transform. And since <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code>
5981             inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;&gt;</span></code>,
5982             <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code> is also
5983             both a grammar and a function object.
5984           </p>
5985 <div class="note"><table border="0" summary="Note">
5986 <tr>
5987 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
5988 <th align="left">Note</th>
5989 </tr>
5990 <tr><td align="left" valign="top"><p>
5991               The second alternate uses <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code>
5992               as its grammar. Recall that <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code>
5993               is the wildcard grammar that matches any expression. Since alternates
5994               in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;&gt;</span></code>
5995               are tried in order, and since the first alternate handles all terminals,
5996               the second alternate handles all (and only) non-terminals. Often enough,
5997               <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
5998               <span class="identifier">_</span><span class="special">,</span>
5999               <em class="replaceable"><code>some-transform</code></em> <span class="special">&gt;</span></code>
6000               is the last alternate in a grammar, so for improved readability, you
6001               could use the equivalent <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">otherwise</span><span class="special">&lt;</span> <em class="replaceable"><code>some-transform</code></em>
6002               <span class="special">&gt;</span></code>.
6003             </p></td></tr>
6004 </table></div>
6005 <p>
6006             The next section describes this grammar further.
6007           </p>
6008 </div>
6009 <div class="section">
6010 <div class="titlepage"><div><div><h5 class="title">
6011 <a name="boost_proto.users_guide.back_end.expression_transformation.callable_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.callable_transforms" title="Callable Transforms">Callable
6012           Transforms</a>
6013 </h5></div></div></div>
6014 <p>
6015             In the grammar defined in the preceding section, the transform associated
6016             with non-terminals is a little strange-looking:
6017           </p>
6018 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
6019     <span class="identifier">_</span>
6020   <span class="special">,</span> <span class="bold"><strong>LeftmostLeaf( proto::_child0 )</strong></span>   <span class="comment">// &lt;-- a "callable" transform</span>
6021 <span class="special">&gt;</span>
6022 </pre>
6023 <p>
6024             It has the effect of accepting non-terminal expressions, taking the 0th
6025             (leftmost) child and recursively invoking the <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code>
6026             function on it. But <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span>
6027             <span class="special">)</span></code> is actually a <span class="emphasis"><em>function
6028             type</em></span>. Literally, it is the type of a function that accepts
6029             an object of type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code>
6030             and returns an object of type <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code>.
6031             So how do we make sense of this transform? Clearly, there is no function
6032             that actually has this signature, nor would such a function be useful.
6033             The key is in understanding how <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;&gt;</span></code> <span class="emphasis"><em>interprets</em></span>
6034             its second template parameter.
6035           </p>
6036 <p>
6037             When the second template parameter to <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code>
6038             is a function type, <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code>
6039             interprets the function type as a transform. In this case, <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code> is treated as the type
6040             of a function object to invoke, and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code>
6041             is treated as a transform. First, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code>
6042             is applied to the current expression (the non-terminal that matched this
6043             alternate sub-grammar), and the result (the 0th child) is passed as an
6044             argument to <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code>.
6045           </p>
6046 <div class="note"><table border="0" summary="Note">
6047 <tr>
6048 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
6049 <th align="left">Note</th>
6050 </tr>
6051 <tr><td align="left" valign="top">
6052 <p>
6053               <span class="bold"><strong>Transforms are a Domain-Specific Language</strong></span>
6054             </p>
6055 <p>
6056               <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span>
6057               <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span> <span class="special">)</span></code>
6058               <span class="emphasis"><em>looks</em></span> like an invocation of the <code class="computeroutput"><span class="identifier">LeftmostLeaf</span></code> function object, but
6059               it's not, but then it actually is! Why this confusing subterfuge? Function
6060               types give us a natural and concise syntax for composing more complicated
6061               transforms from simpler ones. The fact that the syntax is suggestive
6062               of a function invocation is on purpose. It is an embedded domain-specific
6063               language for defining expression transformations. If the subterfuge
6064               worked, it may have fooled you into thinking the transform is doing
6065               exactly what it actually does! And that's the point.
6066             </p>
6067 </td></tr>
6068 </table></div>
6069 <p>
6070             The type <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span>
6071             <span class="special">)</span></code> is an example of a <span class="emphasis"><em>callable
6072             transform</em></span>. It is a function type that represents a function
6073             object to call and its arguments. The types <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span></code>
6074             and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code> are <span class="emphasis"><em>primitive transforms</em></span>.
6075             They are plain structs, not unlike function objects, from which callable
6076             transforms can be composed. There is one other type of transform, <span class="emphasis"><em>object
6077             transforms</em></span>, that we'll encounter next.
6078           </p>
6079 </div>
6080 <div class="section">
6081 <div class="titlepage"><div><div><h5 class="title">
6082 <a name="boost_proto.users_guide.back_end.expression_transformation.object_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.object_transforms" title="Object Transforms">Object
6083           Transforms</a>
6084 </h5></div></div></div>
6085 <p>
6086             The very first transform we looked at simply extracted the value of terminals.
6087             Let's do the same thing, but this time we'll promote all ints to longs
6088             first. (Please forgive the contrived-ness of the examples so far; they
6089             get more interesting later.) Here's the grammar:
6090           </p>
6091 <pre class="programlisting"><span class="comment">// A simple Proto grammar that matches all terminals,</span>
6092 <span class="comment">// and a function object that extracts the value from</span>
6093 <span class="comment">// the terminal, promoting ints to longs:</span>
6094 <span class="keyword">struct</span> <span class="identifier">ValueWithPomote</span>
6095   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
6096         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
6097             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;</span>
6098           <span class="special">,</span> <span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>     <span class="comment">// &lt;-- an "object" transform</span>
6099         <span class="special">&gt;</span>
6100       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
6101             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">_</span> <span class="special">&gt;</span>
6102           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>
6103         <span class="special">&gt;</span>
6104     <span class="special">&gt;</span>
6105 <span class="special">{};</span>
6106 </pre>
6107 <p>
6108             You can read the above grammar as follows: when you match an int terminal,
6109             extract the value from the terminal and use it to initialize a long;
6110             otherwise, when you match another kind of terminal, just extract the
6111             value. The type <code class="computeroutput"><span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span></code>
6112             is a so-called <span class="emphasis"><em>object</em></span> transform. It looks like the
6113             creation of a temporary long, but it's really a function type. Just as
6114             a callable transform is a function type that represents a function to
6115             call and its arguments, an object transforms is a function type that
6116             represents an object to construct and the arguments to its constructor.
6117           </p>
6118 <div class="note"><table border="0" summary="Note">
6119 <tr>
6120 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
6121 <th align="left">Note</th>
6122 </tr>
6123 <tr><td align="left" valign="top">
6124 <p>
6125               <span class="bold"><strong>Object Transforms vs. Callable Transforms</strong></span>
6126             </p>
6127 <p>
6128               When using function types as Proto transforms, they can either represent
6129               an object to construct or a function to call. It is similar to "normal"
6130               C++ where the syntax <code class="computeroutput"><span class="identifier">foo</span><span class="special">(</span><span class="string">"arg"</span><span class="special">)</span></code> can either be interpreted as an object
6131               to construct or a function to call, depending on whether <code class="computeroutput"><span class="identifier">foo</span></code> is a type or a function. But
6132               consider two of the transforms we've seen so far:
6133             </p>
6134 <p>
6135 </p>
6136 <pre class="programlisting"><span class="identifier">LeftmostLeaf</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span><span class="special">)</span>  <span class="comment">// &lt;-- a callable transform</span>
6137 <span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>           <span class="comment">// &lt;-- an object transform</span>
6138 </pre>
6139 <p>
6140             </p>
6141 <p>
6142               Proto can't know in general which is which, so it uses a trait, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;&gt;</span></code>,
6143               to differentiate. <code class="computeroutput"><span class="identifier">is_callable</span><span class="special">&lt;</span> <span class="keyword">long</span>
6144               <span class="special">&gt;::</span><span class="identifier">value</span></code>
6145               is false so <code class="computeroutput"><span class="keyword">long</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span></code>
6146               is an object to construct, but <code class="computeroutput"><span class="identifier">is_callable</span><span class="special">&lt;</span> <span class="identifier">LeftmostLeaf</span>
6147               <span class="special">&gt;::</span><span class="identifier">value</span></code>
6148               is true so <code class="computeroutput"><span class="identifier">LeftmostLeaf</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span><span class="special">)</span></code> is a function to call. Later on, we'll
6149               see how Proto recognizes a type as "callable".
6150             </p>
6151 </td></tr>
6152 </table></div>
6153 </div>
6154 <div class="section">
6155 <div class="titlepage"><div><div><h5 class="title">
6156 <a name="boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity" title="Example: Calculator Arity">Example:
6157           Calculator Arity</a>
6158 </h5></div></div></div>
6159 <p>
6160             Now that we have the basics of Proto transforms down, let's consider
6161             a slightly more realistic example. We can use transforms to improve the
6162             type-safety of the <a class="link" href="users_guide.html#boost_proto.users_guide.getting_started.hello_calculator" title="Hello Calculator">calculator
6163             EDSL</a>. If you recall, it lets you write infix arithmetic expressions
6164             involving argument placeholders like <code class="computeroutput"><span class="identifier">_1</span></code>
6165             and <code class="computeroutput"><span class="identifier">_2</span></code> and pass them
6166             to STL algorithms as function objects, as follows:
6167           </p>
6168 <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">56</span><span class="special">,</span> <span class="number">84</span><span class="special">,</span> <span class="number">37</span><span class="special">,</span> <span class="number">69</span> <span class="special">};</span>
6169 <span class="keyword">double</span> <span class="identifier">a2</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">65</span><span class="special">,</span> <span class="number">120</span><span class="special">,</span> <span class="number">60</span><span class="special">,</span> <span class="number">70</span> <span class="special">};</span>
6170 <span class="keyword">double</span> <span class="identifier">a3</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span> <span class="number">0</span> <span class="special">};</span>
6171
6172 <span class="comment">// Use std::transform() and a calculator expression</span>
6173 <span class="comment">// to calculate percentages given two input sequences:</span>
6174 <span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a1</span><span class="special">+</span><span class="number">4</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,</span> <span class="identifier">a3</span><span class="special">,</span> <span class="special">(</span><span class="identifier">_2</span> <span class="special">-</span> <span class="identifier">_1</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">*</span> <span class="number">100</span><span class="special">);</span>
6175 </pre>
6176 <p>
6177             This works because we gave calculator expressions an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> that evaluates the expression, replacing
6178             the placeholders with the arguments to <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>. The overloaded <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;::</span><span class="keyword">operator</span><span class="special">()</span></code> looked like this:
6179           </p>
6180 <pre class="programlisting"><span class="comment">// Overload operator() to invoke proto::eval() with</span>
6181 <span class="comment">// our calculator_context.</span>
6182 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
6183 <span class="keyword">double</span>
6184 <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">const</span>
6185 <span class="special">{</span>
6186     <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
6187     <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span>
6188     <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a2</span><span class="special">);</span>
6189
6190     <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
6191 <span class="special">}</span>
6192 </pre>
6193 <p>
6194             Although this works, it's not ideal because it doesn't warn users if
6195             they supply too many or too few arguments to a calculator expression.
6196             Consider the following mistakes:
6197           </p>
6198 <pre class="programlisting"><span class="special">(</span><span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_1</span><span class="special">)(</span><span class="number">4</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span>  <span class="comment">// Oops, too many arguments!</span>
6199 <span class="special">(</span><span class="identifier">_2</span> <span class="special">*</span> <span class="identifier">_2</span><span class="special">)(</span><span class="number">42</span><span class="special">);</span>    <span class="comment">// Oops, too few arguments!</span>
6200 </pre>
6201 <p>
6202             The expression <code class="computeroutput"><span class="identifier">_1</span> <span class="special">*</span>
6203             <span class="identifier">_1</span></code> defines a unary calculator
6204             expression; it takes one argument and squares it. If we pass more than
6205             one argument, the extra arguments will be silently ignored, which might
6206             be surprising to users. The next expression, <code class="computeroutput"><span class="identifier">_2</span>
6207             <span class="special">*</span> <span class="identifier">_2</span></code>
6208             defines a binary calculator expression; it takes two arguments, ignores
6209             the first and squares the second. If we only pass one argument, the code
6210             silently fills in <code class="computeroutput"><span class="number">0.0</span></code> for
6211             the second argument, which is also probably not what users expect. What
6212             can be done?
6213           </p>
6214 <p>
6215             We can say that the <span class="emphasis"><em>arity</em></span> of a calculator expression
6216             is the number of arguments it expects, and it is equal to the largest
6217             placeholder in the expression. So, the arity of <code class="computeroutput"><span class="identifier">_1</span>
6218             <span class="special">*</span> <span class="identifier">_1</span></code>
6219             is one, and the arity of <code class="computeroutput"><span class="identifier">_2</span>
6220             <span class="special">*</span> <span class="identifier">_2</span></code>
6221             is two. We can increase the type-safety of our calculator EDSL by making
6222             sure the arity of an expression equals the actual number of arguments
6223             supplied. Computing the arity of an expression is simple with the help
6224             of Proto transforms.
6225           </p>
6226 <p>
6227             It's straightforward to describe in words how the arity of an expression
6228             should be calculated. Consider that calculator expressions can be made
6229             of <code class="computeroutput"><span class="identifier">_1</span></code>, <code class="computeroutput"><span class="identifier">_2</span></code>, literals, unary expressions and
6230             binary expressions. The following table shows the arities for each of
6231             these 5 constituents.
6232           </p>
6233 <div class="table">
6234 <a name="boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.t0"></a><p class="title"><b>Table&#160;33.8.&#160;Calculator Sub-Expression Arities</b></p>
6235 <div class="table-contents"><table class="table" summary="Calculator Sub-Expression Arities">
6236 <colgroup>
6237 <col>
6238 <col>
6239 </colgroup>
6240 <thead><tr>
6241 <th>
6242                     <p>
6243                       Sub-Expression
6244                     </p>
6245                   </th>
6246 <th>
6247                     <p>
6248                       Arity
6249                     </p>
6250                   </th>
6251 </tr></thead>
6252 <tbody>
6253 <tr>
6254 <td>
6255                     <p>
6256                       Placeholder 1
6257                     </p>
6258                   </td>
6259 <td>
6260                     <p>
6261                       <code class="computeroutput"><span class="number">1</span></code>
6262                     </p>
6263                   </td>
6264 </tr>
6265 <tr>
6266 <td>
6267                     <p>
6268                       Placeholder 2
6269                     </p>
6270                   </td>
6271 <td>
6272                     <p>
6273                       <code class="computeroutput"><span class="number">2</span></code>
6274                     </p>
6275                   </td>
6276 </tr>
6277 <tr>
6278 <td>
6279                     <p>
6280                       Literal
6281                     </p>
6282                   </td>
6283 <td>
6284                     <p>
6285                       <code class="computeroutput"><span class="number">0</span></code>
6286                     </p>
6287                   </td>
6288 </tr>
6289 <tr>
6290 <td>
6291                     <p>
6292                       Unary Expression
6293                     </p>
6294                   </td>
6295 <td>
6296                     <p>
6297                       <span class="emphasis"><em>arity of the operand</em></span>
6298                     </p>
6299                   </td>
6300 </tr>
6301 <tr>
6302 <td>
6303                     <p>
6304                       Binary Expression
6305                     </p>
6306                   </td>
6307 <td>
6308                     <p>
6309                       <span class="emphasis"><em>max arity of the two operands</em></span>
6310                     </p>
6311                   </td>
6312 </tr>
6313 </tbody>
6314 </table></div>
6315 </div>
6316 <br class="table-break"><p>
6317             Using this information, we can write the grammar for calculator expressions
6318             and attach transforms for computing the arity of each constituent. The
6319             code below computes the expression arity as a compile-time integer, using
6320             integral wrappers and metafunctions from the Boost MPL Library. The grammar
6321             is described below.
6322           </p>
6323 <p>
6324 </p>
6325 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CalcArity</span>
6326   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
6327         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;,</span>
6328             <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;()</span>
6329         <span class="special">&gt;</span>
6330       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;,</span>
6331             <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">2</span><span class="special">&gt;()</span>
6332         <span class="special">&gt;</span>
6333       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;,</span>
6334             <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;()</span>
6335         <span class="special">&gt;</span>
6336       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">CalcArity</span><span class="special">&gt;,</span>
6337             <span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child</span><span class="special">)</span>
6338         <span class="special">&gt;</span>
6339       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">binary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">CalcArity</span><span class="special">,</span> <span class="identifier">CalcArity</span><span class="special">&gt;,</span>
6340             <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;</span><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span>
6341                      <span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)&gt;()</span>
6342         <span class="special">&gt;</span>
6343     <span class="special">&gt;</span>
6344 <span class="special">{};</span>
6345 </pre>
6346 <p>
6347           </p>
6348 <p>
6349             When we find a placeholder terminal or a literal, we use an <span class="emphasis"><em>object
6350             transform</em></span> such as <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;()</span></code>
6351             to create a (default-constructed) compile-time integer representing the
6352             arity of that terminal.
6353           </p>
6354 <p>
6355             For unary expressions, we use <code class="computeroutput"><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child</span><span class="special">)</span></code> which is a <span class="emphasis"><em>callable transform</em></span>
6356             that computes the arity of the expression's child.
6357           </p>
6358 <p>
6359             The transform for binary expressions has a few new tricks. Let's look
6360             more closely:
6361           </p>
6362 <pre class="programlisting"><span class="comment">// Compute the left and right arities and</span>
6363 <span class="comment">// take the larger of the two.</span>
6364 <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;</span><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span>
6365          <span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)&gt;()</span>
6366 </pre>
6367 <p>
6368             This is an object transform; it default-constructs ... what exactly?
6369             The <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;&gt;</span></code>
6370             template is an MPL metafunction that accepts two compile-time integers.
6371             It has a nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>
6372             typedef (not shown) that is the maximum of the two. But here, we appear
6373             to be passing it two things that are <span class="emphasis"><em>not</em></span> compile-time
6374             integers; they're Proto callable transforms. Proto is smart enough to
6375             recognize that fact. It first evaluates the two nested callable transforms,
6376             computing the arities of the left and right child expressions. Then it
6377             puts the resulting integers into <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;&gt;</span></code> and evaluates the metafunction
6378             by asking for the nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>. That is the type of the object
6379             that gets default-constructed and returned.
6380           </p>
6381 <p>
6382             More generally, when evaluating object transforms, Proto looks at the
6383             object type and checks whether it is a template specialization, like
6384             <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;&gt;</span></code>.
6385             If it is, Proto looks for nested transforms that it can evaluate. After
6386             any nested transforms have been evaluated and substituted back into the
6387             template, the new template specialization is the result type, unless
6388             that type has a nested <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>, in which case that becomes the
6389             result.
6390           </p>
6391 <p>
6392             Now that we can calculate the arity of a calculator expression, let's
6393             redefine the <code class="computeroutput"><span class="identifier">calculator</span><span class="special">&lt;&gt;</span></code> expression wrapper we wrote in
6394             the Getting Started guide to use the <code class="computeroutput"><span class="identifier">CalcArity</span></code>
6395             grammar and some macros from Boost.MPL to issue compile-time errors when
6396             users specify too many or too few arguments.
6397           </p>
6398 <pre class="programlisting"><span class="comment">// The calculator expression wrapper, as defined in the Hello</span>
6399 <span class="comment">// Calculator example in the Getting Started guide. It behaves</span>
6400 <span class="comment">// just like the expression it wraps, but with extra operator()</span>
6401 <span class="comment">// member functions that evaluate the expression.</span>
6402 <span class="comment">//   NEW: Use the CalcArity grammar to ensure that the correct</span>
6403 <span class="comment">//   number of arguments are supplied.</span>
6404 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
6405 <span class="keyword">struct</span> <span class="identifier">calculator</span>
6406   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
6407 <span class="special">{</span>
6408     <span class="keyword">typedef</span>
6409         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
6410     <span class="identifier">base_type</span><span class="special">;</span>
6411
6412     <span class="identifier">calculator</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span>
6413       <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span>
6414     <span class="special">{}</span>
6415
6416     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
6417
6418     <span class="comment">// Use CalcArity to compute the arity of Expr: </span>
6419     <span class="keyword">static</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="identifier">arity</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">CalcArity</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">)&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">;</span>
6420
6421     <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span>
6422     <span class="special">{</span>
6423         <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">arity</span><span class="special">);</span>
6424         <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
6425         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
6426     <span class="special">}</span>
6427
6428     <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">)</span> <span class="keyword">const</span>
6429     <span class="special">{</span>
6430         <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">arity</span><span class="special">);</span>
6431         <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
6432         <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span>
6433         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
6434     <span class="special">}</span>
6435
6436     <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span> <span class="identifier">a1</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a2</span><span class="special">)</span> <span class="keyword">const</span>
6437     <span class="special">{</span>
6438         <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">2</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">arity</span><span class="special">);</span>
6439         <span class="identifier">calculator_context</span> <span class="identifier">ctx</span><span class="special">;</span>
6440         <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span>
6441         <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">args</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">a2</span><span class="special">);</span>
6442         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
6443     <span class="special">}</span>
6444 <span class="special">};</span>
6445 </pre>
6446 <p>
6447             Note the use of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;&gt;</span></code> to access the return type of
6448             the <code class="computeroutput"><span class="identifier">CalcArity</span></code> function
6449             object. Since we used compile-time integers in our transforms, the arity
6450             of the expression is encoded in the return type of the <code class="computeroutput"><span class="identifier">CalcArity</span></code> function object. Proto grammars
6451             are valid TR1-style function objects, so you can use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;&gt;</span></code> to figure out their return types.
6452           </p>
6453 <p>
6454             With our compile-time assertions in place, when users provide too many
6455             or too few arguments to a calculator expression, as in:
6456           </p>
6457 <pre class="programlisting"><span class="special">(</span><span class="identifier">_2</span> <span class="special">*</span> <span class="identifier">_2</span><span class="special">)(</span><span class="number">42</span><span class="special">);</span> <span class="comment">// Oops, too few arguments!</span>
6458 </pre>
6459 <p>
6460             ... they will get a compile-time error message on the line with the assertion
6461             that reads something like this<a href="#ftn.boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0" class="footnote" name="boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0"><sup class="footnote">[35]</sup></a>:
6462           </p>
6463 <pre class="programlisting">c:\boost\org\trunk\libs\proto\scratch\main.cpp(97) : error C2664: 'boost::mpl::asse
6464 rtion_failed' : cannot convert parameter 1 from 'boost::mpl::failed ************boo
6465 st::mpl::assert_relation&lt;x,y,__formal&gt;::************' to 'boost::mpl::assert&lt;false&gt;
6466 ::type'
6467    with
6468    [
6469        x=1,
6470        y=2,
6471        __formal=bool boost::mpl::operator==(boost::mpl::failed,boost::mpl::failed)
6472    ]
6473 </pre>
6474 <p>
6475             The point of this exercise was to show that we can write a fairly simple
6476             Proto grammar with embedded transforms that is declarative and readable
6477             and can compute interesting properties of arbitrarily complicated expressions.
6478             But transforms can do more than that. Boost.Xpressive uses transforms
6479             to turn expressions into finite state automata for matching regular expressions,
6480             and Boost.Spirit uses transforms to build recursive descent parser generators.
6481             Proto comes with a collection of built-in transforms that you can use
6482             to perform very sophisticated expression manipulations like these. In
6483             the next few sections we'll see some of them in action.
6484           </p>
6485 </div>
6486 <div class="section">
6487 <div class="titlepage"><div><div><h5 class="title">
6488 <a name="boost_proto.users_guide.back_end.expression_transformation.state"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.state" title="Transforms With State Accumulation">Transforms
6489           With State Accumulation</a>
6490 </h5></div></div></div>
6491 <p>
6492             So far, we've only seen examples of grammars with transforms that accept
6493             one argument: the expression to transform. But consider for a moment
6494             how, in ordinary procedural code, you would turn a binary tree into a
6495             linked list. You would start with an empty list. Then, you would recursively
6496             convert the right branch to a list, and use the result as the initial
6497             state while converting the left branch to a list. That is, you would
6498             need a function that takes two parameters: the current node and the list
6499             so far. These sorts of <span class="emphasis"><em>accumulation</em></span> problems are
6500             quite common when processing trees. The linked list is an example of
6501             an accumulation variable or <span class="emphasis"><em>state</em></span>. Each iteration
6502             of the algorithm takes the current element and state, applies some binary
6503             function to the two and creates a new state. In the STL, this algorithm
6504             is called <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">accumulate</span><span class="special">()</span></code>.
6505             In many other languages, it is called <span class="emphasis"><em>fold</em></span>. Let's
6506             see how to implement a fold algorithm with Proto transforms.
6507           </p>
6508 <p>
6509             All Proto grammars can optionally accept a state parameter in addition
6510             to the expression to transform. If you want to fold a tree to a list,
6511             you'll need to make use of the state parameter to pass around the list
6512             you've built so far. As for the list, the Boost.Fusion library provides
6513             a <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;&gt;</span></code>
6514             type from which you can build heterogeneous lists. The type <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span></code> represents an empty list.
6515           </p>
6516 <p>
6517             Below is a grammar that recognizes output expressions like <code class="computeroutput"><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span>
6518             <span class="number">42</span> <span class="special">&lt;&lt;</span>
6519             <span class="char">'\n'</span></code> and puts the arguments into
6520             a Fusion list. It is explained below.
6521           </p>
6522 <pre class="programlisting"><span class="comment">// Fold the terminals in output statements like</span>
6523 <span class="comment">// "cout_ &lt;&lt; 42 &lt;&lt; '\n'" into a Fusion cons-list.</span>
6524 <span class="keyword">struct</span> <span class="identifier">FoldToList</span>
6525   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
6526         <span class="comment">// Don't add the ostream terminal to the list</span>
6527         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
6528             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;</span>
6529           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span>
6530         <span class="special">&gt;</span>
6531         <span class="comment">// Put all other terminals at the head of the</span>
6532         <span class="comment">// list that we're building in the "state" parameter</span>
6533       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
6534             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
6535           <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">&gt;(</span>
6536                 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span>
6537             <span class="special">)</span>
6538         <span class="special">&gt;</span>
6539         <span class="comment">// For left-shift operations, first fold the right</span>
6540         <span class="comment">// child to a list using the current state. Use</span>
6541         <span class="comment">// the result as the state parameter when folding</span>
6542         <span class="comment">// the left child to a list.</span>
6543       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
6544             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span><span class="identifier">FoldToList</span><span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">&gt;</span>
6545           <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span>
6546                 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
6547               <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)</span>
6548             <span class="special">)</span>
6549         <span class="special">&gt;</span>
6550     <span class="special">&gt;</span>
6551 <span class="special">{};</span>
6552 </pre>
6553 <p>
6554             Before reading on, see if you can apply what you know already about object,
6555             callable and primitive transforms to figure out how this grammar works.
6556           </p>
6557 <p>
6558             When you use the <code class="computeroutput"><span class="identifier">FoldToList</span></code>
6559             function, you'll need to pass two arguments: the expression to fold,
6560             and the initial state: an empty list. Those two arguments get passed
6561             around to each transform. We learned previously that <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code>
6562             is a primitive transform that accepts a terminal expression and extracts
6563             its value. What we didn't know until now was that it also accepts the
6564             current state <span class="emphasis"><em>and ignores it</em></span>. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span></code>
6565             is also a primitive transform. It accepts the current expression, which
6566             it ignores, and the current state, which it returns.
6567           </p>
6568 <p>
6569             When we find a terminal, we stick it at the head of the cons list, using
6570             the current state as the tail of the list. (The first alternate causes
6571             the <code class="computeroutput"><span class="identifier">ostream</span></code> to be skipped.
6572             We don't want <code class="computeroutput"><span class="identifier">cout</span></code> in
6573             the list.) When we find a shift-left node, we apply the following transform:
6574           </p>
6575 <pre class="programlisting"><span class="comment">// Fold the right child and use the result as</span>
6576 <span class="comment">// state while folding the right.</span>
6577 <span class="identifier">FoldToList</span><span class="special">(</span>
6578     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
6579   <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)</span>
6580 <span class="special">)</span>
6581 </pre>
6582 <p>
6583             You can read this transform as follows: using the current state, fold
6584             the right child to a list. Use the new list as the state while folding
6585             the left child to a list.
6586           </p>
6587 <div class="tip"><table border="0" summary="Tip">
6588 <tr>
6589 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td>
6590 <th align="left">Tip</th>
6591 </tr>
6592 <tr><td align="left" valign="top">
6593 <p>
6594               If your compiler is Microsoft Visual C++, you'll find that the above
6595               transform does not compile. The compiler has bugs with its handling
6596               of nested function types. You can work around the bug by wrapping the
6597               inner transform in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;&gt;</span></code> as follows:
6598             </p>
6599 <p>
6600 </p>
6601 <pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span>
6602     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
6603   <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)&gt;</span>
6604 <span class="special">)</span>
6605 </pre>
6606 <p>
6607             </p>
6608 <p>
6609               <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;&gt;</span></code>
6610               turns a callable transform into a primitive transform, but more on
6611               that later.
6612             </p>
6613 </td></tr>
6614 </table></div>
6615 <p>
6616             Now that we have defined the <code class="computeroutput"><span class="identifier">FoldToList</span></code>
6617             function object, we can use it to turn output expressions into lists
6618             as follows:
6619           </p>
6620 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span>
6621
6622 <span class="comment">// This is the type of the list we build below</span>
6623 <span class="keyword">typedef</span>
6624     <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
6625         <span class="keyword">int</span>
6626       <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
6627             <span class="keyword">double</span>
6628           <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
6629                 <span class="keyword">char</span>
6630               <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span>
6631             <span class="special">&gt;</span>
6632         <span class="special">&gt;</span>
6633     <span class="special">&gt;</span>
6634 <span class="identifier">result_type</span><span class="special">;</span>
6635
6636 <span class="comment">// Fold an output expression into a Fusion list, using</span>
6637 <span class="comment">// fusion::nil as the initial state of the transformation.</span>
6638 <span class="identifier">FoldToList</span> <span class="identifier">to_list</span><span class="special">;</span>
6639 <span class="identifier">result_type</span> <span class="identifier">args</span> <span class="special">=</span> <span class="identifier">to_list</span><span class="special">(</span><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="number">1</span> <span class="special">&lt;&lt;</span> <span class="number">3.14</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span><span class="special">());</span>
6640
6641 <span class="comment">// Now "args" is the list: {1, 3.14, '\n'}</span>
6642 </pre>
6643 <p>
6644             When writing transforms, "fold" is such a basic operation that
6645             Proto provides a number of built-in fold transforms. We'll get to them
6646             later. For now, rest assured that you won't always have to stretch your
6647             brain so far to do such basic things.
6648           </p>
6649 </div>
6650 <div class="section">
6651 <div class="titlepage"><div><div><h5 class="title">
6652 <a name="boost_proto.users_guide.back_end.expression_transformation.data"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.data" title="Passing Auxiliary Data to Transforms">Passing
6653           Auxiliary Data to Transforms</a>
6654 </h5></div></div></div>
6655 <p>
6656             In the last section, we saw that we can pass a second parameter to grammars
6657             with transforms: an accumulation variable or <span class="emphasis"><em>state</em></span>
6658             that gets updated as your transform executes. There are times when your
6659             transforms will need to access auxiliary data that does <span class="emphasis"><em>not</em></span>
6660             accumulate, so bundling it with the state parameter is impractical. Instead,
6661             you can pass auxiliary data as a third parameter, known as the <span class="emphasis"><em>data</em></span>
6662             parameter.
6663           </p>
6664 <p>
6665             Let's modify our previous example so that it writes each terminal to
6666             <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></code> before it puts it into a list.
6667             This could be handy for debugging your transforms, for instance. We can
6668             make it general by passing a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>
6669             into the transform in the data parameter. Within the transform itself,
6670             we can retrieve the <code class="computeroutput"><span class="identifier">ostream</span></code>
6671             with the <code class="computeroutput"><a class="link" href="../boost/proto/_data.html" title="Struct _data">proto::_data</a></code>
6672             transform. The strategy is as follows: use the <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code>
6673             transform to chain two actions. The second action will create the <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;&gt;</span></code>
6674             node as before. The first action, however, will display the current expression.
6675             For that, we first construct an instance of <code class="computeroutput"><a class="link" href="../boost/proto/functional/display_expr.html" title="Struct display_expr">proto::functional::display_expr</a></code> and then call
6676             it.
6677           </p>
6678 <pre class="programlisting"><span class="comment">// Fold the terminals in output statements like</span>
6679 <span class="comment">// "cout_ &lt;&lt; 42 &lt;&lt; '\n'" into a Fusion cons-list.</span>
6680 <span class="keyword">struct</span> <span class="identifier">FoldToList</span>
6681   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
6682         <span class="comment">// Don't add the ostream terminal to the list</span>
6683         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
6684             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;</span>
6685           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span>
6686         <span class="special">&gt;</span>
6687         <span class="comment">// Put all other terminals at the head of the</span>
6688         <span class="comment">// list that we're building in the "state" parameter</span>
6689       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
6690             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
6691           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special">&lt;</span>
6692                 <span class="comment">// First, write the terminal to an ostream passed</span>
6693                 <span class="comment">// in the data parameter</span>
6694                 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">lazy</span><span class="special">&lt;</span>
6695                     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)&gt;(</span><span class="identifier">_</span><span class="special">)</span>
6696                 <span class="special">&gt;</span>
6697                 <span class="comment">// Then, constuct the new cons list.</span>
6698               <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">&gt;(</span>
6699                     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span>
6700                 <span class="special">)</span>
6701             <span class="special">&gt;</span>
6702         <span class="special">&gt;</span>
6703         <span class="comment">// For left-shift operations, first fold the right</span>
6704         <span class="comment">// child to a list using the current state. Use</span>
6705         <span class="comment">// the result as the state parameter when folding</span>
6706         <span class="comment">// the left child to a list.</span>
6707       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
6708             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;</span><span class="identifier">FoldToList</span><span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">&gt;</span>
6709           <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span>
6710                 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
6711               <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)</span>
6712               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span>
6713             <span class="special">)</span>
6714         <span class="special">&gt;</span>
6715     <span class="special">&gt;</span>
6716 <span class="special">{};</span>
6717 </pre>
6718 <p>
6719             This is a lot to take in, no doubt. But focus on the second <code class="computeroutput"><span class="identifier">when</span></code> clause above. It says: when you
6720             find a terminal, first display the terminal using the <code class="computeroutput"><span class="identifier">ostream</span></code>
6721             you find in the data parameter, then take the value of the terminal and
6722             the current state to build a new <code class="computeroutput"><span class="identifier">cons</span></code>
6723             list. The function object <code class="computeroutput"><span class="identifier">display_expr</span></code>
6724             does the job of printing the terminal, and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special">&lt;&gt;</span></code> chains the actions together and
6725             executes them in sequence, returning the result of the last one.
6726           </p>
6727 <div class="note"><table border="0" summary="Note">
6728 <tr>
6729 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
6730 <th align="left">Note</th>
6731 </tr>
6732 <tr><td align="left" valign="top"><p>
6733               Also new is <code class="computeroutput"><a class="link" href="../boost/proto/lazy.html" title="Struct template lazy">proto::lazy&lt;&gt;</a></code>. Sometimes you
6734               don't have a ready-made callable object to execute. Instead, you want
6735               to first make one and <span class="emphasis"><em>then</em></span> execute it. Above,
6736               we need to create a <code class="computeroutput"><span class="identifier">display_expr</span></code>,
6737               initializing it with our <code class="computeroutput"><span class="identifier">ostream</span></code>.
6738               After that, we want to invoke it by passing it the current expression.
6739               It's as if we were doing <code class="computeroutput"><span class="identifier">display_expr</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">)(</span><span class="emphasis"><em>the-expr</em></span><span class="special">)</span></code>.
6740               We achieve this two-phase evaluation using <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">lazy</span><span class="special">&lt;&gt;</span></code>. If this doesn't make sense
6741               yet, don't worry about it.
6742             </p></td></tr>
6743 </table></div>
6744 <p>
6745             We can use the above transform as before, but now we can pass an <code class="computeroutput"><span class="identifier">ostream</span></code> as the third parameter and
6746             get to watch the transform in action. Here's a sample usage:
6747           </p>
6748 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span>
6749
6750 <span class="comment">// This is the type of the list we build below</span>
6751 <span class="keyword">typedef</span>
6752     <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
6753         <span class="keyword">int</span>
6754       <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
6755             <span class="keyword">double</span>
6756           <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">cons</span><span class="special">&lt;</span>
6757                 <span class="keyword">char</span>
6758               <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span>
6759             <span class="special">&gt;</span>
6760         <span class="special">&gt;</span>
6761     <span class="special">&gt;</span>
6762 <span class="identifier">result_type</span><span class="special">;</span>
6763
6764 <span class="comment">// Fold an output expression into a Fusion list, using</span>
6765 <span class="comment">// fusion::nil as the initial state of the transformation.</span>
6766 <span class="comment">// Pass std::cout as the data parameter so that we can track</span>
6767 <span class="comment">// the progress of the transform on the console.</span>
6768 <span class="identifier">FoldToList</span> <span class="identifier">to_list</span><span class="special">;</span>
6769 <span class="identifier">result_type</span> <span class="identifier">args</span> <span class="special">=</span> <span class="identifier">to_list</span><span class="special">(</span><span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="number">1</span> <span class="special">&lt;&lt;</span> <span class="number">3.14</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">nil</span><span class="special">(),</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">);</span>
6770
6771 <span class="comment">// Now "args" is the list: {1, 3.14, '\n'}</span>
6772 </pre>
6773 <p>
6774             This code displays the following:
6775           </p>
6776 <pre class="programlisting">terminal(
6777 )
6778 terminal(3.14)
6779 terminal(1)</pre>
6780 <p>
6781             This is a rather round-about way of demonstrating that you can pass extra
6782             data to a transform as a third parameter. There are no restrictions on
6783             what this parameter can be, and, unlike the state parameter, Proto will
6784             never mess with it.
6785           </p>
6786 <h6>
6787 <a name="boost_proto.users_guide.back_end.expression_transformation.data.h0"></a>
6788             <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.data.transform_environment_variables"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.data.transform_environment_variables">Transform
6789             Environment Variables</a>
6790           </h6>
6791 <div class="note"><table border="0" summary="Note">
6792 <tr>
6793 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
6794 <th align="left">Note</th>
6795 </tr>
6796 <tr><td align="left" valign="top"><p>
6797               <span class="emphasis"><em>This is an advanced topic. Feel free to skip if you are new
6798               to Proto.</em></span>
6799             </p></td></tr>
6800 </table></div>
6801 <p>
6802             The example above uses the data parameter as a transport mechanism for
6803             an unstructured blob of data; in this case, a reference to an <code class="computeroutput"><span class="identifier">ostream</span></code>. As your Proto algorithms become
6804             more sophisticated, you may find that an unstructured blob of data isn't
6805             terribly convenient to work with. Different parts of your algorithm may
6806             be interested in different bits of data. What you want, instead, is a
6807             way to pass in a collection of <span class="emphasis"><em>environment variables</em></span>
6808             to a transform, like a collection of key/value pairs. Then, you can easily
6809             get at the piece of data you want by asking the data parameter for the
6810             value associated with a particular key. Proto's <span class="emphasis"><em>transform environments</em></span>
6811             give you just that.
6812           </p>
6813 <p>
6814             Let's start by defining a key.
6815           </p>
6816 <pre class="programlisting"><span class="identifier">BOOST_PROTO_DEFINE_ENV_VAR</span><span class="special">(</span><span class="identifier">mykey_type</span><span class="special">,</span> <span class="identifier">mykey</span><span class="special">);</span>
6817 </pre>
6818 <p>
6819             This defines a global constant <code class="computeroutput"><span class="identifier">mykey</span></code>
6820             with the type <code class="computeroutput"><span class="identifier">mykey_type</span></code>.
6821             We can use <code class="computeroutput"><span class="identifier">mykey</span></code> to store
6822             a piece of assiciated data in a transform environment, as so:
6823           </p>
6824 <pre class="programlisting"><span class="comment">// Call the MyEval algorithm with a transform environment containing</span>
6825 <span class="comment">// two key/value pairs: one for proto::data and one for mykey</span>
6826 <span class="identifier">MyEval</span><span class="special">()(</span> <span class="identifier">expr</span><span class="special">,</span> <span class="identifier">state</span><span class="special">,</span> <span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span> <span class="special">=</span> <span class="number">42</span><span class="special">,</span> <span class="identifier">mykey</span> <span class="special">=</span> <span class="string">"hello world"</span><span class="special">)</span> <span class="special">);</span>
6827 </pre>
6828 <p>
6829             The above means to invoke the <code class="computeroutput"><span class="identifier">MyEval</span></code>
6830             algorithm with three parameters: an expression, an initial state, and
6831             a transform environment containing two key/value pairs.
6832           </p>
6833 <p>
6834             From within a Proto algorithm, you can access the values associated with
6835             different keys using the <code class="computeroutput"><a class="link" href="../boost/proto/_env_var.html" title="Struct template _env_var">proto::_env_var&lt;&gt;</a></code>
6836             transform. For instance, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_env_var</span><span class="special">&lt;</span><span class="identifier">mykey_type</span><span class="special">&gt;</span></code> would fetch the value <code class="computeroutput"><span class="string">"hello world"</span></code> from the transform
6837             environment created above.
6838           </p>
6839 <p>
6840             The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span></code> transform has some additional
6841             smarts. Rather than always returning the third parameter regarless of
6842             whether it is a blob or a transform environment, it checks first to see
6843             if it's a blob or not. If so, that's what gets returned. If not, it returns
6844             the value associated with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span></code>
6845             key. In the above example, that would be the value <code class="computeroutput"><span class="number">42</span></code>.
6846           </p>
6847 <p>
6848             There's a small host of functions, metafunction, and classes that you
6849             can use to create and manipulate transform environments, some for testing
6850             whether an object is a transform environment, some for coercing an object
6851             to be a transform environment, and some for querying a transform environment
6852             whether or not is has a value for a particular key. For an exhaustive
6853             treatment of the topic, check out the reference for the <code class="computeroutput"><a class="link" href="reference.html#header.boost.proto.transform.env_hpp" title="Header &lt;boost/proto/transform/env.hpp&gt;">boost/proto/transform/env.hpp</a></code>
6854             header.
6855           </p>
6856 </div>
6857 <div class="section">
6858 <div class="titlepage"><div><div><h5 class="title">
6859 <a name="boost_proto.users_guide.back_end.expression_transformation.implicit_params"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.implicit_params" title="Implicit Parameters to Primitive Transforms">Implicit
6860           Parameters to Primitive Transforms</a>
6861 </h5></div></div></div>
6862 <p>
6863             Let's use <code class="computeroutput"><span class="identifier">FoldToList</span></code>
6864             example from the previous two sections to illustrate some other niceties
6865             of Proto transforms. We've seen that grammars, when used as function
6866             objects, can accept up to 3 parameters, and that when using these grammars
6867             in callable transforms, you can also specify up to 3 parameters. Let's
6868             take another look at the transform associated with non-terminals from
6869             the last section:
6870           </p>
6871 <pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span>
6872     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
6873   <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)</span>
6874   <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span>
6875 <span class="special">)</span>
6876 </pre>
6877 <p>
6878             Here we specify all three parameters to both invocations of the <code class="computeroutput"><span class="identifier">FoldToList</span></code> grammar. But we don't have
6879             to specify all three. If we don't specify a third parameter, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span></code> is assumed. Likewise for the
6880             second parameter and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span></code>.
6881             So the above transform could have been written more simply as:
6882           </p>
6883 <pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span>
6884     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
6885   <span class="special">,</span> <span class="identifier">StringCopy</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)</span>
6886 <span class="special">)</span>
6887 </pre>
6888 <p>
6889             The same is true for any primitive transform. The following are all equivalent:
6890           </p>
6891 <div class="table">
6892 <a name="boost_proto.users_guide.back_end.expression_transformation.implicit_params.t0"></a><p class="title"><b>Table&#160;33.9.&#160;Implicit Parameters to Primitive Transforms</b></p>
6893 <div class="table-contents"><table class="table" summary="Implicit Parameters to Primitive Transforms">
6894 <colgroup><col></colgroup>
6895 <thead><tr><th>
6896                     <p>
6897                       Equivalent Transforms
6898                     </p>
6899                   </th></tr></thead>
6900 <tbody>
6901 <tr><td>
6902                     <p>
6903                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span>
6904                       <span class="identifier">FoldToList</span><span class="special">&gt;</span></code>
6905                     </p>
6906                   </td></tr>
6907 <tr><td>
6908                     <p>
6909                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span>
6910                       <span class="identifier">FoldToList</span><span class="special">()&gt;</span></code>
6911                     </p>
6912                   </td></tr>
6913 <tr><td>
6914                     <p>
6915                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span>
6916                       <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">_</span><span class="special">)&gt;</span></code>
6917                     </p>
6918                   </td></tr>
6919 <tr><td>
6920                     <p>
6921                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span>
6922                       <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">_</span><span class="special">,</span>
6923                       <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)&gt;</span></code>
6924                     </p>
6925                   </td></tr>
6926 <tr><td>
6927                     <p>
6928                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span>
6929                       <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">_</span><span class="special">,</span>
6930                       <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span>
6931                       <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)&gt;</span></code>
6932                     </p>
6933                   </td></tr>
6934 </tbody>
6935 </table></div>
6936 </div>
6937 <br class="table-break"><div class="note"><table border="0" summary="Note">
6938 <tr>
6939 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
6940 <th align="left">Note</th>
6941 </tr>
6942 <tr><td align="left" valign="top">
6943 <p>
6944               <span class="bold"><strong>Grammars Are Primitive Transforms Are Function
6945               Objects</strong></span>
6946             </p>
6947 <p>
6948               So far, we've said that all Proto grammars are function objects. But
6949               it's more accurate to say that Proto grammars are primitive transforms
6950               -- a special kind of function object that takes between 1 and 3 arguments,
6951               and that Proto knows to treat specially when used in a callable transform,
6952               as in the table above.
6953             </p>
6954 </td></tr>
6955 </table></div>
6956 <div class="note"><table border="0" summary="Note">
6957 <tr>
6958 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
6959 <th align="left">Note</th>
6960 </tr>
6961 <tr><td align="left" valign="top">
6962 <p>
6963               <span class="bold"><strong>Not All Function Objects Are Primitive Transforms</strong></span>
6964             </p>
6965 <p>
6966               You might be tempted now to drop the <code class="computeroutput"><span class="identifier">_state</span></code>
6967               and <code class="computeroutput"><span class="identifier">_data</span></code> parameters
6968               for all your callable transforms. That would be an error. You can only
6969               do that for primitive transforms, and not all callables are primitive
6970               transforms. Later on, we'll see what distinguishes ordinary callables
6971               from their more powerful primitive transfor cousins, but the short
6972               version is this: primitive transforms inherit from <code class="computeroutput"><a class="link" href="../boost/proto/transform.html" title="Struct template transform">proto::transform&lt;&gt;</a></code>.
6973             </p>
6974 </td></tr>
6975 </table></div>
6976 <p>
6977             Once you know that primitive transforms will always receive all three
6978             parameters -- expression, state, and data -- it makes things possible
6979             that wouldn't be otherwise. For instance, consider that for binary expressions,
6980             these two transforms are equivalent. Can you see why?
6981           </p>
6982 <div class="table">
6983 <a name="boost_proto.users_guide.back_end.expression_transformation.implicit_params.t1"></a><p class="title"><b>Table&#160;33.10.&#160;Two Equivalent Transforms</b></p>
6984 <div class="table-contents"><table class="table" summary="Two Equivalent Transforms">
6985 <colgroup>
6986 <col>
6987 <col>
6988 </colgroup>
6989 <thead><tr>
6990 <th>
6991                     <p>
6992                       Without <code class="literal">proto::reverse_fold&lt;&gt;</code>
6993                     </p>
6994                   </th>
6995 <th>
6996                     <p>
6997                       With <code class="literal">proto::reverse_fold&lt;&gt;</code>
6998                     </p>
6999                   </th>
7000 </tr></thead>
7001 <tbody><tr>
7002 <td>
7003                     <p>
7004 </p>
7005 <pre class="programlisting"><span class="identifier">FoldToList</span><span class="special">(</span>
7006     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span>
7007   <span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span><span class="special">)</span>
7008   <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span>
7009 <span class="special">)</span></pre>
7010 <p>
7011                     </p>
7012                   </td>
7013 <td>
7014                     <p>
7015 </p>
7016 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">reverse_fold</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">FoldToList</span><span class="special">&gt;</span></pre>
7017 <p>
7018                     </p>
7019                   </td>
7020 </tr></tbody>
7021 </table></div>
7022 </div>
7023 <br class="table-break">
7024 </div>
7025 <div class="section">
7026 <div class="titlepage"><div><div><h5 class="title">
7027 <a name="boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions" title="Unpacking Expressions">Unpacking
7028           Expressions</a>
7029 </h5></div></div></div>
7030 <p>
7031             Processing expressions with an arbitrary number of children can be a
7032             pain. What if you want to do something to each child, then pass the results
7033             as arguments to some other function? Can you do it just once without
7034             worrying about how many children an expression has? Yes. This is where
7035             Proto's <span class="emphasis"><em>unpacking expressions</em></span> come in handy. Unpacking
7036             expressions give you a way to write callable and object transforms that
7037             handle <span class="emphasis"><em>n</em></span>-ary expressions.
7038           </p>
7039 <div class="note"><table border="0" summary="Note">
7040 <tr>
7041 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
7042 <th align="left">Note</th>
7043 </tr>
7044 <tr><td align="left" valign="top">
7045 <p>
7046               <span class="bold"><strong>Inspired by C++11 Variadic Templates</strong></span>
7047             </p>
7048 <p>
7049               Proto's unpacking expressions take inspiration from the C++11 feature
7050               of the same name. If you are familiar with variadic functions, and
7051               in particular how to expand a function parameter pack, this discussion
7052               should seem very familiar. However, this feature doesn't actually use
7053               any C++11 features, so the code describe here will work with any compliant
7054               C++98 compiler.
7055             </p>
7056 </td></tr>
7057 </table></div>
7058 <h6>
7059 <a name="boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions.h0"></a>
7060             <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions.example__a_c___expression_evaluator"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.unpacking_expressions.example__a_c___expression_evaluator">Example:
7061             A C++ Expression Evaluator</a>
7062           </h6>
7063 <p>
7064             Proto has the built-in <code class="computeroutput"><a class="link" href="../boost/proto/_default.html" title="Struct template _default">proto::_default&lt;&gt;</a></code>
7065             transform for evaluating Proto expressions in a C++-ish way. But if it
7066             didn't, it wouldn't be too hard to implement one from scratch using Proto's
7067             unpacking patterns. The transform <code class="computeroutput"><span class="identifier">eval</span></code>
7068             below does just that.
7069           </p>
7070 <pre class="programlisting"><span class="comment">// A callable polymorphic function object that takes an unpacked expression</span>
7071 <span class="comment">// and a tag, and evaluates the expression. A plus tag and two operands adds</span>
7072 <span class="comment">// them with operator +, for instance.</span>
7073 <span class="keyword">struct</span> <span class="identifier">do_eval</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
7074 <span class="special">{</span>
7075     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
7076
7077 <span class="preprocessor">#define</span> <span class="identifier">UNARY_OP</span><span class="special">(</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">OP</span><span class="special">)</span>                                                       <span class="special">\</span>
7078     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">&gt;</span>                                                      <span class="special">\</span>
7079     <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">Arg</span> <span class="identifier">arg</span><span class="special">)</span> <span class="keyword">const</span>                           <span class="special">\</span>
7080     <span class="special">{</span>                                                                           <span class="special">\</span>
7081         <span class="keyword">return</span> <span class="identifier">OP</span> <span class="identifier">arg</span><span class="special">;</span>                                                          <span class="special">\</span>
7082     <span class="special">}</span>                                                                           <span class="special">\</span>
7083     <span class="comment">/**/</span>
7084
7085 <span class="preprocessor">#define</span> <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">OP</span><span class="special">)</span>                                                      <span class="special">\</span>
7086     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Left</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Right</span><span class="special">&gt;</span>                                     <span class="special">\</span>
7087     <span class="keyword">double</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">TAG</span><span class="special">,</span> <span class="identifier">Left</span> <span class="identifier">left</span><span class="special">,</span> <span class="identifier">Right</span> <span class="identifier">right</span><span class="special">)</span> <span class="keyword">const</span>            <span class="special">\</span>
7088     <span class="special">{</span>                                                                           <span class="special">\</span>
7089         <span class="keyword">return</span> <span class="identifier">left</span> <span class="identifier">OP</span> <span class="identifier">right</span><span class="special">;</span>                                                   <span class="special">\</span>
7090     <span class="special">}</span>                                                                           <span class="special">\</span>
7091     <span class="comment">/**/</span>
7092
7093     <span class="identifier">UNARY_OP</span><span class="special">(</span><span class="identifier">negate</span><span class="special">,</span> <span class="special">-)</span>
7094     <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">plus</span><span class="special">,</span> <span class="special">+)</span>
7095     <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">minus</span><span class="special">,</span> <span class="special">-)</span>
7096     <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">multiplies</span><span class="special">,</span> <span class="special">*)</span>
7097     <span class="identifier">BINARY_OP</span><span class="special">(</span><span class="identifier">divides</span><span class="special">,</span> <span class="special">/)</span>
7098     <span class="comment">/*... others ...*/</span>
7099 <span class="special">};</span>
7100
7101 <span class="keyword">struct</span> <span class="identifier">eval</span>
7102   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
7103         <span class="comment">// Evaluate terminals by simply returning their value</span>
7104         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">&gt;</span>
7105
7106         <span class="comment">// Non-terminals are handled by unpacking the expression,</span>
7107         <span class="comment">// recursively calling eval on each child, and passing</span>
7108         <span class="comment">// the results along with the expression's tag to do_eval</span>
7109         <span class="comment">// defined above.</span>
7110       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">otherwise</span><span class="special">&lt;</span><span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">))...)&gt;</span>
7111         <span class="comment">// UNPACKING PATTERN HERE -------------------^^^^^^^^^^^^^^^^^^^^^^^^</span>
7112     <span class="special">&gt;</span>
7113 <span class="special">{};</span>
7114 </pre>
7115 <p>
7116             The bulk of the above code is devoted to the <code class="computeroutput"><span class="identifier">do_eval</span></code>
7117             function object that maps tag types to behaviors, but the interesting
7118             bit is the definition of the <code class="computeroutput"><span class="identifier">eval</span></code>
7119             algorithm at the bottom. Terminals are handled quite simply, but non-terminals
7120             could be unary, binary, ternary, even <span class="emphasis"><em>n</em></span>-ary if we
7121             consider function call expressions. The <code class="computeroutput"><span class="identifier">eval</span></code>
7122             algorithm handles this uniformly with the help of an unpacking pattern.
7123           </p>
7124 <p>
7125             Non-terminals are evaluated with this callable transform:
7126           </p>
7127 <pre class="programlisting"><span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">))...)</span>
7128 </pre>
7129 <p>
7130             You can read this as: call the <code class="computeroutput"><span class="identifier">do_eval</span></code>
7131             function object with the tag of the current expression and all its children
7132             after they have each been evaluated with <code class="computeroutput"><span class="identifier">eval</span></code>.
7133             The unpacking pattern is the bit just before the ellipsis: <code class="computeroutput"><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">))</span></code>.
7134           </p>
7135 <p>
7136             What's going on here is this. The unpacking expression gets repeated
7137             once for each child in the expression currently being evaluated. In each
7138             repetition, the type <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">)</span></code> gets replaced with <code class="literal">proto::_child_c&lt;<span class="emphasis"><em>N</em></span>&gt;</code>.
7139             So, if a unary expression is passed to <code class="computeroutput"><span class="identifier">eval</span></code>,
7140             it actually gets evaluated like this:
7141           </p>
7142 <pre class="programlisting"><span class="comment">// After the unpacking pattern is expanded for a unary expression</span>
7143 <span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;))</span>
7144 </pre>
7145 <p>
7146             And when passed a binary expression, the unpacking pattern expands like
7147             this:
7148           </p>
7149 <pre class="programlisting"><span class="comment">// After the unpacking pattern is expanded for a binary expression</span>
7150 <span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;))</span>
7151 </pre>
7152 <p>
7153             Although it can't happen in our example, when passed a terminal, the
7154             unpacking pattern expands such that it extracts the value from the terminal
7155             instead of the children. So it gets handled like this:
7156           </p>
7157 <pre class="programlisting"><span class="comment">// If a terminal were passed to this transform, Proto would try</span>
7158 <span class="comment">// to evaluate it like this, which would fail:</span>
7159 <span class="identifier">do_eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag_of</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;(),</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">))</span>
7160 </pre>
7161 <p>
7162             That doesn't make sense. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span></code>
7163             would return something that isn't a Proto expression, and <code class="computeroutput"><span class="identifier">eval</span></code> wouldn't be able to evaluate it.
7164             Proto algorithms don't work unless you pass them Proto expressions.
7165           </p>
7166 <div class="note"><table border="0" summary="Note">
7167 <tr>
7168 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
7169 <th align="left">Note</th>
7170 </tr>
7171 <tr><td align="left" valign="top">
7172 <p>
7173               <span class="bold"><strong>Kickin' It Old School</strong></span>
7174             </p>
7175 <p>
7176               You may be thinking, my compiler doesn't support C++11 variadic templates!
7177               How can this possibly work? The answer is simple: The <code class="computeroutput"><span class="special">...</span></code> above isn't a C++11 pack expansion.
7178               It's actually an old-school C-style vararg. Remember that callable
7179               and object transforms are <span class="emphasis"><em>function types</em></span>. A transform
7180               with one of these pseudo-pack expansions is really just the type of
7181               a boring, old vararg function. Proto just interprets it differently.
7182             </p>
7183 </td></tr>
7184 </table></div>
7185 <p>
7186             Unpacking patterns are very expressive. Any callable or object transform
7187             can be used as an unpacking pattern, so long as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pack</span><span class="special">(</span><span class="identifier">_</span><span class="special">)</span></code> appears exactly once somewhere within
7188             it. This gives you a lot of flexibility in how you want to process the
7189             children of an expression before passing them on to some function object
7190             or object constructor.
7191           </p>
7192 </div>
7193 <div class="section">
7194 <div class="titlepage"><div><div><h5 class="title">
7195 <a name="boost_proto.users_guide.back_end.expression_transformation.external_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.external_transforms" title="Separating Grammars And Transforms">Separating
7196           Grammars And Transforms</a>
7197 </h5></div></div></div>
7198 <div class="note"><table border="0" summary="Note">
7199 <tr>
7200 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
7201 <th align="left">Note</th>
7202 </tr>
7203 <tr><td align="left" valign="top"><p>
7204               This is an advanced topic that is only necessary for people defining
7205               large EDSLs. Feel free to skip this if you're just getting started
7206               with Proto.
7207             </p></td></tr>
7208 </table></div>
7209 <p>
7210             So far, we've seen examples of grammars with embedded transforms. In
7211             practice, grammars can get pretty large, and you may want to use them
7212             to drive several different computations. For instance, you may have a
7213             grammar for a linear algebra domain, and you may want to use it to compute
7214             the shape of the result (vector or matrix?) and also to compute the result
7215             optimally. You don't want to have to copy and paste the whole shebang
7216             just to tweak one of the embedded transforms. What you want instead is
7217             to define the grammar once, and specify the transforms later when you're
7218             ready to evaluate an expression. For that, you use <span class="emphasis"><em>external
7219             transforms</em></span>. The pattern you'll use is this: replace one or
7220             more of the transforms in your grammar with the special placeholder
7221             <code class="computeroutput"><a class="link" href="../boost/proto/external_transform.html" title="Struct external_transform">proto::external_transform</a></code>.
7222             Then, you'll create a bundle of transforms that you will pass to the
7223             grammar in the data parameter (the 3rd parameter after the expression
7224             and state) when evaluating it.
7225           </p>
7226 <p>
7227             To illustrate external transforms, we'll build a calculator evaluator
7228             that can be configured to throw an exception on division by zero. Here
7229             is a bare-bones front end that defines a domain, a grammar, an expression
7230             wrapper, and some placeholder terminals.
7231           </p>
7232 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
7233 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
7234 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
7235 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">generation</span><span class="special">/</span><span class="identifier">make_vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
7236 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
7237 <span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
7238 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
7239 <span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span>
7240
7241 <span class="comment">// The argument placeholder type</span>
7242 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">:</span> <span class="identifier">I</span> <span class="special">{};</span>
7243
7244 <span class="comment">// The grammar for valid calculator expressions</span>
7245 <span class="keyword">struct</span> <span class="identifier">calc_grammar</span>
7246   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
7247         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
7248       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
7249       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7250       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7251       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7252       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7253     <span class="special">&gt;</span>
7254 <span class="special">{};</span>
7255
7256 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">calc_expr</span><span class="special">;</span>
7257 <span class="keyword">struct</span> <span class="identifier">calc_domain</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calc_expr</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">{};</span>
7258
7259 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span>
7260 <span class="keyword">struct</span> <span class="identifier">calc_expr</span>
7261   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">&gt;,</span> <span class="identifier">calc_domain</span><span class="special">&gt;</span>
7262 <span class="special">{</span>
7263     <span class="identifier">calc_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">calc_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span>
7264 <span class="special">};</span>
7265
7266 <span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">_1</span><span class="special">;</span>
7267 <span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">_2</span><span class="special">;</span>
7268
7269 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
7270 <span class="special">{</span>
7271     <span class="comment">// Build a calculator expression, and do nothing with it.</span>
7272     <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="identifier">_2</span><span class="special">);</span>
7273 <span class="special">}</span>
7274 </pre>
7275 <p>
7276             Now, let's embed transforms into <code class="computeroutput"><span class="identifier">calc_grammar</span></code>
7277             so that we can use it to evaluate calculator expressions:
7278           </p>
7279 <pre class="programlisting"><span class="comment">// The calculator grammar with embedded transforms for evaluating expression.</span>
7280 <span class="keyword">struct</span> <span class="identifier">calc_grammar</span>
7281   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
7282         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
7283             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
7284           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">at</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
7285         <span class="special">&gt;</span>
7286       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
7287             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
7288           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>
7289         <span class="special">&gt;</span>
7290       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
7291             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7292           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7293         <span class="special">&gt;</span>
7294       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
7295             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7296           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7297         <span class="special">&gt;</span>
7298       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
7299             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7300           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7301         <span class="special">&gt;</span>
7302       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
7303             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7304           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
7305         <span class="special">&gt;</span>
7306     <span class="special">&gt;</span>
7307 <span class="special">{};</span>
7308 </pre>
7309 <p>
7310             With this definition of <code class="computeroutput"><span class="identifier">calc_grammar</span></code>
7311             we can evaluate expressions by passing along a Fusion vector containing
7312             the values to use for the <code class="computeroutput"><span class="identifier">_1</span></code>
7313             and <code class="computeroutput"><span class="identifier">_2</span></code> placeholders:
7314           </p>
7315 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">3</span><span class="special">,</span> <span class="number">4</span><span class="special">));</span>
7316 <span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">result</span> <span class="special">==</span> <span class="number">7</span><span class="special">);</span>
7317 </pre>
7318 <p>
7319             We also want an alternative evaluation strategy that checks for division
7320             by zero and throws an exception. Just how ridiculous would it be to copy
7321             the entire <code class="computeroutput"><span class="identifier">calc_grammar</span></code>
7322             just to change the one line that transforms division expressions?! External
7323             transforms are ideally suited to this problem.
7324           </p>
7325 <p>
7326             First, we give the division rule in our grammar a "name"; that
7327             is, we make it a struct. We'll use this unique type later to dispatch
7328             to the right transforms.
7329           </p>
7330 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">calc_grammar</span><span class="special">;</span>
7331 <span class="keyword">struct</span> <span class="identifier">divides_rule</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span> <span class="special">{};</span>
7332 </pre>
7333 <p>
7334             Next, we change <code class="computeroutput"><span class="identifier">calc_grammar</span></code>
7335             to make the handling of division expressions external.
7336           </p>
7337 <pre class="programlisting"><span class="comment">// The calculator grammar with an external transform for evaluating</span>
7338 <span class="comment">// division expressions.</span>
7339 <span class="keyword">struct</span> <span class="identifier">calc_grammar</span>
7340   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
7341         <span class="comment">/* ... as before ... */</span>
7342       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
7343             <span class="identifier">divides_rule</span>
7344           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transform</span>
7345         <span class="special">&gt;</span>
7346     <span class="special">&gt;</span>
7347 <span class="special">{};</span>
7348 </pre>
7349 <p>
7350             The use of <code class="computeroutput"><a class="link" href="../boost/proto/external_transform.html" title="Struct external_transform">proto::external_transform</a></code> above
7351             makes the handling of division expressions externally parameterizeable.
7352           </p>
7353 <p>
7354             Next, we use <code class="computeroutput"><a class="link" href="../boost/proto/external_transforms.html" title="Struct template external_transforms">proto::external_transforms&lt;&gt;</a></code>
7355             (note the trailing 's') to capture our evaluation strategy in a bundle
7356             that we can pass along to the transform in the data parameter. Read on
7357             for the explanation.
7358           </p>
7359 <pre class="programlisting"><span class="comment">// Evaluate division nodes as before</span>
7360 <span class="keyword">struct</span> <span class="identifier">non_checked_division</span>
7361   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special">&lt;</span>
7362         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">divides_rule</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span> <span class="special">&gt;</span>
7363     <span class="special">&gt;</span>
7364 <span class="special">{};</span>
7365
7366 <span class="comment">/* ... */</span>
7367
7368 <span class="identifier">non_checked_division</span> <span class="identifier">non_checked</span><span class="special">;</span>
7369 <span class="keyword">int</span> <span class="identifier">result2</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">2</span><span class="special">),</span> <span class="identifier">non_checked</span><span class="special">);</span>
7370 </pre>
7371 <p>
7372             The struct <code class="computeroutput"><span class="identifier">non_cecked_division</span></code>
7373             associates the transform <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span></code> with the <code class="computeroutput"><span class="identifier">divides_rule</span></code>
7374             grammar rule. An instance of that struct is passed along as the third
7375             parameter when invoking <code class="computeroutput"><span class="identifier">calc_grammar</span></code>.
7376           </p>
7377 <p>
7378             Now, let's implement checked division. The rest should be unsurprising.
7379           </p>
7380 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">division_by_zero</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="special">{};</span>
7381
7382 <span class="keyword">struct</span> <span class="identifier">do_checked_divide</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
7383 <span class="special">{</span>
7384     <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span>
7385     <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">int</span> <span class="identifier">left</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">right</span><span class="special">)</span> <span class="keyword">const</span>
7386     <span class="special">{</span>
7387         <span class="keyword">if</span> <span class="special">(</span><span class="identifier">right</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">throw</span> <span class="identifier">division_by_zero</span><span class="special">();</span>
7388         <span class="keyword">return</span> <span class="identifier">left</span> <span class="special">/</span> <span class="identifier">right</span><span class="special">;</span>
7389     <span class="special">}</span>
7390 <span class="special">};</span>
7391
7392 <span class="keyword">struct</span> <span class="identifier">checked_division</span>
7393   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special">&lt;</span>
7394         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
7395             <span class="identifier">divides_rule</span>
7396           <span class="special">,</span> <span class="identifier">do_checked_divide</span><span class="special">(</span><span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span> <span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">))</span>
7397         <span class="special">&gt;</span>
7398     <span class="special">&gt;</span>
7399 <span class="special">{};</span>
7400
7401 <span class="comment">/* ... */</span>
7402
7403 <span class="keyword">try</span>
7404 <span class="special">{</span>
7405     <span class="identifier">checked_division</span> <span class="identifier">checked</span><span class="special">;</span>
7406     <span class="keyword">int</span> <span class="identifier">result3</span> <span class="special">=</span> <span class="identifier">calc_grammar_extern</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">0</span><span class="special">),</span> <span class="identifier">checked</span><span class="special">);</span>
7407 <span class="special">}</span>
7408 <span class="keyword">catch</span><span class="special">(</span><span class="identifier">division_by_zero</span><span class="special">)</span>
7409 <span class="special">{</span>
7410     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"caught division by zero!\n"</span><span class="special">;</span>
7411 <span class="special">}</span>
7412 </pre>
7413 <p>
7414             The above code demonstrates how a single grammar can be used with different
7415             transforms specified externally. This makes it possible to reuse a grammar
7416             to drive several different computations.
7417           </p>
7418 <h6>
7419 <a name="boost_proto.users_guide.back_end.expression_transformation.external_transforms.h0"></a>
7420             <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.external_transforms.separating_data_from_external_transforms"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.external_transforms.separating_data_from_external_transforms">Separating
7421             Data From External Transforms</a>
7422           </h6>
7423 <p>
7424             As described above, the external transforms feature usurps the data parameter,
7425             which is intended to be a place where you can pass arbitrary data, and
7426             gives it a specific meaning. But what if you are already using the data
7427             parameter for something else? The answer is to use a transform environment.
7428             By associating your external transforms with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transforms</span></code>
7429             key, you are free to pass arbitrary data in other slots.
7430           </p>
7431 <p>
7432             To continue the above example, what if we also needed to pass a piece
7433             of data into our transform along with the external transforms? It would
7434             look like this:
7435           </p>
7436 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">result3</span> <span class="special">=</span> <span class="identifier">calc_grammar_extern</span><span class="special">()(</span>
7437     <span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span>
7438   <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">0</span><span class="special">)</span>
7439   <span class="special">,</span> <span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span> <span class="special">=</span> <span class="number">42</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">transforms</span> <span class="special">=</span> <span class="identifier">checked</span><span class="special">)</span>
7440 <span class="special">);</span>
7441 </pre>
7442 <p>
7443             In the above invocation of the <code class="computeroutput"><span class="identifier">calc_grammar_extern</span></code>
7444             algorithm, the map of external transforms is associated with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transforms</span></code> key and passed to the algorithm
7445             in a transform environment. Also in the transform environment is a key/value
7446             pair that associates the value <code class="computeroutput"><span class="number">42</span></code>
7447             with the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span></code> key.
7448           </p>
7449 </div>
7450 <div class="section">
7451 <div class="titlepage"><div><div><h5 class="title">
7452 <a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms" title="Proto's Built-In Transforms">Proto's
7453           Built-In Transforms</a>
7454 </h5></div></div></div>
7455 <p>
7456             Primitive transforms are the building blocks for more interesting composite
7457             transforms. Proto defines a bunch of generally useful primitive transforms.
7458             They are summarized below.
7459           </p>
7460 <div class="variablelist">
7461 <p class="title"><b></b></p>
7462 <dl class="variablelist">
7463 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_value.html" title="Struct _value">proto::_value</a></code></span></dt>
7464 <dd><p>
7465                   Given a terminal expression, return the value of the terminal.
7466                 </p></dd>
7467 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_child_c.html" title="Struct template _child_c">proto::_child_c&lt;&gt;</a></code></span></dt>
7468 <dd><p>
7469                   Given a non-terminal expression, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><em class="replaceable"><code>N</code></em><span class="special">&gt;</span></code> returns the <em class="replaceable"><code>N</code></em>-th
7470                   child.
7471                 </p></dd>
7472 <dt><span class="term"> <code class="computeroutput"><a class="link" href="reference.html#boost.proto._child">proto::_child</a></code></span></dt>
7473 <dd><p>
7474                   A synonym for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span></code>.
7475                 </p></dd>
7476 <dt><span class="term"> <code class="computeroutput"><a class="link" href="reference.html#boost.proto._left">proto::_left</a></code></span></dt>
7477 <dd><p>
7478                   A synonym for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span></code>.
7479                 </p></dd>
7480 <dt><span class="term"> <code class="computeroutput"><a class="link" href="reference.html#boost.proto._right">proto::_right</a></code></span></dt>
7481 <dd><p>
7482                   A synonym for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span></code>.
7483                 </p></dd>
7484 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_expr.html" title="Struct _expr">proto::_expr</a></code></span></dt>
7485 <dd><p>
7486                   Returns the current expression unmodified.
7487                 </p></dd>
7488 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_state.html" title="Struct _state">proto::_state</a></code></span></dt>
7489 <dd><p>
7490                   Returns the current state unmodified.
7491                 </p></dd>
7492 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_data.html" title="Struct _data">proto::_data</a></code></span></dt>
7493 <dd><p>
7494                   Returns the current data unmodified.
7495                 </p></dd>
7496 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/call.html" title="Struct template call">proto::call&lt;&gt;</a></code></span></dt>
7497 <dd><p>
7498                   For a given callable transform <code class="computeroutput"><em class="replaceable"><code>CT</code></em></code>,
7499                   <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><em class="replaceable"><code>CT</code></em><span class="special">&gt;</span></code> turns the callable transform
7500                   into a primitive transform. This is useful for disambiguating callable
7501                   transforms from object transforms, and also for working around
7502                   compiler bugs with nested function types.
7503                 </p></dd>
7504 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/make.html" title="Struct template make">proto::make&lt;&gt;</a></code></span></dt>
7505 <dd><p>
7506                   For a given object transform <code class="computeroutput"><em class="replaceable"><code>OT</code></em></code>,
7507                   <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">make</span><span class="special">&lt;</span><em class="replaceable"><code>OT</code></em><span class="special">&gt;</span></code> turns the object transform
7508                   into a primitive transform. This is useful for disambiguating object
7509                   transforms from callable transforms, and also for working around
7510                   compiler bugs with nested function types.
7511                 </p></dd>
7512 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_default.html" title="Struct template _default">proto::_default&lt;&gt;</a></code></span></dt>
7513 <dd><p>
7514                   Given a grammar <em class="replaceable"><code>G</code></em>, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><em class="replaceable"><code>G</code></em><span class="special">&gt;</span></code> evaluates the current node
7515                   according to the standard C++ meaning of the operation the node
7516                   represents. For instance, if the current node is a binary plus
7517                   node, the two children will both be evaluated according to <code class="computeroutput"><em class="replaceable"><code>G</code></em></code>
7518                   and the results will be added and returned. The return type is
7519                   deduced with the help of the Boost.Typeof library.
7520                 </p></dd>
7521 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/fold.html" title="Struct template fold">proto::fold&lt;&gt;</a></code></span></dt>
7522 <dd><p>
7523                   Given three transforms <code class="computeroutput"><em class="replaceable"><code>ET</code></em></code>,
7524                   <code class="computeroutput"><em class="replaceable"><code>ST</code></em></code>, and <code class="computeroutput"><em class="replaceable"><code>FT</code></em></code>,
7525                   <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special">&lt;</span><em class="replaceable"><code>ET</code></em><span class="special">,</span> <em class="replaceable"><code>ST</code></em><span class="special">,</span> <em class="replaceable"><code>FT</code></em><span class="special">&gt;</span></code> first evaluates <code class="computeroutput"><em class="replaceable"><code>ET</code></em></code>
7526                   to obtain a Fusion sequence and <code class="computeroutput"><em class="replaceable"><code>ST</code></em></code>
7527                   to obtain an initial state for the fold, and then evaluates <code class="computeroutput"><em class="replaceable"><code>FT</code></em></code>
7528                   for each element in the sequence to generate the next state from
7529                   the previous.
7530                 </p></dd>
7531 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold&lt;&gt;</a></code></span></dt>
7532 <dd><p>
7533                   Like <code class="computeroutput"><a class="link" href="../boost/proto/fold.html" title="Struct template fold">proto::fold&lt;&gt;</a></code>, except the
7534                   elements in the Fusion sequence are iterated in reverse order.
7535                 </p></dd>
7536 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/fold_tree.html" title="Struct template fold_tree">proto::fold_tree&lt;&gt;</a></code></span></dt>
7537 <dd><p>
7538                   Like <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special">&lt;</span><em class="replaceable"><code>ET</code></em><span class="special">,</span> <em class="replaceable"><code>ST</code></em><span class="special">,</span> <em class="replaceable"><code>FT</code></em><span class="special">&gt;</span></code>, except that the result of
7539                   the <code class="computeroutput"><em class="replaceable"><code>ET</code></em></code> transform is treated
7540                   as an expression tree that is <span class="emphasis"><em>flattened</em></span> to
7541                   generate the sequence to be folded. Flattening an expression tree
7542                   causes child nodes with the same tag type as the parent to be put
7543                   into sequence. For instance, <code class="computeroutput"><span class="identifier">a</span>
7544                   <span class="special">&gt;&gt;</span> <span class="identifier">b</span>
7545                   <span class="special">&gt;&gt;</span> <span class="identifier">c</span></code>
7546                   would be flattened to the sequence [<code class="computeroutput"><span class="identifier">a</span></code>,
7547                   <code class="computeroutput"><span class="identifier">b</span></code>, <code class="computeroutput"><span class="identifier">c</span></code>], and this is the sequence
7548                   that would be folded.
7549                 </p></dd>
7550 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/reverse_fold_tree.html" title="Struct template reverse_fold_tree">proto::reverse_fold_tree&lt;&gt;</a></code></span></dt>
7551 <dd><p>
7552                   Like <code class="computeroutput"><a class="link" href="../boost/proto/fold_tree.html" title="Struct template fold_tree">proto::fold_tree&lt;&gt;</a></code>, except that
7553                   the flattened sequence is iterated in reverse order.
7554                 </p></dd>
7555 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/lazy.html" title="Struct template lazy">proto::lazy&lt;&gt;</a></code></span></dt>
7556 <dd><p>
7557                   A combination of <code class="computeroutput"><a class="link" href="../boost/proto/make.html" title="Struct template make">proto::make&lt;&gt;</a></code>
7558                   and <code class="computeroutput"><a class="link" href="../boost/proto/call.html" title="Struct template call">proto::call&lt;&gt;</a></code> that is useful
7559                   when the nature of the transform depends on the expression, state
7560                   and/or data parameters. <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">lazy</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">...</span><span class="identifier">An</span><span class="special">)&gt;</span></code> first evaluates <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">make</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">()&gt;</span></code>
7561                   to compute a callable type <code class="computeroutput"><span class="identifier">R2</span></code>.
7562                   Then, it evaluates <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">R2</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">...</span><span class="identifier">An</span><span class="special">)&gt;</span></code>.
7563                 </p></dd>
7564 </dl>
7565 </div>
7566 <h6>
7567 <a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.h0"></a>
7568             <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.all_grammars_are_primitive_transforms"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms.all_grammars_are_primitive_transforms">All
7569             Grammars Are Primitive Transforms</a>
7570           </h6>
7571 <p>
7572             In addition to the above primitive transforms, all of Proto's grammar
7573             elements are also primitive transforms. Their behaviors are described
7574             below.
7575           </p>
7576 <div class="variablelist">
7577 <p class="title"><b></b></p>
7578 <dl class="variablelist">
7579 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/_.html" title="Struct _">proto::_</a></code></span></dt>
7580 <dd><p>
7581                   Return the current expression unmodified.
7582                 </p></dd>
7583 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code></span></dt>
7584 <dd><p>
7585                   For the specified set of alternate sub-grammars, find the one that
7586                   matches the given expression and apply its associated transform.
7587                 </p></dd>
7588 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/and_.html" title="Struct template and_">proto::and_&lt;&gt;</a></code></span></dt>
7589 <dd><p>
7590                   For the given set of sub-grammars, apply all the associated transforms
7591                   and return the result of the last.
7592                 </p></dd>
7593 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/not_.html" title="Struct template not_">proto::not_&lt;&gt;</a></code></span></dt>
7594 <dd><p>
7595                   Return the current expression unmodified.
7596                 </p></dd>
7597 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/if_.html" title="Struct template if_">proto::if_&lt;&gt;</a></code></span></dt>
7598 <dd><p>
7599                   Given three transforms, evaluate the first and treat the result
7600                   as a compile-time Boolean value. If it is true, evaluate the second
7601                   transform. Otherwise, evaluate the third.
7602                 </p></dd>
7603 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/switch_.html" title="Struct template switch_">proto::switch_&lt;&gt;</a></code></span></dt>
7604 <dd><p>
7605                   As with <code class="computeroutput"><a class="link" href="../boost/proto/or_.html" title="Struct template or_">proto::or_&lt;&gt;</a></code>, find the sub-grammar
7606                   that matches the given expression and apply its associated transform.
7607                 </p></dd>
7608 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/terminal.html" title="Struct template terminal">proto::terminal&lt;&gt;</a></code></span></dt>
7609 <dd><p>
7610                   Return the current terminal expression unmodified.
7611                 </p></dd>
7612 <dt><span class="term"> <code class="computeroutput"><a class="link" href="../boost/proto/plus.html" title="Struct template plus">proto::plus&lt;&gt;</a></code>, <code class="computeroutput"><a class="link" href="../boost/proto/nary_expr.html" title="Struct template nary_expr">proto::nary_expr&lt;&gt;</a></code>,
7613               et. al.</span></dt>
7614 <dd><p>
7615                   A Proto grammar that matches a non-terminal such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><em class="replaceable"><code>G0</code></em><span class="special">,</span> <em class="replaceable"><code>G1</code></em><span class="special">&gt;</span></code>, when used as a primitive transform,
7616                   creates a new plus node where the left child is transformed according
7617                   to <code class="computeroutput"><em class="replaceable"><code>G0</code></em></code> and the right child
7618                   with <code class="computeroutput"><em class="replaceable"><code>G1</code></em></code>.
7619                 </p></dd>
7620 </dl>
7621 </div>
7622 <h6>
7623 <a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.h1"></a>
7624             <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_pass_through_transform"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_pass_through_transform">The
7625             Pass-Through Transform</a>
7626           </h6>
7627 <p>
7628             Note the primitive transform associated with grammar elements such as
7629             <code class="computeroutput"><a class="link" href="../boost/proto/plus.html" title="Struct template plus">proto::plus&lt;&gt;</a></code> described above.
7630             They possess a so-called <span class="emphasis"><em>pass-through</em></span> transform.
7631             The pass-through transform accepts an expression of a certain tag type
7632             (say, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code>) and creates a new expression
7633             of the same tag type, where each child expression is transformed according
7634             to the corresponding child grammar of the pass-through transform. So
7635             for instance this grammar ...
7636           </p>
7637 <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span> <span class="identifier">X</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">Y</span><span class="special">&gt;</span> <span class="special">&gt;</span>
7638 </pre>
7639 <p>
7640             ... matches function expressions where the first child matches the <code class="computeroutput"><span class="identifier">X</span></code> grammar and the rest match the <code class="computeroutput"><span class="identifier">Y</span></code> grammar. When used as a transform,
7641             the above grammar will create a new function expression where the first
7642             child is transformed according to <code class="computeroutput"><span class="identifier">X</span></code>
7643             and the rest are transformed according to <code class="computeroutput"><span class="identifier">Y</span></code>.
7644           </p>
7645 <p>
7646             The following class templates in Proto can be used as grammars with pass-through
7647             transforms:
7648           </p>
7649 <div class="table">
7650 <a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.t0"></a><p class="title"><b>Table&#160;33.11.&#160;Class Templates With Pass-Through Transforms</b></p>
7651 <div class="table-contents"><table class="table" summary="Class Templates With Pass-Through Transforms">
7652 <colgroup><col></colgroup>
7653 <thead><tr><th>
7654                     <p>
7655                       Templates with Pass-Through Transforms
7656                     </p>
7657                   </th></tr></thead>
7658 <tbody>
7659 <tr><td>
7660                     <p>
7661                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&lt;&gt;</span></code>
7662                     </p>
7663                   </td></tr>
7664 <tr><td>
7665                     <p>
7666                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">negate</span><span class="special">&lt;&gt;</span></code>
7667                     </p>
7668                   </td></tr>
7669 <tr><td>
7670                     <p>
7671                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">dereference</span><span class="special">&lt;&gt;</span></code>
7672                     </p>
7673                   </td></tr>
7674 <tr><td>
7675                     <p>
7676                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">complement</span><span class="special">&lt;&gt;</span></code>
7677                     </p>
7678                   </td></tr>
7679 <tr><td>
7680                     <p>
7681                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">address_of</span><span class="special">&lt;&gt;</span></code>
7682                     </p>
7683                   </td></tr>
7684 <tr><td>
7685                     <p>
7686                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_not</span><span class="special">&lt;&gt;</span></code>
7687                     </p>
7688                   </td></tr>
7689 <tr><td>
7690                     <p>
7691                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_inc</span><span class="special">&lt;&gt;</span></code>
7692                     </p>
7693                   </td></tr>
7694 <tr><td>
7695                     <p>
7696                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pre_dec</span><span class="special">&lt;&gt;</span></code>
7697                     </p>
7698                   </td></tr>
7699 <tr><td>
7700                     <p>
7701                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_inc</span><span class="special">&lt;&gt;</span></code>
7702                     </p>
7703                   </td></tr>
7704 <tr><td>
7705                     <p>
7706                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">post_dec</span><span class="special">&lt;&gt;</span></code>
7707                     </p>
7708                   </td></tr>
7709 <tr><td>
7710                     <p>
7711                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left</span><span class="special">&lt;&gt;</span></code>
7712                     </p>
7713                   </td></tr>
7714 <tr><td>
7715                     <p>
7716                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right</span><span class="special">&lt;&gt;</span></code>
7717                     </p>
7718                   </td></tr>
7719 <tr><td>
7720                     <p>
7721                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;&gt;</span></code>
7722                     </p>
7723                   </td></tr>
7724 <tr><td>
7725                     <p>
7726                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;&gt;</span></code>
7727                     </p>
7728                   </td></tr>
7729 <tr><td>
7730                     <p>
7731                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus</span><span class="special">&lt;&gt;</span></code>
7732                     </p>
7733                   </td></tr>
7734 <tr><td>
7735                     <p>
7736                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;&gt;</span></code>
7737                     </p>
7738                   </td></tr>
7739 <tr><td>
7740                     <p>
7741                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;&gt;</span></code>
7742                     </p>
7743                   </td></tr>
7744 <tr><td>
7745                     <p>
7746                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less</span><span class="special">&lt;&gt;</span></code>
7747                     </p>
7748                   </td></tr>
7749 <tr><td>
7750                     <p>
7751                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater</span><span class="special">&lt;&gt;</span></code>
7752                     </p>
7753                   </td></tr>
7754 <tr><td>
7755                     <p>
7756                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">less_equal</span><span class="special">&lt;&gt;</span></code>
7757                     </p>
7758                   </td></tr>
7759 <tr><td>
7760                     <p>
7761                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">greater_equal</span><span class="special">&lt;&gt;</span></code>
7762                     </p>
7763                   </td></tr>
7764 <tr><td>
7765                     <p>
7766                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special">&lt;&gt;</span></code>
7767                     </p>
7768                   </td></tr>
7769 <tr><td>
7770                     <p>
7771                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_equal_to</span><span class="special">&lt;&gt;</span></code>
7772                     </p>
7773                   </td></tr>
7774 <tr><td>
7775                     <p>
7776                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_or</span><span class="special">&lt;&gt;</span></code>
7777                     </p>
7778                   </td></tr>
7779 <tr><td>
7780                     <p>
7781                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_and</span><span class="special">&lt;&gt;</span></code>
7782                     </p>
7783                   </td></tr>
7784 <tr><td>
7785                     <p>
7786                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and</span><span class="special">&lt;&gt;</span></code>
7787                     </p>
7788                   </td></tr>
7789 <tr><td>
7790                     <p>
7791                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or</span><span class="special">&lt;&gt;</span></code>
7792                     </p>
7793                   </td></tr>
7794 <tr><td>
7795                     <p>
7796                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor</span><span class="special">&lt;&gt;</span></code>
7797                     </p>
7798                   </td></tr>
7799 <tr><td>
7800                     <p>
7801                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">comma</span><span class="special">&lt;&gt;</span></code>
7802                     </p>
7803                   </td></tr>
7804 <tr><td>
7805                     <p>
7806                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">mem_ptr</span><span class="special">&lt;&gt;</span></code>
7807                     </p>
7808                   </td></tr>
7809 <tr><td>
7810                     <p>
7811                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">assign</span><span class="special">&lt;&gt;</span></code>
7812                     </p>
7813                   </td></tr>
7814 <tr><td>
7815                     <p>
7816                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special">&lt;&gt;</span></code>
7817                     </p>
7818                   </td></tr>
7819 <tr><td>
7820                     <p>
7821                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special">&lt;&gt;</span></code>
7822                     </p>
7823                   </td></tr>
7824 <tr><td>
7825                     <p>
7826                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special">&lt;&gt;</span></code>
7827                     </p>
7828                   </td></tr>
7829 <tr><td>
7830                     <p>
7831                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special">&lt;&gt;</span></code>
7832                     </p>
7833                   </td></tr>
7834 <tr><td>
7835                     <p>
7836                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special">&lt;&gt;</span></code>
7837                     </p>
7838                   </td></tr>
7839 <tr><td>
7840                     <p>
7841                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special">&lt;&gt;</span></code>
7842                     </p>
7843                   </td></tr>
7844 <tr><td>
7845                     <p>
7846                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special">&lt;&gt;</span></code>
7847                     </p>
7848                   </td></tr>
7849 <tr><td>
7850                     <p>
7851                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special">&lt;&gt;</span></code>
7852                     </p>
7853                   </td></tr>
7854 <tr><td>
7855                     <p>
7856                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special">&lt;&gt;</span></code>
7857                     </p>
7858                   </td></tr>
7859 <tr><td>
7860                     <p>
7861                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special">&lt;&gt;</span></code>
7862                     </p>
7863                   </td></tr>
7864 <tr><td>
7865                     <p>
7866                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">subscript</span><span class="special">&lt;&gt;</span></code>
7867                     </p>
7868                   </td></tr>
7869 <tr><td>
7870                     <p>
7871                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">if_else_</span><span class="special">&lt;&gt;</span></code>
7872                     </p>
7873                   </td></tr>
7874 <tr><td>
7875                     <p>
7876                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;&gt;</span></code>
7877                     </p>
7878                   </td></tr>
7879 <tr><td>
7880                     <p>
7881                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_expr</span><span class="special">&lt;&gt;</span></code>
7882                     </p>
7883                   </td></tr>
7884 <tr><td>
7885                     <p>
7886                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">binary_expr</span><span class="special">&lt;&gt;</span></code>
7887                     </p>
7888                   </td></tr>
7889 <tr><td>
7890                     <p>
7891                       <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;&gt;</span></code>
7892                     </p>
7893                   </td></tr>
7894 </tbody>
7895 </table></div>
7896 </div>
7897 <br class="table-break"><h6>
7898 <a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.h2"></a>
7899             <span class="phrase"><a name="boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_many_roles_of_proto_operator_metafunctions"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.canned_transforms.the_many_roles_of_proto_operator_metafunctions">The
7900             Many Roles of Proto Operator Metafunctions</a>
7901           </h6>
7902 <p>
7903             We've seen templates such as <code class="computeroutput"><a class="link" href="../boost/proto/terminal.html" title="Struct template terminal">proto::terminal&lt;&gt;</a></code>,
7904             <code class="computeroutput"><a class="link" href="../boost/proto/plus.html" title="Struct template plus">proto::plus&lt;&gt;</a></code> and <code class="computeroutput"><a class="link" href="../boost/proto/nary_expr.html" title="Struct template nary_expr">proto::nary_expr&lt;&gt;</a></code>
7905             fill many roles. They are metafunction that generate expression types.
7906             They are grammars that match expression types. And they are primitive
7907             transforms. The following code samples show examples of each.
7908           </p>
7909 <p>
7910             <span class="bold"><strong>As Metafunctions ...</strong></span>
7911           </p>
7912 <pre class="programlisting"><span class="comment">// proto::terminal&lt;&gt; and proto::plus&lt;&gt; are metafunctions</span>
7913 <span class="comment">// that generate expression types:</span>
7914 <span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">int_</span><span class="special">;</span>
7915 <span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">int_</span><span class="special">,</span> <span class="identifier">int_</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">plus_</span><span class="special">;</span>
7916
7917 <span class="identifier">int_</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">42</span><span class="special">},</span> <span class="identifier">j</span> <span class="special">=</span> <span class="special">{</span><span class="number">24</span><span class="special">};</span>
7918 <span class="identifier">plus_</span> <span class="identifier">p</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">j</span><span class="special">};</span>
7919 </pre>
7920 <p>
7921             <span class="bold"><strong>As Grammars ...</strong></span>
7922           </p>
7923 <pre class="programlisting"><span class="comment">// proto::terminal&lt;&gt; and proto::plus&lt;&gt; are grammars that</span>
7924 <span class="comment">// match expression types</span>
7925 <span class="keyword">struct</span> <span class="identifier">Int</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">{};</span>
7926 <span class="keyword">struct</span> <span class="identifier">Plus</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Int</span><span class="special">,</span> <span class="identifier">Int</span><span class="special">&gt;</span> <span class="special">{};</span>
7927
7928 <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">int_</span><span class="special">,</span> <span class="identifier">Int</span> <span class="special">&gt;</span> <span class="special">));</span>
7929 <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span> <span class="identifier">plus_</span><span class="special">,</span> <span class="identifier">Plus</span> <span class="special">&gt;</span> <span class="special">));</span>
7930 </pre>
7931 <p>
7932             <span class="bold"><strong>As Primitive Transforms ...</strong></span>
7933           </p>
7934 <pre class="programlisting"><span class="comment">// A transform that removes all unary_plus nodes in an expression</span>
7935 <span class="keyword">struct</span> <span class="identifier">RemoveUnaryPlus</span>
7936   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
7937         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
7938             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">unary_plus</span><span class="special">&lt;</span><span class="identifier">RemoveUnaryPlus</span><span class="special">&gt;</span>
7939           <span class="special">,</span> <span class="identifier">RemoveUnaryPlus</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child</span><span class="special">)</span>
7940         <span class="special">&gt;</span>
7941         <span class="comment">// Use proto::terminal&lt;&gt; and proto::nary_expr&lt;&gt;</span>
7942         <span class="comment">// both as grammars and as primitive transforms.</span>
7943       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
7944       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">RemoveUnaryPlus</span><span class="special">&gt;</span> <span class="special">&gt;</span>
7945     <span class="special">&gt;</span>
7946 <span class="special">{};</span>
7947
7948 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
7949 <span class="special">{</span>
7950     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">literal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
7951
7952     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span>
7953         <span class="special">+</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+(</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+</span><span class="identifier">i</span><span class="special">)</span>
7954     <span class="special">);</span>
7955
7956     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span>
7957         <span class="identifier">RemoveUnaryPlus</span><span class="special">()(</span> <span class="special">+</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+(</span><span class="identifier">i</span> <span class="special">-</span> <span class="special">+</span><span class="identifier">i</span><span class="special">)</span> <span class="special">)</span>
7958     <span class="special">);</span>
7959 <span class="special">}</span>
7960 </pre>
7961 <p>
7962             The above code displays the following, which shows that unary plus nodes
7963             have been stripped from the expression:
7964           </p>
7965 <pre class="programlisting">minus(
7966     unary_plus(
7967         terminal(0)
7968     )
7969   , unary_plus(
7970         minus(
7971             terminal(0)
7972           , unary_plus(
7973                 terminal(0)
7974             )
7975         )
7976     )
7977 )
7978 minus(
7979     terminal(0)
7980   , minus(
7981         terminal(0)
7982       , terminal(0)
7983     )
7984 )
7985 </pre>
7986 </div>
7987 <div class="section">
7988 <div class="titlepage"><div><div><h5 class="title">
7989 <a name="boost_proto.users_guide.back_end.expression_transformation.primitives"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.primitives" title="Building Custom Primitive Transforms">Building
7990           Custom Primitive Transforms</a>
7991 </h5></div></div></div>
7992 <p>
7993             In previous sections, we've seen how to compose larger transforms out
7994             of smaller transforms using function types. The smaller transforms from
7995             which larger transforms are composed are <span class="emphasis"><em>primitive transforms</em></span>,
7996             and Proto provides a bunch of common ones such as <code class="computeroutput"><span class="identifier">_child0</span></code>
7997             and <code class="computeroutput"><span class="identifier">_value</span></code>. In this section
7998             we'll see how to author your own primitive transforms.
7999           </p>
8000 <div class="note"><table border="0" summary="Note">
8001 <tr>
8002 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
8003 <th align="left">Note</th>
8004 </tr>
8005 <tr><td align="left" valign="top"><p>
8006               There are a few reasons why you might want to write your own primitive
8007               transforms. For instance, your transform may be complicated, and composing
8008               it out of primitives becomes unwieldy. You might also need to work
8009               around compiler bugs on legacy compilers that make composing transforms
8010               using function types problematic. Finally, you might also decide to
8011               define your own primitive transforms to improve compile times. Since
8012               Proto can simply invoke a primitive transform directly without having
8013               to process arguments or differentiate callable transforms from object
8014               transforms, primitive transforms are more efficient.
8015             </p></td></tr>
8016 </table></div>
8017 <p>
8018             Primitive transforms inherit from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform</span><span class="special">&lt;&gt;</span></code> and have a nested <code class="computeroutput"><span class="identifier">impl</span><span class="special">&lt;&gt;</span></code>
8019             template that inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform_impl</span><span class="special">&lt;&gt;</span></code>. For example, this is how Proto
8020             defines the <code class="computeroutput"><span class="identifier">_child_c</span><span class="special">&lt;</span><em class="replaceable"><code>N</code></em><span class="special">&gt;</span></code>
8021             transform, which returns the <em class="replaceable"><code>N</code></em>-th child of
8022             the current expression:
8023           </p>
8024 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">proto</span>
8025 <span class="special">{</span>
8026     <span class="comment">// A primitive transform that returns N-th child</span>
8027     <span class="comment">// of the current expression.</span>
8028     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">&gt;</span>
8029     <span class="keyword">struct</span> <span class="identifier">_child_c</span> <span class="special">:</span> <span class="identifier">transform</span><span class="special">&lt;</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">&gt;</span> <span class="special">&gt;</span>
8030     <span class="special">{</span>
8031         <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">State</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Data</span><span class="special">&gt;</span>
8032         <span class="keyword">struct</span> <span class="identifier">impl</span> <span class="special">:</span> <span class="identifier">transform_impl</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">State</span><span class="special">,</span> <span class="identifier">Data</span><span class="special">&gt;</span>
8033         <span class="special">{</span>
8034             <span class="keyword">typedef</span>
8035                 <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span>
8036             <span class="identifier">result_type</span><span class="special">;</span>
8037
8038             <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span>
8039                 <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">expr_param</span> <span class="identifier">expr</span>
8040               <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">state_param</span> <span class="identifier">state</span>
8041               <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">data_param</span> <span class="identifier">data</span>
8042             <span class="special">)</span> <span class="keyword">const</span>
8043             <span class="special">{</span>
8044                 <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">child_c</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">);</span>
8045             <span class="special">}</span>
8046         <span class="special">};</span>
8047     <span class="special">};</span>
8048
8049     <span class="comment">// Note that _child_c&lt;N&gt; is callable, so that</span>
8050     <span class="comment">// it can be used in callable transforms, as:</span>
8051     <span class="comment">//   _child_c&lt;0&gt;(_child_c&lt;1&gt;)</span>
8052     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">&gt;</span>
8053     <span class="keyword">struct</span> <span class="identifier">is_callable</span><span class="special">&lt;</span><span class="identifier">_child_c</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">&gt;</span> <span class="special">&gt;</span>
8054       <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
8055     <span class="special">{};</span>
8056 <span class="special">}}</span>
8057 </pre>
8058 <p>
8059             The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform</span><span class="special">&lt;&gt;</span></code>
8060             base class provides the <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> overloads and the nested <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code>
8061             template that make your transform a valid function object. These are
8062             implemented in terms of the nested <code class="computeroutput"><span class="identifier">impl</span><span class="special">&lt;&gt;</span></code> template you define.
8063           </p>
8064 <p>
8065             The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform_impl</span><span class="special">&lt;&gt;</span></code>
8066             base class is a convenience. It provides some nested typedefs that are
8067             generally useful. They are specified in the table below:
8068           </p>
8069 <div class="table">
8070 <a name="boost_proto.users_guide.back_end.expression_transformation.primitives.t0"></a><p class="title"><b>Table&#160;33.12.&#160;proto::transform_impl&lt;Expr, State, Data&gt; typedefs</b></p>
8071 <div class="table-contents"><table class="table" summary="proto::transform_impl&lt;Expr, State, Data&gt; typedefs">
8072 <colgroup>
8073 <col>
8074 <col>
8075 </colgroup>
8076 <thead><tr>
8077 <th>
8078                     <p>
8079                       typedef
8080                     </p>
8081                   </th>
8082 <th>
8083                     <p>
8084                       Equivalent To
8085                     </p>
8086                   </th>
8087 </tr></thead>
8088 <tbody>
8089 <tr>
8090 <td>
8091                     <p>
8092                       <code class="computeroutput"><span class="identifier">expr</span></code>
8093                     </p>
8094                   </td>
8095 <td>
8096                     <p>
8097                       <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
8098                     </p>
8099                   </td>
8100 </tr>
8101 <tr>
8102 <td>
8103                     <p>
8104                       <code class="computeroutput"><span class="identifier">state</span></code>
8105                     </p>
8106                   </td>
8107 <td>
8108                     <p>
8109                       <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">State</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
8110                     </p>
8111                   </td>
8112 </tr>
8113 <tr>
8114 <td>
8115                     <p>
8116                       <code class="computeroutput"><span class="identifier">data</span></code>
8117                     </p>
8118                   </td>
8119 <td>
8120                     <p>
8121                       <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">Data</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
8122                     </p>
8123                   </td>
8124 </tr>
8125 <tr>
8126 <td>
8127                     <p>
8128                       <code class="computeroutput"><span class="identifier">expr_param</span></code>
8129                     </p>
8130                   </td>
8131 <td>
8132                     <p>
8133                       <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">add_reference</span><span class="special">&lt;</span><span class="keyword">typename</span>
8134                       <span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
8135                     </p>
8136                   </td>
8137 </tr>
8138 <tr>
8139 <td>
8140                     <p>
8141                       <code class="computeroutput"><span class="identifier">state_param</span></code>
8142                     </p>
8143                   </td>
8144 <td>
8145                     <p>
8146                       <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">add_reference</span><span class="special">&lt;</span><span class="keyword">typename</span>
8147                       <span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">State</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
8148                     </p>
8149                   </td>
8150 </tr>
8151 <tr>
8152 <td>
8153                     <p>
8154                       <code class="computeroutput"><span class="identifier">data_param</span></code>
8155                     </p>
8156                   </td>
8157 <td>
8158                     <p>
8159                       <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">add_reference</span><span class="special">&lt;</span><span class="keyword">typename</span>
8160                       <span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">Data</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
8161                     </p>
8162                   </td>
8163 </tr>
8164 </tbody>
8165 </table></div>
8166 </div>
8167 <br class="table-break"><p>
8168             You'll notice that <code class="computeroutput"><span class="identifier">_child_c</span><span class="special">::</span><span class="identifier">impl</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> takes arguments of types <code class="computeroutput"><span class="identifier">expr_param</span></code>, <code class="computeroutput"><span class="identifier">state_param</span></code>,
8169             and <code class="computeroutput"><span class="identifier">data_param</span></code>. The typedefs
8170             make it easy to accept arguments by reference or const reference accordingly.
8171           </p>
8172 <p>
8173             The only other interesting bit is the <code class="computeroutput"><span class="identifier">is_callable</span><span class="special">&lt;&gt;</span></code> specialization, which will be
8174             described in the <a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.is_callable" title="Making Your Transform Callable">next
8175             section</a>.
8176           </p>
8177 </div>
8178 <div class="section">
8179 <div class="titlepage"><div><div><h5 class="title">
8180 <a name="boost_proto.users_guide.back_end.expression_transformation.is_callable"></a><a class="link" href="users_guide.html#boost_proto.users_guide.back_end.expression_transformation.is_callable" title="Making Your Transform Callable">Making
8181           Your Transform Callable</a>
8182 </h5></div></div></div>
8183 <p>
8184             Transforms are typically of the form <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">Something</span><span class="special">,</span> <span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">,...)</span> <span class="special">&gt;</span></code>.
8185             The question is whether <code class="computeroutput"><span class="identifier">R</span></code>
8186             represents a function to call or an object to construct, and the answer
8187             determines how <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code> evaluates the transform.
8188             <code class="computeroutput"><a class="link" href="../boost/proto/when.html" title="Struct template when">proto::when&lt;&gt;</a></code> uses the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;&gt;</span></code>
8189             trait to disambiguate between the two. Proto does its best to guess whether
8190             a type is callable or not, but it doesn't always get it right. It's best
8191             to know the rules Proto uses, so that you know when you need to be more
8192             explicit.
8193           </p>
8194 <p>
8195             For most types <code class="computeroutput"><span class="identifier">R</span></code>, <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">&gt;</span></code>
8196             checks for inheritance from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>.
8197             However, if the type <code class="computeroutput"><span class="identifier">R</span></code>
8198             is a template specialization, Proto assumes that it is <span class="emphasis"><em>not</em></span>
8199             callable <span class="emphasis"><em>even if the template inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code></em></span>.
8200             We'll see why in a minute. Consider the following erroneous callable
8201             object:
8202           </p>
8203 <pre class="programlisting"><span class="comment">// Proto can't tell this defines something callable!</span>
8204 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
8205 <span class="keyword">struct</span> <span class="identifier">times2</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
8206 <span class="special">{</span>
8207     <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">result_type</span><span class="special">;</span>
8208
8209     <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
8210     <span class="special">{</span>
8211         <span class="keyword">return</span> <span class="identifier">i</span> <span class="special">*</span> <span class="number">2</span><span class="special">;</span>
8212     <span class="special">}</span>
8213 <span class="special">};</span>
8214
8215 <span class="comment">// ERROR! This is not going to multiply the int by 2:</span>
8216 <span class="keyword">struct</span> <span class="identifier">IntTimes2</span>
8217   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
8218         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
8219       <span class="special">,</span> <span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
8220     <span class="special">&gt;</span>
8221 <span class="special">{};</span>
8222 </pre>
8223 <p>
8224             The problem is that Proto doesn't know that <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code> is callable, so rather that invoking
8225             the <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
8226             function object, Proto will try to construct a <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code> object and initialize it will an
8227             <code class="computeroutput"><span class="keyword">int</span></code>. That will not compile.
8228           </p>
8229 <div class="note"><table border="0" summary="Note">
8230 <tr>
8231 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
8232 <th align="left">Note</th>
8233 </tr>
8234 <tr><td align="left" valign="top"><p>
8235               Why can't Proto tell that <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code> is callable? After all, it inherits
8236               from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>, and that is detectable,
8237               right? The problem is that merely asking whether some type <code class="computeroutput"><span class="identifier">X</span><span class="special">&lt;</span><span class="identifier">Y</span><span class="special">&gt;</span></code>
8238               inherits from <code class="computeroutput"><span class="identifier">callable</span></code>
8239               will cause the template <code class="computeroutput"><span class="identifier">X</span><span class="special">&lt;</span><span class="identifier">Y</span><span class="special">&gt;</span></code> to be instantiated. That's a problem
8240               for a type like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">_child1</span><span class="special">)&gt;</span></code>. <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;&gt;</span></code> will not suffer to be instantiated
8241               with <code class="computeroutput"><span class="identifier">_value</span><span class="special">(</span><span class="identifier">_child1</span><span class="special">)</span></code>
8242               as a template parameter. Since merely asking the question will sometimes
8243               result in a hard error, Proto can't ask; it has to assume that <code class="computeroutput"><span class="identifier">X</span><span class="special">&lt;</span><span class="identifier">Y</span><span class="special">&gt;</span></code>
8244               represents an object to construct and not a function to call.
8245             </p></td></tr>
8246 </table></div>
8247 <p>
8248             There are a couple of solutions to the <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code> problem. One solution is to wrap
8249             the transform in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;&gt;</span></code>. This forces Proto to treat
8250             <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
8251             as callable:
8252           </p>
8253 <pre class="programlisting"><span class="comment">// OK, calls times2&lt;int&gt;</span>
8254 <span class="keyword">struct</span> <span class="identifier">IntTimes2</span>
8255   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
8256         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
8257       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)&gt;</span>
8258     <span class="special">&gt;</span>
8259 <span class="special">{};</span>
8260 </pre>
8261 <p>
8262             This can be a bit of a pain, because we need to wrap every use of <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>,
8263             which can be tedious and error prone, and makes our grammar cluttered
8264             and harder to read.
8265           </p>
8266 <p>
8267             Another solution is to specialize <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;&gt;</span></code> on our <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;&gt;</span></code> template:
8268           </p>
8269 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">proto</span>
8270 <span class="special">{</span>
8271     <span class="comment">// Tell Proto that times2&lt;&gt; is callable</span>
8272     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
8273     <span class="keyword">struct</span> <span class="identifier">is_callable</span><span class="special">&lt;</span><span class="identifier">times2</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;</span>
8274       <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
8275     <span class="special">{};</span>
8276 <span class="special">}}</span>
8277
8278 <span class="comment">// OK, times2&lt;&gt; is callable</span>
8279 <span class="keyword">struct</span> <span class="identifier">IntTimes2</span>
8280   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
8281         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
8282       <span class="special">,</span> <span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
8283     <span class="special">&gt;</span>
8284 <span class="special">{};</span>
8285 </pre>
8286 <p>
8287             This is better, but still a pain because of the need to open Proto's
8288             namespace.
8289           </p>
8290 <p>
8291             You could simply make sure that the callable type is not a template specialization.
8292             Consider the following:
8293           </p>
8294 <pre class="programlisting"><span class="comment">// No longer a template specialization!</span>
8295 <span class="keyword">struct</span> <span class="identifier">times2int</span> <span class="special">:</span> <span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">{};</span>
8296
8297 <span class="comment">// OK, times2int is callable</span>
8298 <span class="keyword">struct</span> <span class="identifier">IntTimes2</span>
8299   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
8300         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
8301       <span class="special">,</span> <span class="identifier">times2int</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
8302     <span class="special">&gt;</span>
8303 <span class="special">{};</span>
8304 </pre>
8305 <p>
8306             This works because now Proto can tell that <code class="computeroutput"><span class="identifier">times2int</span></code>
8307             inherits (indirectly) from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>.
8308             Any non-template types can be safely checked for inheritance because,
8309             as they are not templates, there is no worry about instantiation errors.
8310           </p>
8311 <p>
8312             There is one last way to tell Proto that <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;&gt;</span></code> is callable. You could add an
8313             extra dummy template parameter that defaults to <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>:
8314           </p>
8315 <pre class="programlisting"><span class="comment">// Proto will recognize this as callable</span>
8316 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Callable</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span><span class="special">&gt;</span>
8317 <span class="keyword">struct</span> <span class="identifier">times2</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
8318 <span class="special">{</span>
8319     <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">result_type</span><span class="special">;</span>
8320
8321     <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
8322     <span class="special">{</span>
8323         <span class="keyword">return</span> <span class="identifier">i</span> <span class="special">*</span> <span class="number">2</span><span class="special">;</span>
8324     <span class="special">}</span>
8325 <span class="special">};</span>
8326
8327 <span class="comment">// OK, this works!</span>
8328 <span class="keyword">struct</span> <span class="identifier">IntTimes2</span>
8329   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
8330         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
8331       <span class="special">,</span> <span class="identifier">times2</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
8332     <span class="special">&gt;</span>
8333 <span class="special">{};</span>
8334 </pre>
8335 <p>
8336             Note that in addition to the extra template parameter, <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;&gt;</span></code>
8337             still inherits from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span></code>.
8338             That's not necessary in this example but it is good style because any
8339             types derived from <code class="computeroutput"><span class="identifier">times2</span><span class="special">&lt;&gt;</span></code> (as <code class="computeroutput"><span class="identifier">times2int</span></code>
8340             defined above) will still be considered callable.
8341           </p>
8342 </div>
8343 </div>
8344 </div>
8345 <div class="section">
8346 <div class="titlepage"><div><div><h3 class="title">
8347 <a name="boost_proto.users_guide.examples"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples" title="Examples">Examples</a>
8348 </h3></div></div></div>
8349 <div class="toc"><dl class="toc">
8350 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.hello_world">Hello
8351         World: Building an Expression Template and Evaluating It</a></span></dt>
8352 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.calc1">Calc1: Defining
8353         an Evaluation Context</a></span></dt>
8354 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.calc2">Calc2: Adding
8355         Members Using <code class="literal">proto::extends&lt;&gt;</code></a></span></dt>
8356 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.calc3">Calc3: Defining
8357         a Simple Transform</a></span></dt>
8358 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.lazy_vector">Lazy
8359         Vector: Controlling Operator Overloads</a></span></dt>
8360 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.rgb">RGB: Type Manipulations
8361         with Proto Transforms</a></span></dt>
8362 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.tarray">TArray: A
8363         Simple Linear Algebra Library</a></span></dt>
8364 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.vec3">Vec3: Computing
8365         With Transforms and Contexts</a></span></dt>
8366 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.vector">Vector: Adapting
8367         a Non-Proto Terminal Type</a></span></dt>
8368 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.mixed">Mixed: Adapting
8369         Several Non-Proto Terminal Types</a></span></dt>
8370 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.map_assign">Map Assign:
8371         An Intermediate Transform</a></span></dt>
8372 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.future_group">Future
8373         Group: A More Advanced Transform</a></span></dt>
8374 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.lambda">Lambda: A
8375         Simple Lambda Library with Proto</a></span></dt>
8376 <dt><span class="section"><a href="users_guide.html#boost_proto.users_guide.examples.checked_calc">Checked
8377         Calculator: A Simple Example of External Transforms</a></span></dt>
8378 </dl></div>
8379 <p>
8380         A code example is worth a thousand words ...
8381       </p>
8382 <div class="section">
8383 <div class="titlepage"><div><div><h4 class="title">
8384 <a name="boost_proto.users_guide.examples.hello_world"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.hello_world" title="Hello World: Building an Expression Template and Evaluating It">Hello
8385         World: Building an Expression Template and Evaluating It</a>
8386 </h4></div></div></div>
8387 <p>
8388           A trivial example which builds and expression template and evaluates it.
8389         </p>
8390 <p>
8391 </p>
8392 <pre class="programlisting"><span class="comment">////////////////////////////////////////////////////////////////////</span>
8393 <span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
8394 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
8395 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
8396
8397 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
8398 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8399 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8400 <span class="comment">// This #include is only needed for compilers that use typeof emulation:</span>
8401 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8402 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
8403
8404 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">cout_</span> <span class="special">=</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">};</span>
8405
8406 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
8407 <span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">)</span>
8408 <span class="special">{</span>
8409     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_context</span> <span class="identifier">ctx</span><span class="special">;</span>
8410     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
8411 <span class="special">}</span>
8412
8413 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
8414 <span class="special">{</span>
8415     <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">cout_</span> <span class="special">&lt;&lt;</span> <span class="string">"hello"</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="string">" world"</span> <span class="special">);</span>
8416     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
8417 <span class="special">}</span>
8418 </pre>
8419 <p>
8420         </p>
8421 </div>
8422 <div class="section">
8423 <div class="titlepage"><div><div><h4 class="title">
8424 <a name="boost_proto.users_guide.examples.calc1"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.calc1" title="Calc1: Defining an Evaluation Context">Calc1: Defining
8425         an Evaluation Context</a>
8426 </h4></div></div></div>
8427 <p>
8428           A simple example that builds a miniature embedded domain-specific language
8429           for lazy arithmetic expressions, with TR1 bind-style argument placeholders.
8430         </p>
8431 <p>
8432 </p>
8433 <pre class="programlisting"><span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
8434 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
8435 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
8436 <span class="comment">//</span>
8437 <span class="comment">// This is a simple example of how to build an arithmetic expression</span>
8438 <span class="comment">// evaluator with placeholders.</span>
8439
8440 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
8441 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8442 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8443 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
8444 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>
8445
8446 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">{};</span>
8447
8448 <span class="comment">// Define some placeholders</span>
8449 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="number">1</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
8450 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="number">2</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span>
8451
8452 <span class="comment">// Define a calculator context, for evaluating arithmetic expressions</span>
8453 <span class="keyword">struct</span> <span class="identifier">calculator_context</span>
8454   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">&gt;</span>
8455 <span class="special">{</span>
8456     <span class="comment">// The values bound to the placeholders</span>
8457     <span class="keyword">double</span> <span class="identifier">d</span><span class="special">[</span><span class="number">2</span><span class="special">];</span>
8458
8459     <span class="comment">// The result of evaluating arithmetic expressions</span>
8460     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
8461
8462     <span class="keyword">explicit</span> <span class="identifier">calculator_context</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span><span class="special">)</span>
8463     <span class="special">{</span>
8464         <span class="identifier">d</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d1</span><span class="special">;</span>
8465         <span class="identifier">d</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d2</span><span class="special">;</span>
8466     <span class="special">}</span>
8467
8468     <span class="comment">// Handle the evaluation of the placeholder terminals</span>
8469     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
8470     <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;)</span> <span class="keyword">const</span>
8471     <span class="special">{</span>
8472         <span class="keyword">return</span> <span class="identifier">d</span><span class="special">[</span> <span class="identifier">I</span> <span class="special">-</span> <span class="number">1</span> <span class="special">];</span>
8473     <span class="special">}</span>
8474 <span class="special">};</span>
8475
8476 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
8477 <span class="keyword">double</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span> <span class="special">)</span>
8478 <span class="special">{</span>
8479     <span class="comment">// Create a calculator context with d1 and d2 substituted for _1 and _2</span>
8480     <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">,</span> <span class="identifier">d2</span><span class="special">);</span>
8481
8482     <span class="comment">// Evaluate the calculator expression with the calculator_context</span>
8483     <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
8484 <span class="special">}</span>
8485
8486 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
8487 <span class="special">{</span>
8488     <span class="comment">// Displays "5"</span>
8489     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">_1</span> <span class="special">+</span> <span class="number">2.0</span><span class="special">,</span> <span class="number">3.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
8490
8491     <span class="comment">// Displays "6"</span>
8492     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span><span class="special">,</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
8493
8494     <span class="comment">// Displays "0.5"</span>
8495     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">evaluate</span><span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
8496
8497     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
8498 <span class="special">}</span>
8499 </pre>
8500 <p>
8501         </p>
8502 </div>
8503 <div class="section">
8504 <div class="titlepage"><div><div><h4 class="title">
8505 <a name="boost_proto.users_guide.examples.calc2"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.calc2" title="Calc2: Adding Members Using proto::extends&lt;&gt;">Calc2: Adding
8506         Members Using <code class="literal">proto::extends&lt;&gt;</code></a>
8507 </h4></div></div></div>
8508 <p>
8509           An extension of the Calc1 example that uses <code class="computeroutput"><a class="link" href="../boost/proto/extends.html" title="Struct template extends">proto::extends&lt;&gt;</a></code>
8510           to make calculator expressions valid function objects that can be used
8511           with STL algorithms.
8512         </p>
8513 <p>
8514 </p>
8515 <pre class="programlisting"><span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
8516 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
8517 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
8518 <span class="comment">//</span>
8519 <span class="comment">// This example enhances the simple arithmetic expression evaluator</span>
8520 <span class="comment">// in calc1.cpp by using proto::extends to make arithmetic</span>
8521 <span class="comment">// expressions immediately evaluable with operator (), a-la a</span>
8522 <span class="comment">// function object</span>
8523
8524 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
8525 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8526 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8527 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
8528 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>
8529
8530 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
8531 <span class="keyword">struct</span> <span class="identifier">calculator_expression</span><span class="special">;</span>
8532
8533 <span class="comment">// Tell proto how to generate expressions in the calculator_domain</span>
8534 <span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
8535   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calculator_expression</span><span class="special">&gt;</span> <span class="special">&gt;</span>
8536 <span class="special">{};</span>
8537
8538 <span class="comment">// Will be used to define the placeholders _1 and _2</span>
8539 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">{};</span>
8540
8541 <span class="comment">// Define a calculator context, for evaluating arithmetic expressions</span>
8542 <span class="comment">// (This is as before, in calc1.cpp)</span>
8543 <span class="keyword">struct</span> <span class="identifier">calculator_context</span>
8544   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">&gt;</span>
8545 <span class="special">{</span>
8546     <span class="comment">// The values bound to the placeholders</span>
8547     <span class="keyword">double</span> <span class="identifier">d</span><span class="special">[</span><span class="number">2</span><span class="special">];</span>
8548
8549     <span class="comment">// The result of evaluating arithmetic expressions</span>
8550     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
8551
8552     <span class="keyword">explicit</span> <span class="identifier">calculator_context</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span><span class="special">)</span>
8553     <span class="special">{</span>
8554         <span class="identifier">d</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d1</span><span class="special">;</span>
8555         <span class="identifier">d</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d2</span><span class="special">;</span>
8556     <span class="special">}</span>
8557
8558     <span class="comment">// Handle the evaluation of the placeholder terminals</span>
8559     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">&gt;</span>
8560     <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;)</span> <span class="keyword">const</span>
8561     <span class="special">{</span>
8562         <span class="keyword">return</span> <span class="identifier">d</span><span class="special">[</span> <span class="identifier">I</span> <span class="special">-</span> <span class="number">1</span> <span class="special">];</span>
8563     <span class="special">}</span>
8564 <span class="special">};</span>
8565
8566 <span class="comment">// Wrap all calculator expressions in this type, which defines</span>
8567 <span class="comment">// operator () to evaluate the expression.</span>
8568 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
8569 <span class="keyword">struct</span> <span class="identifier">calculator_expression</span>
8570   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
8571 <span class="special">{</span>
8572     <span class="keyword">explicit</span> <span class="identifier">calculator_expression</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span>
8573       <span class="special">:</span> <span class="identifier">calculator_expression</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span>
8574     <span class="special">{}</span>
8575
8576     <span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">(</span><span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;)</span>
8577
8578     <span class="comment">// Override operator () to evaluate the expression</span>
8579     <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()()</span> <span class="keyword">const</span>
8580     <span class="special">{</span>
8581         <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">;</span>
8582         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
8583     <span class="special">}</span>
8584
8585     <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">)</span> <span class="keyword">const</span>
8586     <span class="special">{</span>
8587         <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">);</span>
8588         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
8589     <span class="special">}</span>
8590
8591     <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span><span class="special">)</span> <span class="keyword">const</span>
8592     <span class="special">{</span>
8593         <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">,</span> <span class="identifier">d2</span><span class="special">);</span>
8594         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
8595     <span class="special">}</span>
8596 <span class="special">};</span>
8597
8598 <span class="comment">// Define some placeholders (notice they're wrapped in calculator_expression&lt;&gt;)</span>
8599 <span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="number">1</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span>
8600 <span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="number">2</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span>
8601
8602 <span class="comment">// Now, our arithmetic expressions are immediately executable function objects:</span>
8603 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
8604 <span class="special">{</span>
8605     <span class="comment">// Displays "5"</span>
8606     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2.0</span><span class="special">)(</span> <span class="number">3.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
8607
8608     <span class="comment">// Displays "6"</span>
8609     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
8610
8611     <span class="comment">// Displays "0.5"</span>
8612     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
8613
8614     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
8615 <span class="special">}</span>
8616 </pre>
8617 <p>
8618         </p>
8619 </div>
8620 <div class="section">
8621 <div class="titlepage"><div><div><h4 class="title">
8622 <a name="boost_proto.users_guide.examples.calc3"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.calc3" title="Calc3: Defining a Simple Transform">Calc3: Defining
8623         a Simple Transform</a>
8624 </h4></div></div></div>
8625 <p>
8626           An extension of the Calc2 example that uses a Proto transform to calculate
8627           the arity of a calculator expression and statically assert that the correct
8628           number of arguments are passed.
8629         </p>
8630 <p>
8631 </p>
8632 <pre class="programlisting"><span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
8633 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
8634 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
8635 <span class="comment">//</span>
8636 <span class="comment">// This example enhances the arithmetic expression evaluator</span>
8637 <span class="comment">// in calc2.cpp by using a proto transform to calculate the</span>
8638 <span class="comment">// number of arguments an expression requires and using a</span>
8639 <span class="comment">// compile-time assert to guarantee that the right number of</span>
8640 <span class="comment">// arguments are actually specified.</span>
8641
8642 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
8643 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8644 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8645 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">min_max</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8646 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8647 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8648 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8649 <span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
8650 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
8651 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>
8652
8653 <span class="comment">// Will be used to define the placeholders _1 and _2</span>
8654 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">:</span> <span class="identifier">I</span> <span class="special">{};</span>
8655
8656 <span class="comment">// This grammar basically says that a calculator expression is one of:</span>
8657 <span class="comment">//   - A placeholder terminal</span>
8658 <span class="comment">//   - Some other terminal</span>
8659 <span class="comment">//   - Some non-terminal whose children are calculator expressions</span>
8660 <span class="comment">// In addition, it has transforms that say how to calculate the</span>
8661 <span class="comment">// expression arity for each of the three cases.</span>
8662 <span class="keyword">struct</span> <span class="identifier">CalculatorGrammar</span>
8663   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
8664
8665         <span class="comment">// placeholders have a non-zero arity ...</span>
8666         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span> <span class="special">&gt;</span>
8667
8668         <span class="comment">// Any other terminals have arity 0 ...</span>
8669       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;()</span> <span class="special">&gt;</span>
8670
8671         <span class="comment">// For any non-terminals, find the arity of the children and</span>
8672         <span class="comment">// take the maximum. This is recursive.</span>
8673       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
8674              <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(),</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;</span><span class="identifier">CalculatorGrammar</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">&gt;()</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
8675
8676     <span class="special">&gt;</span>
8677 <span class="special">{};</span>
8678
8679 <span class="comment">// Simple wrapper for calculating a calculator expression's arity.</span>
8680 <span class="comment">// It specifies mpl::int_&lt;0&gt; as the initial state. The data, which</span>
8681 <span class="comment">// is not used, is mpl::void_.</span>
8682 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
8683 <span class="keyword">struct</span> <span class="identifier">calculator_arity</span>
8684   <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">CalculatorGrammar</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">)&gt;</span>
8685 <span class="special">{};</span>
8686
8687 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
8688 <span class="keyword">struct</span> <span class="identifier">calculator_expression</span><span class="special">;</span>
8689
8690 <span class="comment">// Tell proto how to generate expressions in the calculator_domain</span>
8691 <span class="keyword">struct</span> <span class="identifier">calculator_domain</span>
8692   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calculator_expression</span><span class="special">&gt;</span> <span class="special">&gt;</span>
8693 <span class="special">{};</span>
8694
8695 <span class="comment">// Define a calculator context, for evaluating arithmetic expressions</span>
8696 <span class="comment">// (This is as before, in calc1.cpp and calc2.cpp)</span>
8697 <span class="keyword">struct</span> <span class="identifier">calculator_context</span>
8698   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="special">&gt;</span>
8699 <span class="special">{</span>
8700     <span class="comment">// The values bound to the placeholders</span>
8701     <span class="keyword">double</span> <span class="identifier">d</span><span class="special">[</span><span class="number">2</span><span class="special">];</span>
8702
8703     <span class="comment">// The result of evaluating arithmetic expressions</span>
8704     <span class="keyword">typedef</span> <span class="keyword">double</span> <span class="identifier">result_type</span><span class="special">;</span>
8705
8706     <span class="keyword">explicit</span> <span class="identifier">calculator_context</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.</span><span class="special">)</span>
8707     <span class="special">{</span>
8708         <span class="identifier">d</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d1</span><span class="special">;</span>
8709         <span class="identifier">d</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">d2</span><span class="special">;</span>
8710     <span class="special">}</span>
8711
8712     <span class="comment">// Handle the evaluation of the placeholder terminals</span>
8713     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span>
8714     <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;)</span> <span class="keyword">const</span>
8715     <span class="special">{</span>
8716         <span class="keyword">return</span> <span class="identifier">d</span><span class="special">[</span> <span class="identifier">I</span><span class="special">()</span> <span class="special">-</span> <span class="number">1</span> <span class="special">];</span>
8717     <span class="special">}</span>
8718 <span class="special">};</span>
8719
8720 <span class="comment">// Wrap all calculator expressions in this type, which defines</span>
8721 <span class="comment">// operator () to evaluate the expression.</span>
8722 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
8723 <span class="keyword">struct</span> <span class="identifier">calculator_expression</span>
8724   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
8725 <span class="special">{</span>
8726     <span class="keyword">typedef</span>
8727         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">calculator_domain</span><span class="special">&gt;</span>
8728     <span class="identifier">base_type</span><span class="special">;</span>
8729
8730     <span class="keyword">explicit</span> <span class="identifier">calculator_expression</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">())</span>
8731       <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span>
8732     <span class="special">{}</span>
8733
8734     <span class="identifier">BOOST_PROTO_EXTENDS_USING_ASSIGN</span><span class="special">(</span><span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;)</span>
8735
8736     <span class="comment">// Override operator () to evaluate the expression</span>
8737     <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()()</span> <span class="keyword">const</span>
8738     <span class="special">{</span>
8739         <span class="comment">// Assert that the expression has arity 0</span>
8740         <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">calculator_arity</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">);</span>
8741         <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">;</span>
8742         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
8743     <span class="special">}</span>
8744
8745     <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">)</span> <span class="keyword">const</span>
8746     <span class="special">{</span>
8747         <span class="comment">// Assert that the expression has arity 1</span>
8748         <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">calculator_arity</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">);</span>
8749         <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">);</span>
8750         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
8751     <span class="special">}</span>
8752
8753     <span class="keyword">double</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="keyword">double</span> <span class="identifier">d1</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d2</span><span class="special">)</span> <span class="keyword">const</span>
8754     <span class="special">{</span>
8755         <span class="comment">// Assert that the expression has arity 2</span>
8756         <span class="identifier">BOOST_MPL_ASSERT_RELATION</span><span class="special">(</span><span class="number">2</span><span class="special">,</span> <span class="special">==,</span> <span class="identifier">calculator_arity</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">);</span>
8757         <span class="identifier">calculator_context</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">d1</span><span class="special">,</span> <span class="identifier">d2</span><span class="special">);</span>
8758         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
8759     <span class="special">}</span>
8760 <span class="special">};</span>
8761
8762 <span class="comment">// Define some placeholders (notice they're wrapped in calculator_expression&lt;&gt;)</span>
8763 <span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span><span class="special">;</span>
8764 <span class="identifier">calculator_expression</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">2</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span><span class="special">;</span>
8765
8766 <span class="comment">// Now, our arithmetic expressions are immediately executable function objects:</span>
8767 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
8768 <span class="special">{</span>
8769     <span class="comment">// Displays "5"</span>
8770     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2.0</span><span class="special">)(</span> <span class="number">3.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
8771
8772     <span class="comment">// Displays "6"</span>
8773     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span> <span class="identifier">_1</span> <span class="special">*</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
8774
8775     <span class="comment">// Displays "0.5"</span>
8776     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">/</span> <span class="identifier">_2</span> <span class="special">)(</span> <span class="number">3.0</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
8777
8778     <span class="comment">// This won't compile because the arity of the</span>
8779     <span class="comment">// expression doesn't match the number of arguments</span>
8780     <span class="comment">// ( (_1 - _2) / _2 )( 3.0 );</span>
8781
8782     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
8783 <span class="special">}</span>
8784 </pre>
8785 <p>
8786         </p>
8787 </div>
8788 <div class="section">
8789 <div class="titlepage"><div><div><h4 class="title">
8790 <a name="boost_proto.users_guide.examples.lazy_vector"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.lazy_vector" title="Lazy Vector: Controlling Operator Overloads">Lazy
8791         Vector: Controlling Operator Overloads</a>
8792 </h4></div></div></div>
8793 <p>
8794           This example constructs a mini-library for linear algebra, using expression
8795           templates to eliminate the need for temporaries when adding vectors of
8796           numbers.
8797         </p>
8798 <p>
8799           This example uses a domain with a grammar to prune the set of overloaded
8800           operators. Only those operators that produce valid lazy vector expressions
8801           are allowed.
8802         </p>
8803 <p>
8804 </p>
8805 <pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
8806 <span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
8807 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
8808 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
8809 <span class="comment">//</span>
8810 <span class="comment">// This example constructs a mini-library for linear algebra, using</span>
8811 <span class="comment">// expression templates to eliminate the need for temporaries when</span>
8812 <span class="comment">// adding vectors of numbers.</span>
8813 <span class="comment">//</span>
8814 <span class="comment">// This example uses a domain with a grammar to prune the set</span>
8815 <span class="comment">// of overloaded operators. Only those operators that produce</span>
8816 <span class="comment">// valid lazy vector expressions are allowed.</span>
8817
8818 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
8819 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
8820 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8821 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8822 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8823 <span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
8824 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
8825 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>
8826
8827 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
8828 <span class="keyword">struct</span> <span class="identifier">lazy_vector_expr</span><span class="special">;</span>
8829
8830 <span class="comment">// This grammar describes which lazy vector expressions</span>
8831 <span class="comment">// are allowed; namely, vector terminals and addition</span>
8832 <span class="comment">// and subtraction of lazy vector expressions.</span>
8833 <span class="keyword">struct</span> <span class="identifier">LazyVectorGrammar</span>
8834   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
8835         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
8836       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span> <span class="identifier">LazyVectorGrammar</span><span class="special">,</span> <span class="identifier">LazyVectorGrammar</span> <span class="special">&gt;</span>
8837       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span> <span class="identifier">LazyVectorGrammar</span><span class="special">,</span> <span class="identifier">LazyVectorGrammar</span> <span class="special">&gt;</span>
8838     <span class="special">&gt;</span>
8839 <span class="special">{};</span>
8840
8841 <span class="comment">// Tell proto that in the lazy_vector_domain, all</span>
8842 <span class="comment">// expressions should be wrapped in laxy_vector_expr&lt;&gt;</span>
8843 <span class="comment">// and must conform to the lazy vector grammar.</span>
8844 <span class="keyword">struct</span> <span class="identifier">lazy_vector_domain</span>
8845   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">lazy_vector_expr</span><span class="special">&gt;,</span> <span class="identifier">LazyVectorGrammar</span><span class="special">&gt;</span>
8846 <span class="special">{};</span>
8847
8848 <span class="comment">// Here is an evaluation context that indexes into a lazy vector</span>
8849 <span class="comment">// expression, and combines the result.</span>
8850 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Size</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">&gt;</span>
8851 <span class="keyword">struct</span> <span class="identifier">lazy_subscript_context</span>
8852 <span class="special">{</span>
8853     <span class="identifier">lazy_subscript_context</span><span class="special">(</span><span class="identifier">Size</span> <span class="identifier">subscript</span><span class="special">)</span>
8854       <span class="special">:</span> <span class="identifier">subscript_</span><span class="special">(</span><span class="identifier">subscript</span><span class="special">)</span>
8855     <span class="special">{}</span>
8856
8857     <span class="comment">// Use default_eval for all the operations ...</span>
8858     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Tag</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">&gt;</span>
8859     <span class="keyword">struct</span> <span class="identifier">eval</span>
8860       <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">lazy_subscript_context</span><span class="special">&gt;</span>
8861     <span class="special">{};</span>
8862
8863     <span class="comment">// ... except for terminals, which we index with our subscript</span>
8864     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
8865     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&gt;</span>
8866     <span class="special">{</span>
8867         <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">result_type</span><span class="special">;</span>
8868
8869         <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">,</span> <span class="identifier">lazy_subscript_context</span> <span class="special">&amp;</span> <span class="identifier">ctx</span> <span class="special">)</span> <span class="keyword">const</span>
8870         <span class="special">{</span>
8871             <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)[</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">subscript_</span> <span class="special">];</span>
8872         <span class="special">}</span>
8873     <span class="special">};</span>
8874
8875     <span class="identifier">Size</span> <span class="identifier">subscript_</span><span class="special">;</span>
8876 <span class="special">};</span>
8877
8878 <span class="comment">// Here is the domain-specific expression wrapper, which overrides</span>
8879 <span class="comment">// operator [] to evaluate the expression using the lazy_subscript_context.</span>
8880 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
8881 <span class="keyword">struct</span> <span class="identifier">lazy_vector_expr</span>
8882   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">lazy_vector_expr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">lazy_vector_domain</span><span class="special">&gt;</span>
8883 <span class="special">{</span>
8884     <span class="identifier">lazy_vector_expr</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">()</span> <span class="special">)</span>
8885       <span class="special">:</span> <span class="identifier">lazy_vector_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)</span>
8886     <span class="special">{}</span>
8887
8888     <span class="comment">// Use the lazy_subscript_context&lt;&gt; to implement subscripting</span>
8889     <span class="comment">// of a lazy vector expression tree.</span>
8890     <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Size</span> <span class="special">&gt;</span>
8891     <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">lazy_subscript_context</span><span class="special">&lt;</span><span class="identifier">Size</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span>
8892     <span class="keyword">operator</span> <span class="special">[](</span> <span class="identifier">Size</span> <span class="identifier">subscript</span> <span class="special">)</span> <span class="keyword">const</span>
8893     <span class="special">{</span>
8894         <span class="identifier">lazy_subscript_context</span><span class="special">&lt;</span><span class="identifier">Size</span><span class="special">&gt;</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">subscript</span><span class="special">);</span>
8895         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
8896     <span class="special">}</span>
8897 <span class="special">};</span>
8898
8899 <span class="comment">// Here is our lazy_vector terminal, implemented in terms of lazy_vector_expr</span>
8900 <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">&gt;</span>
8901 <span class="keyword">struct</span> <span class="identifier">lazy_vector</span>
8902   <span class="special">:</span> <span class="identifier">lazy_vector_expr</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span>
8903 <span class="special">{</span>
8904     <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">expr_type</span><span class="special">;</span>
8905
8906     <span class="identifier">lazy_vector</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">T</span><span class="special">()</span> <span class="special">)</span>
8907       <span class="special">:</span> <span class="identifier">lazy_vector_expr</span><span class="special">&lt;</span><span class="identifier">expr_type</span><span class="special">&gt;(</span> <span class="identifier">expr_type</span><span class="special">::</span><span class="identifier">make</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;(</span> <span class="identifier">size</span><span class="special">,</span> <span class="identifier">value</span> <span class="special">)</span> <span class="special">)</span> <span class="special">)</span>
8908     <span class="special">{}</span>
8909
8910     <span class="comment">// Here we define a += operator for lazy vector terminals that</span>
8911     <span class="comment">// takes a lazy vector expression and indexes it. expr[i] here</span>
8912     <span class="comment">// uses lazy_subscript_context&lt;&gt; under the covers.</span>
8913     <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
8914     <span class="identifier">lazy_vector</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">+=</span> <span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
8915     <span class="special">{</span>
8916         <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">).</span><span class="identifier">size</span><span class="special">();</span>
8917         <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">size</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
8918         <span class="special">{</span>
8919             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">+=</span> <span class="identifier">expr</span><span class="special">[</span><span class="identifier">i</span><span class="special">];</span>
8920         <span class="special">}</span>
8921         <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
8922     <span class="special">}</span>
8923 <span class="special">};</span>
8924
8925 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
8926 <span class="special">{</span>
8927     <span class="comment">// lazy_vectors with 4 elements each.</span>
8928     <span class="identifier">lazy_vector</span><span class="special">&lt;</span> <span class="keyword">double</span> <span class="special">&gt;</span> <span class="identifier">v1</span><span class="special">(</span> <span class="number">4</span><span class="special">,</span> <span class="number">1.0</span> <span class="special">),</span> <span class="identifier">v2</span><span class="special">(</span> <span class="number">4</span><span class="special">,</span> <span class="number">2.0</span> <span class="special">),</span> <span class="identifier">v3</span><span class="special">(</span> <span class="number">4</span><span class="special">,</span> <span class="number">3.0</span> <span class="special">);</span>
8929
8930     <span class="comment">// Add two vectors lazily and get the 2nd element.</span>
8931     <span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">v2</span> <span class="special">+</span> <span class="identifier">v3</span> <span class="special">)[</span> <span class="number">2</span> <span class="special">];</span>   <span class="comment">// Look ma, no temporaries!</span>
8932     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">d1</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
8933
8934     <span class="comment">// Subtract two vectors and add the result to a third vector.</span>
8935     <span class="identifier">v1</span> <span class="special">+=</span> <span class="identifier">v2</span> <span class="special">-</span> <span class="identifier">v3</span><span class="special">;</span>                  <span class="comment">// Still no temporaries!</span>
8936     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="char">'{'</span> <span class="special">&lt;&lt;</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">1</span><span class="special">]</span>
8937               <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="char">','</span> <span class="special">&lt;&lt;</span> <span class="identifier">v1</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="char">'}'</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
8938
8939     <span class="comment">// This expression is disallowed because it does not conform</span>
8940     <span class="comment">// to the LazyVectorGrammar</span>
8941     <span class="comment">//(v2 + v3) += v1;</span>
8942
8943     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
8944 <span class="special">}</span>
8945 </pre>
8946 <p>
8947         </p>
8948 </div>
8949 <div class="section">
8950 <div class="titlepage"><div><div><h4 class="title">
8951 <a name="boost_proto.users_guide.examples.rgb"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.rgb" title="RGB: Type Manipulations with Proto Transforms">RGB: Type Manipulations
8952         with Proto Transforms</a>
8953 </h4></div></div></div>
8954 <p>
8955           This is a simple example of doing arbitrary type manipulations with Proto
8956           transforms. It takes some expression involving primary colors and combines
8957           the colors according to arbitrary rules. It is a port of the RGB example
8958           from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>.
8959         </p>
8960 <p>
8961 </p>
8962 <pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
8963 <span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
8964 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
8965 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
8966 <span class="comment">//</span>
8967 <span class="comment">// This is a simple example of doing arbitrary type manipulations with proto</span>
8968 <span class="comment">// transforms. It takes some expression involving primary colors and combines</span>
8969 <span class="comment">// the colors according to arbitrary rules. It is a port of the RGB example</span>
8970 <span class="comment">// from PETE (http://www.codesourcery.com/pooma/download.html).</span>
8971
8972 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
8973 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8974 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
8975 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
8976
8977 <span class="keyword">struct</span> <span class="identifier">RedTag</span>
8978 <span class="special">{</span>
8979     <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">RedTag</span><span class="special">)</span>
8980     <span class="special">{</span>
8981         <span class="keyword">return</span> <span class="identifier">sout</span> <span class="special">&lt;&lt;</span> <span class="string">"This expression is red."</span><span class="special">;</span>
8982     <span class="special">}</span>
8983 <span class="special">};</span>
8984
8985 <span class="keyword">struct</span> <span class="identifier">BlueTag</span>
8986 <span class="special">{</span>
8987     <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">BlueTag</span><span class="special">)</span>
8988     <span class="special">{</span>
8989         <span class="keyword">return</span> <span class="identifier">sout</span> <span class="special">&lt;&lt;</span> <span class="string">"This expression is blue."</span><span class="special">;</span>
8990     <span class="special">}</span>
8991 <span class="special">};</span>
8992
8993 <span class="keyword">struct</span> <span class="identifier">GreenTag</span>
8994 <span class="special">{</span>
8995     <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">GreenTag</span><span class="special">)</span>
8996     <span class="special">{</span>
8997         <span class="keyword">return</span> <span class="identifier">sout</span> <span class="special">&lt;&lt;</span> <span class="string">"This expression is green."</span><span class="special">;</span>
8998     <span class="special">}</span>
8999 <span class="special">};</span>
9000
9001 <span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">RedTag</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">RedT</span><span class="special">;</span>
9002 <span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">BlueTag</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">BlueT</span><span class="special">;</span>
9003 <span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">GreenTag</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">GreenT</span><span class="special">;</span>
9004
9005 <span class="keyword">struct</span> <span class="identifier">Red</span><span class="special">;</span>
9006 <span class="keyword">struct</span> <span class="identifier">Blue</span><span class="special">;</span>
9007 <span class="keyword">struct</span> <span class="identifier">Green</span><span class="special">;</span>
9008
9009 <span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
9010 <span class="comment">// A transform that produces new colors according to some arbitrary rules:</span>
9011 <span class="comment">// red &amp; green give blue, red &amp; blue give green, blue and green give red.</span>
9012 <span class="keyword">struct</span> <span class="identifier">Red</span>
9013   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
9014         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Green</span><span class="special">,</span> <span class="identifier">Blue</span><span class="special">&gt;</span>
9015       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">Green</span><span class="special">&gt;</span>
9016       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Red</span><span class="special">,</span> <span class="identifier">Red</span><span class="special">&gt;</span>
9017       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">RedTag</span><span class="special">&gt;</span>
9018     <span class="special">&gt;</span>
9019 <span class="special">{};</span>
9020
9021 <span class="keyword">struct</span> <span class="identifier">Green</span>
9022   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
9023         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Red</span><span class="special">,</span> <span class="identifier">Blue</span><span class="special">&gt;</span>
9024       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">Red</span><span class="special">&gt;</span>
9025       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Green</span><span class="special">,</span> <span class="identifier">Green</span><span class="special">&gt;</span>
9026       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">GreenTag</span><span class="special">&gt;</span>
9027     <span class="special">&gt;</span>
9028 <span class="special">{};</span>
9029
9030 <span class="keyword">struct</span> <span class="identifier">Blue</span>
9031   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
9032         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Red</span><span class="special">,</span> <span class="identifier">Green</span><span class="special">&gt;</span>
9033       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Green</span><span class="special">,</span> <span class="identifier">Red</span><span class="special">&gt;</span>
9034       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">Blue</span><span class="special">&gt;</span>
9035       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">BlueTag</span><span class="special">&gt;</span>
9036     <span class="special">&gt;</span>
9037 <span class="special">{};</span>
9038
9039 <span class="keyword">struct</span> <span class="identifier">RGB</span>
9040   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
9041         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">Red</span><span class="special">,</span> <span class="identifier">RedTag</span><span class="special">()</span> <span class="special">&gt;</span>
9042       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">Blue</span><span class="special">,</span> <span class="identifier">BlueTag</span><span class="special">()</span> <span class="special">&gt;</span>
9043       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">Green</span><span class="special">,</span> <span class="identifier">GreenTag</span><span class="special">()</span> <span class="special">&gt;</span>
9044     <span class="special">&gt;</span>
9045 <span class="special">{};</span>
9046
9047 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9048 <span class="keyword">void</span> <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
9049 <span class="special">{</span>
9050     <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="comment">// dummy state and data parameter, not used</span>
9051     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">RGB</span><span class="special">()(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9052 <span class="special">}</span>
9053
9054 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
9055 <span class="special">{</span>
9056     <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">RedT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">GreenT</span><span class="special">());</span>
9057     <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">RedT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">GreenT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">BlueT</span><span class="special">());</span>
9058     <span class="identifier">printColor</span><span class="special">(</span><span class="identifier">RedT</span><span class="special">()</span> <span class="special">+</span> <span class="special">(</span><span class="identifier">GreenT</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">BlueT</span><span class="special">()));</span>
9059
9060     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
9061 <span class="special">}</span>
9062 </pre>
9063 <p>
9064         </p>
9065 </div>
9066 <div class="section">
9067 <div class="titlepage"><div><div><h4 class="title">
9068 <a name="boost_proto.users_guide.examples.tarray"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.tarray" title="TArray: A Simple Linear Algebra Library">TArray: A
9069         Simple Linear Algebra Library</a>
9070 </h4></div></div></div>
9071 <p>
9072           This example constructs a mini-library for linear algebra, using expression
9073           templates to eliminate the need for temporaries when adding arrays of numbers.
9074           It duplicates the TArray example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>.
9075         </p>
9076 <p>
9077 </p>
9078 <pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
9079 <span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
9080 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
9081 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
9082 <span class="comment">//</span>
9083 <span class="comment">// This example constructs a mini-library for linear algebra, using</span>
9084 <span class="comment">// expression templates to eliminate the need for temporaries when</span>
9085 <span class="comment">// adding arrays of numbers. It duplicates the TArray example from</span>
9086 <span class="comment">// PETE (http://www.codesourcery.com/pooma/download.html)</span>
9087
9088 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
9089 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9090 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9091 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9092 <span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
9093 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
9094 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>
9095
9096 <span class="comment">// This grammar describes which TArray expressions</span>
9097 <span class="comment">// are allowed; namely, int and array terminals</span>
9098 <span class="comment">// plus, minus, multiplies and divides of TArray expressions.</span>
9099 <span class="keyword">struct</span> <span class="identifier">TArrayGrammar</span>
9100   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
9101         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;</span>
9102       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special">&gt;</span>
9103       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">&gt;</span>
9104       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">&gt;</span>
9105       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">&gt;</span>
9106       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span> <span class="identifier">TArrayGrammar</span><span class="special">,</span> <span class="identifier">TArrayGrammar</span> <span class="special">&gt;</span>
9107     <span class="special">&gt;</span>
9108 <span class="special">{};</span>
9109
9110 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9111 <span class="keyword">struct</span> <span class="identifier">TArrayExpr</span><span class="special">;</span>
9112
9113 <span class="comment">// Tell proto that in the TArrayDomain, all</span>
9114 <span class="comment">// expressions should be wrapped in TArrayExpr&lt;&gt; and</span>
9115 <span class="comment">// must conform to the TArrayGrammar</span>
9116 <span class="keyword">struct</span> <span class="identifier">TArrayDomain</span>
9117   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">TArrayExpr</span><span class="special">&gt;,</span> <span class="identifier">TArrayGrammar</span><span class="special">&gt;</span>
9118 <span class="special">{};</span>
9119
9120 <span class="comment">// Here is an evaluation context that indexes into a TArray</span>
9121 <span class="comment">// expression, and combines the result.</span>
9122 <span class="keyword">struct</span> <span class="identifier">TArraySubscriptCtx</span>
9123   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">TArraySubscriptCtx</span> <span class="keyword">const</span> <span class="special">&gt;</span>
9124 <span class="special">{</span>
9125     <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span>
9126
9127     <span class="identifier">TArraySubscriptCtx</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span><span class="special">)</span>
9128       <span class="special">:</span> <span class="identifier">i_</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span>
9129     <span class="special">{}</span>
9130
9131     <span class="comment">// Index array terminals with our subscript. Everything</span>
9132     <span class="comment">// else will be handled by the default evaluation context.</span>
9133     <span class="keyword">int</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">(&amp;</span><span class="identifier">data</span><span class="special">)[</span><span class="number">3</span><span class="special">])</span> <span class="keyword">const</span>
9134     <span class="special">{</span>
9135         <span class="keyword">return</span> <span class="identifier">data</span><span class="special">[</span><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">i_</span><span class="special">];</span>
9136     <span class="special">}</span>
9137
9138     <span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i_</span><span class="special">;</span>
9139 <span class="special">};</span>
9140
9141 <span class="comment">// Here is an evaluation context that prints a TArray expression.</span>
9142 <span class="keyword">struct</span> <span class="identifier">TArrayPrintCtx</span>
9143   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">TArrayPrintCtx</span> <span class="keyword">const</span> <span class="special">&gt;</span>
9144 <span class="special">{</span>
9145     <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">result_type</span><span class="special">;</span>
9146
9147     <span class="identifier">TArrayPrintCtx</span><span class="special">()</span> <span class="special">{}</span>
9148
9149     <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
9150     <span class="special">{</span>
9151         <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span><span class="special">;</span>
9152     <span class="special">}</span>
9153
9154     <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">(&amp;</span><span class="identifier">arr</span><span class="special">)[</span><span class="number">3</span><span class="special">])</span> <span class="keyword">const</span>
9155     <span class="special">{</span>
9156         <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="char">'{'</span> <span class="special">&lt;&lt;</span> <span class="identifier">arr</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="string">", "</span> <span class="special">&lt;&lt;</span> <span class="identifier">arr</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="string">", "</span> <span class="special">&lt;&lt;</span> <span class="identifier">arr</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="char">'}'</span><span class="special">;</span>
9157     <span class="special">}</span>
9158
9159     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">&gt;</span>
9160     <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span>
9161     <span class="special">{</span>
9162         <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="char">'('</span> <span class="special">&lt;&lt;</span> <span class="identifier">l</span> <span class="special">&lt;&lt;</span> <span class="string">" + "</span> <span class="special">&lt;&lt;</span> <span class="identifier">r</span> <span class="special">&lt;&lt;</span> <span class="char">')'</span><span class="special">;</span>
9163     <span class="special">}</span>
9164
9165     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">&gt;</span>
9166     <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span>
9167     <span class="special">{</span>
9168         <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="char">'('</span> <span class="special">&lt;&lt;</span> <span class="identifier">l</span> <span class="special">&lt;&lt;</span> <span class="string">" - "</span> <span class="special">&lt;&lt;</span> <span class="identifier">r</span> <span class="special">&lt;&lt;</span> <span class="char">')'</span><span class="special">;</span>
9169     <span class="special">}</span>
9170
9171     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">&gt;</span>
9172     <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span>
9173     <span class="special">{</span>
9174         <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">l</span> <span class="special">&lt;&lt;</span> <span class="string">" * "</span> <span class="special">&lt;&lt;</span> <span class="identifier">r</span><span class="special">;</span>
9175     <span class="special">}</span>
9176
9177     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">L</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">R</span><span class="special">&gt;</span>
9178     <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides</span><span class="special">,</span> <span class="identifier">L</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">l</span><span class="special">,</span> <span class="identifier">R</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">r</span><span class="special">)</span> <span class="keyword">const</span>
9179     <span class="special">{</span>
9180         <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">l</span> <span class="special">&lt;&lt;</span> <span class="string">" / "</span> <span class="special">&lt;&lt;</span> <span class="identifier">r</span><span class="special">;</span>
9181     <span class="special">}</span>
9182 <span class="special">};</span>
9183
9184 <span class="comment">// Here is the domain-specific expression wrapper, which overrides</span>
9185 <span class="comment">// operator [] to evaluate the expression using the TArraySubscriptCtx.</span>
9186 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9187 <span class="keyword">struct</span> <span class="identifier">TArrayExpr</span>
9188   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">TArrayExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">TArrayDomain</span><span class="special">&gt;</span>
9189 <span class="special">{</span>
9190     <span class="keyword">typedef</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">TArrayExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">TArrayDomain</span><span class="special">&gt;</span> <span class="identifier">base_type</span><span class="special">;</span>
9191
9192     <span class="identifier">TArrayExpr</span><span class="special">(</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span> <span class="special">=</span> <span class="identifier">Expr</span><span class="special">()</span> <span class="special">)</span>
9193       <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span> <span class="identifier">expr</span> <span class="special">)</span>
9194     <span class="special">{}</span>
9195
9196     <span class="comment">// Use the TArraySubscriptCtx to implement subscripting</span>
9197     <span class="comment">// of a TArray expression tree.</span>
9198     <span class="keyword">int</span> <span class="keyword">operator</span> <span class="special">[](</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span> <span class="special">)</span> <span class="keyword">const</span>
9199     <span class="special">{</span>
9200         <span class="identifier">TArraySubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
9201         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
9202     <span class="special">}</span>
9203
9204     <span class="comment">// Use the TArrayPrintCtx to display a TArray expression tree.</span>
9205     <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">&lt;&lt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&amp;</span><span class="identifier">sout</span><span class="special">,</span> <span class="identifier">TArrayExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
9206     <span class="special">{</span>
9207         <span class="identifier">TArrayPrintCtx</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">;</span>
9208         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
9209     <span class="special">}</span>
9210 <span class="special">};</span>
9211
9212 <span class="comment">// Here is our TArray terminal, implemented in terms of TArrayExpr</span>
9213 <span class="comment">// It is basically just an array of 3 integers.</span>
9214 <span class="keyword">struct</span> <span class="identifier">TArray</span>
9215   <span class="special">:</span> <span class="identifier">TArrayExpr</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;</span>
9216 <span class="special">{</span>
9217     <span class="keyword">explicit</span> <span class="identifier">TArray</span><span class="special">(</span> <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="number">0</span> <span class="special">)</span>
9218     <span class="special">{</span>
9219         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
9220         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">j</span><span class="special">;</span>
9221         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">k</span><span class="special">;</span>
9222     <span class="special">}</span>
9223
9224     <span class="comment">// Here we override operator [] to give read/write access to</span>
9225     <span class="comment">// the elements of the array. (We could use the TArrayExpr</span>
9226     <span class="comment">// operator [] if we made the subscript context smarter about</span>
9227     <span class="comment">// returning non-const reference when appropriate.)</span>
9228     <span class="keyword">int</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span><span class="special">)</span>
9229     <span class="special">{</span>
9230         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
9231     <span class="special">}</span>
9232
9233     <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
9234     <span class="special">{</span>
9235         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
9236     <span class="special">}</span>
9237
9238     <span class="comment">// Here we define a operator = for TArray terminals that</span>
9239     <span class="comment">// takes a TArray expression.</span>
9240     <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
9241     <span class="identifier">TArray</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">=(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
9242     <span class="special">{</span>
9243         <span class="comment">// proto::as_expr&lt;TArrayDomain&gt;(expr) is the same as</span>
9244         <span class="comment">// expr unless expr is an integer, in which case it</span>
9245         <span class="comment">// is made into a TArrayExpr terminal first.</span>
9246         <span class="keyword">return</span> <span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">TArrayDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">));</span>
9247     <span class="special">}</span>
9248
9249     <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
9250     <span class="identifier">TArray</span> <span class="special">&amp;</span><span class="identifier">printAssign</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
9251     <span class="special">{</span>
9252         <span class="special">*</span><span class="keyword">this</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">;</span>
9253         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">*</span><span class="keyword">this</span> <span class="special">&lt;&lt;</span> <span class="string">" = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">expr</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9254         <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
9255     <span class="special">}</span>
9256
9257 <span class="keyword">private</span><span class="special">:</span>
9258     <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
9259     <span class="identifier">TArray</span> <span class="special">&amp;</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
9260     <span class="special">{</span>
9261         <span class="comment">// expr[i] here uses TArraySubscriptCtx under the covers.</span>
9262         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">[</span><span class="number">0</span><span class="special">];</span>
9263         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">[</span><span class="number">1</span><span class="special">];</span>
9264         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">expr</span><span class="special">[</span><span class="number">2</span><span class="special">];</span>
9265         <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
9266     <span class="special">}</span>
9267 <span class="special">};</span>
9268
9269 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
9270 <span class="special">{</span>
9271     <span class="identifier">TArray</span> <span class="identifier">a</span><span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">);</span>
9272
9273     <span class="identifier">TArray</span> <span class="identifier">b</span><span class="special">;</span>
9274
9275     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">a</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9276     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">b</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9277
9278     <span class="identifier">b</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="number">7</span><span class="special">;</span> <span class="identifier">b</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="number">33</span><span class="special">;</span> <span class="identifier">b</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">99</span><span class="special">;</span>
9279
9280     <span class="identifier">TArray</span> <span class="identifier">c</span><span class="special">(</span><span class="identifier">a</span><span class="special">);</span>
9281
9282     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">c</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9283
9284     <span class="identifier">a</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
9285
9286     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">a</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9287     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">b</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9288     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">c</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9289
9290     <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">;</span>
9291
9292     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">a</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9293
9294     <span class="identifier">a</span><span class="special">.</span><span class="identifier">printAssign</span><span class="special">(</span><span class="identifier">b</span><span class="special">+</span><span class="identifier">c</span><span class="special">*(</span><span class="identifier">b</span> <span class="special">+</span> <span class="number">3</span><span class="special">*</span><span class="identifier">c</span><span class="special">));</span>
9295
9296     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
9297 <span class="special">}</span>
9298 </pre>
9299 <p>
9300         </p>
9301 </div>
9302 <div class="section">
9303 <div class="titlepage"><div><div><h4 class="title">
9304 <a name="boost_proto.users_guide.examples.vec3"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.vec3" title="Vec3: Computing With Transforms and Contexts">Vec3: Computing
9305         With Transforms and Contexts</a>
9306 </h4></div></div></div>
9307 <p>
9308           This is a simple example using <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;&gt;</span></code> to extend a terminal type with
9309           additional behaviors, and using custom contexts and <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code> for evaluating expressions. It is a port
9310           of the Vec3 example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>.
9311         </p>
9312 <p>
9313 </p>
9314 <pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
9315 <span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
9316 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
9317 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
9318 <span class="comment">//</span>
9319 <span class="comment">// This is a simple example using proto::extends to extend a terminal type with</span>
9320 <span class="comment">// additional behaviors, and using custom contexts and proto::eval for</span>
9321 <span class="comment">// evaluating expressions. It is a port of the Vec3 example</span>
9322 <span class="comment">// from PETE (http://www.codesourcery.com/pooma/download.html).</span>
9323
9324 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
9325 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">functional</span><span class="special">&gt;</span>
9326 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9327 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9328 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9329 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9330 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto_typeof</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9331 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9332 <span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
9333 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
9334 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>
9335
9336 <span class="comment">// Here is an evaluation context that indexes into a Vec3</span>
9337 <span class="comment">// expression, and combines the result.</span>
9338 <span class="keyword">struct</span> <span class="identifier">Vec3SubscriptCtx</span>
9339   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">Vec3SubscriptCtx</span> <span class="keyword">const</span> <span class="special">&gt;</span>
9340 <span class="special">{</span>
9341     <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span>
9342
9343     <span class="identifier">Vec3SubscriptCtx</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span>
9344       <span class="special">:</span> <span class="identifier">i_</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span>
9345     <span class="special">{}</span>
9346
9347     <span class="comment">// Index array terminals with our subscript. Everything</span>
9348     <span class="comment">// else will be handled by the default evaluation context.</span>
9349     <span class="keyword">int</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">(&amp;</span><span class="identifier">arr</span><span class="special">)[</span><span class="number">3</span><span class="special">])</span> <span class="keyword">const</span>
9350     <span class="special">{</span>
9351         <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">[</span><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">i_</span><span class="special">];</span>
9352     <span class="special">}</span>
9353
9354     <span class="keyword">int</span> <span class="identifier">i_</span><span class="special">;</span>
9355 <span class="special">};</span>
9356
9357 <span class="comment">// Here is an evaluation context that counts the number</span>
9358 <span class="comment">// of Vec3 terminals in an expression.</span>
9359 <span class="keyword">struct</span> <span class="identifier">CountLeavesCtx</span>
9360   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span> <span class="identifier">CountLeavesCtx</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">null_context</span> <span class="special">&gt;</span>
9361 <span class="special">{</span>
9362     <span class="identifier">CountLeavesCtx</span><span class="special">()</span>
9363       <span class="special">:</span> <span class="identifier">count</span><span class="special">(</span><span class="number">0</span><span class="special">)</span>
9364       <span class="special">{}</span>
9365
9366       <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>
9367
9368       <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span><span class="special">(&amp;)[</span><span class="number">3</span><span class="special">])</span>
9369       <span class="special">{</span>
9370           <span class="special">++</span><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">count</span><span class="special">;</span>
9371       <span class="special">}</span>
9372
9373       <span class="keyword">int</span> <span class="identifier">count</span><span class="special">;</span>
9374 <span class="special">};</span>
9375
9376 <span class="keyword">struct</span> <span class="identifier">iplus</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span> <span class="special">{};</span>
9377
9378 <span class="comment">// Here is a transform that does the same thing as the above context.</span>
9379 <span class="comment">// It demonstrates the use of the std::plus&lt;&gt; function object</span>
9380 <span class="comment">// with the fold transform. With minor modifications, this</span>
9381 <span class="comment">// transform could be used to calculate the leaf count at compile</span>
9382 <span class="comment">// time, rather than at runtime.</span>
9383 <span class="keyword">struct</span> <span class="identifier">CountLeaves</span>
9384   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
9385         <span class="comment">// match a Vec3 terminal, return 1</span>
9386         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]&gt;,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;()</span> <span class="special">&gt;</span>
9387         <span class="comment">// match a terminal, return int() (which is 0)</span>
9388       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;,</span> <span class="keyword">int</span><span class="special">()</span> <span class="special">&gt;</span>
9389         <span class="comment">// fold everything else, using std::plus&lt;&gt; to add</span>
9390         <span class="comment">// the leaf count of each child to the accumulated state.</span>
9391       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">otherwise</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="keyword">int</span><span class="special">(),</span> <span class="identifier">iplus</span><span class="special">(</span><span class="identifier">CountLeaves</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">)</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
9392     <span class="special">&gt;</span>
9393 <span class="special">{};</span>
9394
9395 <span class="comment">// Here is the Vec3 struct, which is a vector of 3 integers.</span>
9396 <span class="keyword">struct</span> <span class="identifier">Vec3</span>
9397   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">[</span><span class="number">3</span><span class="special">]&gt;::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">Vec3</span><span class="special">&gt;</span>
9398 <span class="special">{</span>
9399     <span class="keyword">explicit</span> <span class="identifier">Vec3</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">j</span><span class="special">=</span><span class="number">0</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">k</span><span class="special">=</span><span class="number">0</span><span class="special">)</span>
9400     <span class="special">{</span>
9401         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
9402         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">j</span><span class="special">;</span>
9403         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">k</span><span class="special">;</span>
9404     <span class="special">}</span>
9405
9406     <span class="keyword">int</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">[](</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span>
9407     <span class="special">{</span>
9408         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
9409     <span class="special">}</span>
9410
9411     <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">[](</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span>
9412     <span class="special">{</span>
9413         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
9414     <span class="special">}</span>
9415
9416     <span class="comment">// Here we define a operator = for Vec3 terminals that</span>
9417     <span class="comment">// takes a Vec3 expression.</span>
9418     <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">Expr</span> <span class="special">&gt;</span>
9419     <span class="identifier">Vec3</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">=(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
9420     <span class="special">{</span>
9421         <span class="keyword">typedef</span> <span class="identifier">Vec3SubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">;</span>
9422         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">(</span><span class="number">0</span><span class="special">));</span>
9423         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
9424         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">CVec3SubscriptCtx</span><span class="special">(</span><span class="number">2</span><span class="special">));</span>
9425         <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
9426     <span class="special">}</span>
9427
9428     <span class="comment">// This copy-assign is needed because a template is never</span>
9429     <span class="comment">// considered for copy assignment.</span>
9430     <span class="identifier">Vec3</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">Vec3</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">that</span><span class="special">)</span>
9431     <span class="special">{</span>
9432         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">that</span><span class="special">[</span><span class="number">0</span><span class="special">];</span>
9433         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">that</span><span class="special">[</span><span class="number">1</span><span class="special">];</span>
9434         <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">that</span><span class="special">[</span><span class="number">2</span><span class="special">];</span>
9435         <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
9436     <span class="special">}</span>
9437
9438     <span class="keyword">void</span> <span class="identifier">print</span><span class="special">()</span> <span class="keyword">const</span>
9439     <span class="special">{</span>
9440         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="char">'{'</span> <span class="special">&lt;&lt;</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span>
9441                   <span class="special">&lt;&lt;</span> <span class="string">", "</span> <span class="special">&lt;&lt;</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">1</span><span class="special">]</span>
9442                   <span class="special">&lt;&lt;</span> <span class="string">", "</span> <span class="special">&lt;&lt;</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span>
9443                   <span class="special">&lt;&lt;</span> <span class="char">'}'</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9444     <span class="special">}</span>
9445 <span class="special">};</span>
9446
9447 <span class="comment">// The count_leaves() function uses the CountLeaves transform and</span>
9448 <span class="comment">// to count the number of leaves in an expression.</span>
9449 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9450 <span class="keyword">int</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
9451 <span class="special">{</span>
9452     <span class="comment">// Count the number of Vec3 terminals using the</span>
9453     <span class="comment">// CountLeavesCtx evaluation context.</span>
9454     <span class="identifier">CountLeavesCtx</span> <span class="identifier">ctx</span><span class="special">;</span>
9455     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
9456
9457     <span class="comment">// This is another way to count the leaves using a transform.</span>
9458     <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
9459     <span class="identifier">BOOST_ASSERT</span><span class="special">(</span> <span class="identifier">CountLeaves</span><span class="special">()(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">count</span> <span class="special">);</span>
9460
9461     <span class="keyword">return</span> <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">count</span><span class="special">;</span>
9462 <span class="special">}</span>
9463
9464 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
9465 <span class="special">{</span>
9466     <span class="identifier">Vec3</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">c</span><span class="special">;</span>
9467
9468     <span class="identifier">c</span> <span class="special">=</span> <span class="number">4</span><span class="special">;</span>
9469
9470     <span class="identifier">b</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
9471     <span class="identifier">b</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">2</span><span class="special">;</span>
9472     <span class="identifier">b</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="number">3</span><span class="special">;</span>
9473
9474     <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">;</span>
9475
9476     <span class="identifier">a</span><span class="special">.</span><span class="identifier">print</span><span class="special">();</span>
9477
9478     <span class="identifier">Vec3</span> <span class="identifier">d</span><span class="special">;</span>
9479     <span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span><span class="identifier">expr1</span><span class="special">,</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">);</span>
9480     <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">expr1</span><span class="special">;</span>
9481     <span class="identifier">d</span><span class="special">.</span><span class="identifier">print</span><span class="special">();</span>
9482
9483     <span class="keyword">int</span> <span class="identifier">num</span> <span class="special">=</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">expr1</span><span class="special">);</span>
9484     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">num</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9485
9486     <span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">,</span> <span class="identifier">b</span> <span class="special">+</span> <span class="number">3</span> <span class="special">*</span> <span class="identifier">c</span><span class="special">);</span>
9487     <span class="identifier">num</span> <span class="special">=</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">);</span>
9488     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">num</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9489
9490     <span class="identifier">BOOST_PROTO_AUTO</span><span class="special">(</span><span class="identifier">expr3</span><span class="special">,</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span> <span class="special">*</span> <span class="identifier">d</span><span class="special">);</span>
9491     <span class="identifier">num</span> <span class="special">=</span> <span class="identifier">count_leaves</span><span class="special">(</span><span class="identifier">expr3</span><span class="special">);</span>
9492     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">num</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9493
9494     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
9495 <span class="special">}</span>
9496 </pre>
9497 <p>
9498         </p>
9499 </div>
9500 <div class="section">
9501 <div class="titlepage"><div><div><h4 class="title">
9502 <a name="boost_proto.users_guide.examples.vector"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.vector" title="Vector: Adapting a Non-Proto Terminal Type">Vector: Adapting
9503         a Non-Proto Terminal Type</a>
9504 </h4></div></div></div>
9505 <p>
9506           This is an example of using <code class="computeroutput"><span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">()</span></code> to Protofy expressions using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;&gt;</span></code>,
9507           a non-Proto type. It is a port of the Vector example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>.
9508         </p>
9509 <p>
9510 </p>
9511 <pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
9512 <span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
9513 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
9514 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
9515 <span class="comment">//</span>
9516 <span class="comment">// This is an example of using BOOST_PROTO_DEFINE_OPERATORS to Protofy</span>
9517 <span class="comment">// expressions using std::vector&lt;&gt;, a non-proto type. It is a port of the</span>
9518 <span class="comment">// Vector example from PETE (http://www.codesourcery.com/pooma/download.html).</span>
9519
9520 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
9521 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
9522 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">stdexcept</span><span class="special">&gt;</span>
9523 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">bool</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9524 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9525 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">debug</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9526 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9527 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">enable_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9528 <span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
9529 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
9530 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>
9531
9532 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9533 <span class="keyword">struct</span> <span class="identifier">VectorExpr</span><span class="special">;</span>
9534
9535 <span class="comment">// Here is an evaluation context that indexes into a std::vector</span>
9536 <span class="comment">// expression and combines the result.</span>
9537 <span class="keyword">struct</span> <span class="identifier">VectorSubscriptCtx</span>
9538 <span class="special">{</span>
9539     <span class="identifier">VectorSubscriptCtx</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span><span class="special">)</span>
9540       <span class="special">:</span> <span class="identifier">i_</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span>
9541     <span class="special">{}</span>
9542
9543     <span class="comment">// Unless this is a vector terminal, use the</span>
9544     <span class="comment">// default evaluation context</span>
9545     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
9546     <span class="keyword">struct</span> <span class="identifier">eval</span>
9547       <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span><span class="special">&gt;</span>
9548     <span class="special">{};</span>
9549
9550     <span class="comment">// Index vector terminals with our subscript.</span>
9551     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9552     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span>
9553         <span class="identifier">Expr</span>
9554       <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span>
9555             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
9556         <span class="special">&gt;::</span><span class="identifier">type</span>
9557     <span class="special">&gt;</span>
9558     <span class="special">{</span>
9559         <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">result_type</span><span class="special">;</span>
9560
9561         <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
9562         <span class="special">{</span>
9563             <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)[</span><span class="identifier">ctx</span><span class="special">.</span><span class="identifier">i_</span><span class="special">];</span>
9564         <span class="special">}</span>
9565     <span class="special">};</span>
9566
9567     <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i_</span><span class="special">;</span>
9568 <span class="special">};</span>
9569
9570 <span class="comment">// Here is an evaluation context that verifies that all the</span>
9571 <span class="comment">// vectors in an expression have the same size.</span>
9572 <span class="keyword">struct</span> <span class="identifier">VectorSizeCtx</span>
9573 <span class="special">{</span>
9574     <span class="identifier">VectorSizeCtx</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">)</span>
9575       <span class="special">:</span> <span class="identifier">size_</span><span class="special">(</span><span class="identifier">size</span><span class="special">)</span>
9576     <span class="special">{}</span>
9577
9578     <span class="comment">// Unless this is a vector terminal, use the</span>
9579     <span class="comment">// null evaluation context</span>
9580     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
9581     <span class="keyword">struct</span> <span class="identifier">eval</span>
9582       <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">null_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span><span class="special">&gt;</span>
9583     <span class="special">{};</span>
9584
9585     <span class="comment">// Index array terminals with our subscript. Everything</span>
9586     <span class="comment">// else will be handled by the default evaluation context.</span>
9587     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9588     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span>
9589         <span class="identifier">Expr</span>
9590       <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span>
9591             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
9592         <span class="special">&gt;::</span><span class="identifier">type</span>
9593     <span class="special">&gt;</span>
9594     <span class="special">{</span>
9595         <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>
9596
9597         <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
9598         <span class="special">{</span>
9599             <span class="keyword">if</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">.</span><span class="identifier">size_</span> <span class="special">!=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">).</span><span class="identifier">size</span><span class="special">())</span>
9600             <span class="special">{</span>
9601                 <span class="keyword">throw</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span><span class="special">(</span><span class="string">"LHS and RHS are not compatible"</span><span class="special">);</span>
9602             <span class="special">}</span>
9603         <span class="special">}</span>
9604     <span class="special">};</span>
9605
9606     <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size_</span><span class="special">;</span>
9607 <span class="special">};</span>
9608
9609 <span class="comment">// A grammar which matches all the assignment operators,</span>
9610 <span class="comment">// so we can easily disable them.</span>
9611 <span class="keyword">struct</span> <span class="identifier">AssignOps</span>
9612   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special">&lt;</span><span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span><span class="special">&gt;</span>
9613 <span class="special">{};</span>
9614
9615 <span class="comment">// Here are the cases used by the switch_ above.</span>
9616 <span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span>
9617 <span class="special">{</span>
9618     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">D</span> <span class="special">=</span> <span class="number">0</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span>  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">{};</span>
9619
9620     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>         <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9621     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>        <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9622     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9623     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>      <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9624     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>      <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9625     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9626     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9627     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9628     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9629     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9630 <span class="special">};</span>
9631
9632 <span class="comment">// A vector grammar is a terminal or some op that is not an</span>
9633 <span class="comment">// assignment op. (Assignment will be handled specially.)</span>
9634 <span class="keyword">struct</span> <span class="identifier">VectorGrammar</span>
9635   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
9636         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
9637       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">VectorGrammar</span><span class="special">&gt;</span> <span class="special">&gt;,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">AssignOps</span><span class="special">&gt;</span> <span class="special">&gt;</span>
9638     <span class="special">&gt;</span>
9639 <span class="special">{};</span>
9640
9641 <span class="comment">// Expressions in the vector domain will be wrapped in VectorExpr&lt;&gt;</span>
9642 <span class="comment">// and must conform to the VectorGrammar</span>
9643 <span class="keyword">struct</span> <span class="identifier">VectorDomain</span>
9644   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">VectorExpr</span><span class="special">&gt;,</span> <span class="identifier">VectorGrammar</span><span class="special">&gt;</span>
9645 <span class="special">{};</span>
9646
9647 <span class="comment">// Here is VectorExpr, which extends a proto expr type by</span>
9648 <span class="comment">// giving it an operator [] which uses the VectorSubscriptCtx</span>
9649 <span class="comment">// to evaluate an expression with a given index.</span>
9650 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9651 <span class="keyword">struct</span> <span class="identifier">VectorExpr</span>
9652   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">VectorDomain</span><span class="special">&gt;</span>
9653 <span class="special">{</span>
9654     <span class="keyword">explicit</span> <span class="identifier">VectorExpr</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
9655       <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">VectorExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">VectorDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">)</span>
9656     <span class="special">{}</span>
9657
9658     <span class="comment">// Use the VectorSubscriptCtx to implement subscripting</span>
9659     <span class="comment">// of a Vector expression tree.</span>
9660     <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">Expr</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span><span class="special">&gt;::</span><span class="identifier">type</span>
9661     <span class="keyword">operator</span> <span class="special">[](</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">)</span> <span class="keyword">const</span>
9662     <span class="special">{</span>
9663         <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
9664         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
9665     <span class="special">}</span>
9666 <span class="special">};</span>
9667
9668 <span class="comment">// Define a trait type for detecting vector terminals, to</span>
9669 <span class="comment">// be used by the BOOST_PROTO_DEFINE_OPERATORS macro below.</span>
9670 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
9671 <span class="keyword">struct</span> <span class="identifier">IsVector</span>
9672   <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span>
9673 <span class="special">{};</span>
9674
9675 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">&gt;</span>
9676 <span class="keyword">struct</span> <span class="identifier">IsVector</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&gt;</span>
9677   <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
9678 <span class="special">{};</span>
9679
9680 <span class="keyword">namespace</span> <span class="identifier">VectorOps</span>
9681 <span class="special">{</span>
9682     <span class="comment">// This defines all the overloads to make expressions involving</span>
9683     <span class="comment">// std::vector to build expression templates.</span>
9684     <span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">(</span><span class="identifier">IsVector</span><span class="special">,</span> <span class="identifier">VectorDomain</span><span class="special">)</span>
9685
9686     <span class="keyword">typedef</span> <span class="identifier">VectorSubscriptCtx</span> <span class="keyword">const</span> <span class="identifier">CVectorSubscriptCtx</span><span class="special">;</span>
9687
9688     <span class="comment">// Assign to a vector from some expression.</span>
9689     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9690     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
9691     <span class="special">{</span>
9692         <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span>
9693         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">VectorDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">size</span><span class="special">);</span> <span class="comment">// will throw if the sizes don't match</span>
9694         <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
9695         <span class="special">{</span>
9696             <span class="identifier">arr</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">VectorDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
9697         <span class="special">}</span>
9698         <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
9699     <span class="special">}</span>
9700
9701     <span class="comment">// Add-assign to a vector from some expression.</span>
9702     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9703     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">+=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
9704     <span class="special">{</span>
9705         <span class="identifier">VectorSizeCtx</span> <span class="keyword">const</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span>
9706         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">VectorDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">size</span><span class="special">);</span> <span class="comment">// will throw if the sizes don't match</span>
9707         <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
9708         <span class="special">{</span>
9709             <span class="identifier">arr</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">+=</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">VectorDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">)[</span><span class="identifier">i</span><span class="special">];</span>
9710         <span class="special">}</span>
9711         <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
9712     <span class="special">}</span>
9713 <span class="special">}</span>
9714
9715 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
9716 <span class="special">{</span>
9717     <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">VectorOps</span><span class="special">;</span>
9718
9719     <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
9720     <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">n</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
9721     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">a</span><span class="special">,</span><span class="identifier">b</span><span class="special">,</span><span class="identifier">c</span><span class="special">,</span><span class="identifier">d</span><span class="special">;</span>
9722     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">e</span><span class="special">(</span><span class="identifier">n</span><span class="special">);</span>
9723
9724     <span class="keyword">for</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
9725     <span class="special">{</span>
9726         <span class="identifier">a</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
9727         <span class="identifier">b</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">2</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span>
9728         <span class="identifier">c</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">3</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span>
9729         <span class="identifier">d</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
9730     <span class="special">}</span>
9731
9732     <span class="identifier">VectorOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">b</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span>
9733     <span class="identifier">VectorOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span> <span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span> <span class="special">*</span> <span class="identifier">c</span><span class="special">);</span>
9734     <span class="identifier">a</span> <span class="special">+=</span> <span class="identifier">if_else</span><span class="special">(</span><span class="identifier">d</span> <span class="special">&lt;</span> <span class="number">30</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span>
9735
9736     <span class="identifier">VectorOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">e</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span>
9737     <span class="identifier">e</span> <span class="special">+=</span> <span class="identifier">e</span> <span class="special">-</span> <span class="number">4</span> <span class="special">/</span> <span class="special">(</span><span class="identifier">c</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span>
9738
9739     <span class="keyword">for</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
9740     <span class="special">{</span>
9741         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span>
9742             <span class="special">&lt;&lt;</span> <span class="string">" a("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">a</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
9743             <span class="special">&lt;&lt;</span> <span class="string">" b("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">b</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
9744             <span class="special">&lt;&lt;</span> <span class="string">" c("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">c</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
9745             <span class="special">&lt;&lt;</span> <span class="string">" d("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">d</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
9746             <span class="special">&lt;&lt;</span> <span class="string">" e("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">e</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
9747             <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
9748     <span class="special">}</span>
9749 <span class="special">}</span>
9750 </pre>
9751 <p>
9752         </p>
9753 </div>
9754 <div class="section">
9755 <div class="titlepage"><div><div><h4 class="title">
9756 <a name="boost_proto.users_guide.examples.mixed"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.mixed" title="Mixed: Adapting Several Non-Proto Terminal Types">Mixed: Adapting
9757         Several Non-Proto Terminal Types</a>
9758 </h4></div></div></div>
9759 <p>
9760           This is an example of using <code class="computeroutput"><span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">()</span></code> to Protofy expressions using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;&gt;</span></code>
9761           and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;&gt;</span></code>,
9762           non-Proto types. It is a port of the Mixed example from <a href="http://acts.nersc.gov/formertools/pete/index.html" target="_top">PETE</a>.
9763         </p>
9764 <p>
9765 </p>
9766 <pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
9767 <span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
9768 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
9769 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
9770 <span class="comment">//</span>
9771 <span class="comment">// This is an example of using BOOST_PROTO_DEFINE_OPERATORS to Protofy</span>
9772 <span class="comment">// expressions using std::vector&lt;&gt; and std::list, non-proto types. It is a port</span>
9773 <span class="comment">// of the Mixed example from PETE.</span>
9774 <span class="comment">// (http://www.codesourcery.com/pooma/download.html).</span>
9775
9776 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">list</span><span class="special">&gt;</span>
9777 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cmath</span><span class="special">&gt;</span>
9778 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
9779 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">complex</span><span class="special">&gt;</span>
9780 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
9781 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">stdexcept</span><span class="special">&gt;</span>
9782 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9783 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">debug</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9784 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9785 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9786 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">enable_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9787 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9788 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9789 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">complex</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9790 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">remove_reference</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
9791 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
9792 <span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
9793 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>
9794
9795 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9796 <span class="keyword">struct</span> <span class="identifier">MixedExpr</span><span class="special">;</span>
9797
9798 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iter</span><span class="special">&gt;</span>
9799 <span class="keyword">struct</span> <span class="identifier">iterator_wrapper</span>
9800 <span class="special">{</span>
9801     <span class="keyword">typedef</span> <span class="identifier">Iter</span> <span class="identifier">iterator</span><span class="special">;</span>
9802
9803     <span class="keyword">explicit</span> <span class="identifier">iterator_wrapper</span><span class="special">(</span><span class="identifier">Iter</span> <span class="identifier">iter</span><span class="special">)</span>
9804       <span class="special">:</span> <span class="identifier">it</span><span class="special">(</span><span class="identifier">iter</span><span class="special">)</span>
9805     <span class="special">{}</span>
9806
9807     <span class="keyword">mutable</span> <span class="identifier">Iter</span> <span class="identifier">it</span><span class="special">;</span>
9808 <span class="special">};</span>
9809
9810 <span class="keyword">struct</span> <span class="identifier">begin</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
9811 <span class="special">{</span>
9812     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
9813     <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>
9814
9815     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Cont</span><span class="special">&gt;</span>
9816     <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Cont</span><span class="special">)&gt;</span>
9817       <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span>
9818             <span class="identifier">iterator_wrapper</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">Cont</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">const_iterator</span><span class="special">&gt;</span>
9819         <span class="special">&gt;</span>
9820     <span class="special">{};</span>
9821
9822     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Cont</span><span class="special">&gt;</span>
9823     <span class="keyword">typename</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">Cont</span> <span class="keyword">const</span> <span class="special">&amp;)&gt;::</span><span class="identifier">type</span>
9824     <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Cont</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">cont</span><span class="special">)</span> <span class="keyword">const</span>
9825     <span class="special">{</span>
9826         <span class="identifier">iterator_wrapper</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Cont</span><span class="special">::</span><span class="identifier">const_iterator</span><span class="special">&gt;</span> <span class="identifier">it</span><span class="special">(</span><span class="identifier">cont</span><span class="special">.</span><span class="identifier">begin</span><span class="special">());</span>
9827         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">(</span><span class="identifier">it</span><span class="special">);</span>
9828     <span class="special">}</span>
9829 <span class="special">};</span>
9830
9831 <span class="comment">// Here is a grammar that replaces vector and list terminals with their</span>
9832 <span class="comment">// begin iterators</span>
9833 <span class="keyword">struct</span> <span class="identifier">Begin</span>
9834   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
9835         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;,</span> <span class="identifier">begin</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> <span class="special">&gt;</span>
9836       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;,</span> <span class="identifier">begin</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span> <span class="special">&gt;</span>
9837       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
9838       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">Begin</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
9839     <span class="special">&gt;</span>
9840 <span class="special">{};</span>
9841
9842 <span class="comment">// Here is an evaluation context that dereferences iterator</span>
9843 <span class="comment">// terminals.</span>
9844 <span class="keyword">struct</span> <span class="identifier">DereferenceCtx</span>
9845 <span class="special">{</span>
9846     <span class="comment">// Unless this is an iterator terminal, use the</span>
9847     <span class="comment">// default evaluation context</span>
9848     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
9849     <span class="keyword">struct</span> <span class="identifier">eval</span>
9850       <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">DereferenceCtx</span> <span class="keyword">const</span><span class="special">&gt;</span>
9851     <span class="special">{};</span>
9852
9853     <span class="comment">// Dereference iterator terminals.</span>
9854     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9855     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span>
9856         <span class="identifier">Expr</span>
9857       <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span>
9858             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">iterator_wrapper</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
9859         <span class="special">&gt;::</span><span class="identifier">type</span>
9860     <span class="special">&gt;</span>
9861     <span class="special">{</span>
9862         <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">IteratorWrapper</span><span class="special">;</span>
9863         <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">IteratorWrapper</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">iterator</span><span class="special">;</span>
9864         <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">iterator_traits</span><span class="special">&lt;</span><span class="identifier">iterator</span><span class="special">&gt;::</span><span class="identifier">reference</span> <span class="identifier">result_type</span><span class="special">;</span>
9865
9866         <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">DereferenceCtx</span> <span class="keyword">const</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
9867         <span class="special">{</span>
9868             <span class="keyword">return</span> <span class="special">*</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">).</span><span class="identifier">it</span><span class="special">;</span>
9869         <span class="special">}</span>
9870     <span class="special">};</span>
9871 <span class="special">};</span>
9872
9873 <span class="comment">// Here is an evaluation context that increments iterator</span>
9874 <span class="comment">// terminals.</span>
9875 <span class="keyword">struct</span> <span class="identifier">IncrementCtx</span>
9876 <span class="special">{</span>
9877     <span class="comment">// Unless this is an iterator terminal, use the</span>
9878     <span class="comment">// default evaluation context</span>
9879     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
9880     <span class="keyword">struct</span> <span class="identifier">eval</span>
9881       <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">null_eval</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">IncrementCtx</span> <span class="keyword">const</span><span class="special">&gt;</span>
9882     <span class="special">{};</span>
9883
9884     <span class="comment">// advance iterator terminals.</span>
9885     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9886     <span class="keyword">struct</span> <span class="identifier">eval</span><span class="special">&lt;</span>
9887         <span class="identifier">Expr</span>
9888       <span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span><span class="special">&lt;</span>
9889             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">iterator_wrapper</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
9890         <span class="special">&gt;::</span><span class="identifier">type</span>
9891     <span class="special">&gt;</span>
9892     <span class="special">{</span>
9893         <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>
9894
9895         <span class="identifier">result_type</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">IncrementCtx</span> <span class="keyword">const</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
9896         <span class="special">{</span>
9897             <span class="special">++</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(</span><span class="identifier">expr</span><span class="special">).</span><span class="identifier">it</span><span class="special">;</span>
9898         <span class="special">}</span>
9899     <span class="special">};</span>
9900 <span class="special">};</span>
9901
9902 <span class="comment">// A grammar which matches all the assignment operators,</span>
9903 <span class="comment">// so we can easily disable them.</span>
9904 <span class="keyword">struct</span> <span class="identifier">AssignOps</span>
9905   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special">&lt;</span><span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span><span class="special">&gt;</span>
9906 <span class="special">{};</span>
9907
9908 <span class="comment">// Here are the cases used by the switch_ above.</span>
9909 <span class="keyword">struct</span> <span class="identifier">AssignOpsCases</span>
9910 <span class="special">{</span>
9911     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">D</span> <span class="special">=</span> <span class="number">0</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span>  <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">{};</span>
9912
9913     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>         <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9914     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">minus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>        <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9915     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">multiplies_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9916     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">divides_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>      <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9917     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">modulus_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>      <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9918     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_left_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9919     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">shift_right_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9920     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_and_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9921     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_or_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>   <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9922     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">case_</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">bitwise_xor_assign</span><span class="special">,</span> <span class="identifier">D</span> <span class="special">&gt;</span>  <span class="special">:</span> <span class="identifier">_</span> <span class="special">{};</span>
9923 <span class="special">};</span>
9924
9925 <span class="comment">// An expression conforms to the MixedGrammar if it is a terminal or some</span>
9926 <span class="comment">// op that is not an assignment op. (Assignment will be handled specially.)</span>
9927 <span class="keyword">struct</span> <span class="identifier">MixedGrammar</span>
9928   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
9929         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
9930       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">and_</span><span class="special">&lt;</span>
9931             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">MixedGrammar</span><span class="special">&gt;</span> <span class="special">&gt;</span>
9932           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special">&lt;</span><span class="identifier">AssignOps</span><span class="special">&gt;</span>
9933         <span class="special">&gt;</span>
9934     <span class="special">&gt;</span>
9935 <span class="special">{};</span>
9936
9937 <span class="comment">// Expressions in the MixedDomain will be wrapped in MixedExpr&lt;&gt;</span>
9938 <span class="comment">// and must conform to the MixedGrammar</span>
9939 <span class="keyword">struct</span> <span class="identifier">MixedDomain</span>
9940   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">MixedExpr</span><span class="special">&gt;,</span> <span class="identifier">MixedGrammar</span><span class="special">&gt;</span>
9941 <span class="special">{};</span>
9942
9943 <span class="comment">// Here is MixedExpr, a wrapper for expression types in the MixedDomain.</span>
9944 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
9945 <span class="keyword">struct</span> <span class="identifier">MixedExpr</span>
9946   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">MixedExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">MixedDomain</span><span class="special">&gt;</span>
9947 <span class="special">{</span>
9948     <span class="keyword">explicit</span> <span class="identifier">MixedExpr</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
9949       <span class="special">:</span> <span class="identifier">MixedExpr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span>
9950     <span class="special">{}</span>
9951 <span class="keyword">private</span><span class="special">:</span>
9952     <span class="comment">// hide this:</span>
9953     <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">MixedExpr</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;,</span> <span class="identifier">MixedDomain</span><span class="special">&gt;::</span><span class="keyword">operator</span> <span class="special">[];</span>
9954 <span class="special">};</span>
9955
9956 <span class="comment">// Define a trait type for detecting vector and list terminals, to</span>
9957 <span class="comment">// be used by the BOOST_PROTO_DEFINE_OPERATORS macro below.</span>
9958 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
9959 <span class="keyword">struct</span> <span class="identifier">IsMixed</span>
9960   <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span>
9961 <span class="special">{};</span>
9962
9963 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">&gt;</span>
9964 <span class="keyword">struct</span> <span class="identifier">IsMixed</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&gt;</span>
9965   <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
9966 <span class="special">{};</span>
9967
9968 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">&gt;</span>
9969 <span class="keyword">struct</span> <span class="identifier">IsMixed</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&gt;</span>
9970   <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
9971 <span class="special">{};</span>
9972
9973 <span class="keyword">namespace</span> <span class="identifier">MixedOps</span>
9974 <span class="special">{</span>
9975     <span class="comment">// This defines all the overloads to make expressions involving</span>
9976     <span class="comment">// std::vector to build expression templates.</span>
9977     <span class="identifier">BOOST_PROTO_DEFINE_OPERATORS</span><span class="special">(</span><span class="identifier">IsMixed</span><span class="special">,</span> <span class="identifier">MixedDomain</span><span class="special">)</span>
9978
9979     <span class="keyword">struct</span> <span class="identifier">assign_op</span>
9980     <span class="special">{</span>
9981         <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">U</span><span class="special">&gt;</span>
9982         <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">T</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">,</span> <span class="identifier">U</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">u</span><span class="special">)</span> <span class="keyword">const</span>
9983         <span class="special">{</span>
9984             <span class="identifier">t</span> <span class="special">=</span> <span class="identifier">u</span><span class="special">;</span>
9985         <span class="special">}</span>
9986     <span class="special">};</span>
9987
9988     <span class="keyword">struct</span> <span class="identifier">plus_assign_op</span>
9989     <span class="special">{</span>
9990         <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">U</span><span class="special">&gt;</span>
9991         <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">T</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">,</span> <span class="identifier">U</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">u</span><span class="special">)</span> <span class="keyword">const</span>
9992         <span class="special">{</span>
9993             <span class="identifier">t</span> <span class="special">+=</span> <span class="identifier">u</span><span class="special">;</span>
9994         <span class="special">}</span>
9995     <span class="special">};</span>
9996
9997     <span class="keyword">struct</span> <span class="identifier">minus_assign_op</span>
9998     <span class="special">{</span>
9999         <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">U</span><span class="special">&gt;</span>
10000         <span class="keyword">void</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">T</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">,</span> <span class="identifier">U</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">u</span><span class="special">)</span> <span class="keyword">const</span>
10001         <span class="special">{</span>
10002             <span class="identifier">t</span> <span class="special">-=</span> <span class="identifier">u</span><span class="special">;</span>
10003         <span class="special">}</span>
10004     <span class="special">};</span>
10005
10006     <span class="keyword">struct</span> <span class="identifier">sin_</span>
10007     <span class="special">{</span>
10008         <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
10009         <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>
10010
10011         <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">&gt;</span>
10012         <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Arg</span><span class="special">)&gt;</span>
10013           <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_const</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special">&lt;</span><span class="identifier">Arg</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span>
10014         <span class="special">{};</span>
10015
10016         <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">&gt;</span>
10017         <span class="identifier">Arg</span> <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a</span><span class="special">)</span> <span class="keyword">const</span>
10018         <span class="special">{</span>
10019             <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">sin</span><span class="special">(</span><span class="identifier">a</span><span class="special">);</span>
10020         <span class="special">}</span>
10021     <span class="special">};</span>
10022
10023     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">&gt;</span>
10024     <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>
10025         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>
10026       <span class="special">,</span> <span class="identifier">MixedDomain</span>
10027       <span class="special">,</span> <span class="identifier">sin_</span> <span class="keyword">const</span>
10028       <span class="special">,</span> <span class="identifier">A</span> <span class="keyword">const</span> <span class="special">&amp;</span>
10029     <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">sin</span><span class="special">(</span><span class="identifier">A</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a</span><span class="special">)</span>
10030     <span class="special">{</span>
10031         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span><span class="special">,</span> <span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">sin_</span><span class="special">(),</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">a</span><span class="special">));</span>
10032     <span class="special">}</span>
10033
10034     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">FwdIter</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Op</span><span class="special">&gt;</span>
10035     <span class="keyword">void</span> <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">FwdIter</span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">FwdIter</span> <span class="identifier">end</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">Op</span> <span class="identifier">op</span><span class="special">)</span>
10036     <span class="special">{</span>
10037         <span class="identifier">IncrementCtx</span> <span class="keyword">const</span> <span class="identifier">inc</span> <span class="special">=</span> <span class="special">{};</span>
10038         <span class="identifier">DereferenceCtx</span> <span class="keyword">const</span> <span class="identifier">deref</span> <span class="special">=</span> <span class="special">{};</span>
10039         <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">Begin</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;)&gt;::</span><span class="identifier">type</span> <span class="identifier">expr2</span> <span class="special">=</span> <span class="identifier">Begin</span><span class="special">()(</span><span class="identifier">expr</span><span class="special">);</span>
10040         <span class="keyword">for</span><span class="special">(;</span> <span class="identifier">begin</span> <span class="special">!=</span> <span class="identifier">end</span><span class="special">;</span> <span class="special">++</span><span class="identifier">begin</span><span class="special">)</span>
10041         <span class="special">{</span>
10042             <span class="identifier">op</span><span class="special">(*</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">,</span> <span class="identifier">deref</span><span class="special">));</span>
10043             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">expr2</span><span class="special">,</span> <span class="identifier">inc</span><span class="special">);</span>
10044         <span class="special">}</span>
10045     <span class="special">}</span>
10046
10047     <span class="comment">// Add-assign to a vector from some expression.</span>
10048     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
10049     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
10050     <span class="special">{</span>
10051         <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">assign_op</span><span class="special">());</span>
10052         <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
10053     <span class="special">}</span>
10054
10055     <span class="comment">// Add-assign to a list from some expression.</span>
10056     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
10057     <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
10058     <span class="special">{</span>
10059         <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">assign_op</span><span class="special">());</span>
10060         <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
10061     <span class="special">}</span>
10062
10063     <span class="comment">// Add-assign to a vector from some expression.</span>
10064     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
10065     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">+=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
10066     <span class="special">{</span>
10067         <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">plus_assign_op</span><span class="special">());</span>
10068         <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
10069     <span class="special">}</span>
10070
10071     <span class="comment">// Add-assign to a list from some expression.</span>
10072     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
10073     <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">+=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
10074     <span class="special">{</span>
10075         <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">plus_assign_op</span><span class="special">());</span>
10076         <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
10077     <span class="special">}</span>
10078
10079     <span class="comment">// Minus-assign to a vector from some expression.</span>
10080     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
10081     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">-=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
10082     <span class="special">{</span>
10083         <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">minus_assign_op</span><span class="special">());</span>
10084         <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
10085     <span class="special">}</span>
10086
10087     <span class="comment">// Minus-assign to a list from some expression.</span>
10088     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
10089     <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="keyword">operator</span> <span class="special">-=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">arr</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">expr</span><span class="special">)</span>
10090     <span class="special">{</span>
10091         <span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">arr</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">arr</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">as_expr</span><span class="special">&lt;</span><span class="identifier">MixedDomain</span><span class="special">&gt;(</span><span class="identifier">expr</span><span class="special">),</span> <span class="identifier">minus_assign_op</span><span class="special">());</span>
10092         <span class="keyword">return</span> <span class="identifier">arr</span><span class="special">;</span>
10093     <span class="special">}</span>
10094 <span class="special">}</span>
10095
10096 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
10097 <span class="special">{</span>
10098     <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">MixedOps</span><span class="special">;</span>
10099
10100     <span class="keyword">int</span> <span class="identifier">n</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
10101     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">a</span><span class="special">,</span><span class="identifier">b</span><span class="special">,</span><span class="identifier">c</span><span class="special">,</span><span class="identifier">d</span><span class="special">;</span>
10102     <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">e</span><span class="special">;</span>
10103     <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">f</span><span class="special">;</span>
10104
10105     <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
10106     <span class="keyword">for</span><span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span><span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
10107     <span class="special">{</span>
10108         <span class="identifier">a</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
10109         <span class="identifier">b</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">2</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span>
10110         <span class="identifier">c</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">3</span><span class="special">*</span><span class="identifier">i</span><span class="special">);</span>
10111         <span class="identifier">d</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
10112         <span class="identifier">e</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">0.0</span><span class="special">);</span>
10113         <span class="identifier">f</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="number">1.0</span><span class="special">,</span> <span class="number">1.0</span><span class="special">));</span>
10114     <span class="special">}</span>
10115
10116     <span class="identifier">MixedOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">b</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span>
10117     <span class="identifier">MixedOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span> <span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span> <span class="special">*</span> <span class="identifier">c</span><span class="special">);</span>
10118     <span class="identifier">a</span> <span class="special">+=</span> <span class="identifier">if_else</span><span class="special">(</span><span class="identifier">d</span> <span class="special">&lt;</span> <span class="number">30</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span>
10119
10120     <span class="identifier">MixedOps</span><span class="special">::</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">e</span><span class="special">,</span> <span class="identifier">c</span><span class="special">);</span>
10121     <span class="identifier">e</span> <span class="special">+=</span> <span class="identifier">e</span> <span class="special">-</span> <span class="number">4</span> <span class="special">/</span> <span class="special">(</span><span class="identifier">c</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span>
10122
10123     <span class="identifier">f</span> <span class="special">-=</span> <span class="identifier">sin</span><span class="special">(</span><span class="number">0.1</span> <span class="special">*</span> <span class="identifier">e</span> <span class="special">*</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="number">0.2</span><span class="special">,</span> <span class="number">1.2</span><span class="special">));</span>
10124
10125     <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</span><span class="identifier">const_iterator</span> <span class="identifier">ei</span> <span class="special">=</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span>
10126     <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">const_iterator</span> <span class="identifier">fi</span> <span class="special">=</span> <span class="identifier">f</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span>
10127     <span class="keyword">for</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special">&lt;</span> <span class="identifier">n</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
10128     <span class="special">{</span>
10129         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span>
10130             <span class="special">&lt;&lt;</span> <span class="string">"a("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">a</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
10131             <span class="special">&lt;&lt;</span> <span class="string">" b("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">b</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
10132             <span class="special">&lt;&lt;</span> <span class="string">" c("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">c</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
10133             <span class="special">&lt;&lt;</span> <span class="string">" d("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">d</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
10134             <span class="special">&lt;&lt;</span> <span class="string">" e("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="special">*</span><span class="identifier">ei</span><span class="special">++</span>
10135             <span class="special">&lt;&lt;</span> <span class="string">" f("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">") = "</span> <span class="special">&lt;&lt;</span> <span class="special">*</span><span class="identifier">fi</span><span class="special">++</span>
10136             <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
10137     <span class="special">}</span>
10138 <span class="special">}</span>
10139 </pre>
10140 <p>
10141         </p>
10142 </div>
10143 <div class="section">
10144 <div class="titlepage"><div><div><h4 class="title">
10145 <a name="boost_proto.users_guide.examples.map_assign"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.map_assign" title="Map Assign: An Intermediate Transform">Map Assign:
10146         An Intermediate Transform</a>
10147 </h4></div></div></div>
10148 <p>
10149           A demonstration of how to implement <code class="computeroutput"><span class="identifier">map_list_of</span><span class="special">()</span></code> from the Boost.Assign library using Proto.
10150           <code class="computeroutput"><span class="identifier">map_list_assign</span><span class="special">()</span></code>
10151           is used to conveniently initialize a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;&gt;</span></code>. By using Proto, we can avoid any
10152           dynamic allocation while building the intermediate representation.
10153         </p>
10154 <p>
10155 </p>
10156 <pre class="programlisting"><span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
10157 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
10158 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
10159 <span class="comment">//</span>
10160 <span class="comment">// This is a port of map_list_of() from the Boost.Assign library.</span>
10161 <span class="comment">// It has the advantage of being more efficient at runtime by not</span>
10162 <span class="comment">// building any temporary container that requires dynamic allocation.</span>
10163
10164 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">map</span><span class="special">&gt;</span>
10165 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span>
10166 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
10167 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10168 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10169 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">add_reference</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10170 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
10171 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>
10172
10173 <span class="keyword">struct</span> <span class="identifier">map_list_of_tag</span>
10174 <span class="special">{};</span>
10175
10176 <span class="comment">// A simple callable function object that inserts a</span>
10177 <span class="comment">// (key,value) pair into a map.</span>
10178 <span class="keyword">struct</span> <span class="identifier">insert</span>
10179   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
10180 <span class="special">{</span>
10181     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
10182     <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>
10183
10184     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Map</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Value</span><span class="special">&gt;</span>
10185     <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Map</span><span class="special">,</span> <span class="identifier">Key</span><span class="special">,</span> <span class="identifier">Value</span><span class="special">)&gt;</span>
10186       <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_reference</span><span class="special">&lt;</span><span class="identifier">Map</span><span class="special">&gt;</span>
10187     <span class="special">{};</span>
10188
10189     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Map</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Value</span><span class="special">&gt;</span>
10190     <span class="identifier">Map</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Map</span> <span class="special">&amp;</span><span class="identifier">map</span><span class="special">,</span> <span class="identifier">Key</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">key</span><span class="special">,</span> <span class="identifier">Value</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">value</span><span class="special">)</span> <span class="keyword">const</span>
10191     <span class="special">{</span>
10192         <span class="identifier">map</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">Map</span><span class="special">::</span><span class="identifier">value_type</span><span class="special">(</span><span class="identifier">key</span><span class="special">,</span> <span class="identifier">value</span><span class="special">));</span>
10193         <span class="keyword">return</span> <span class="identifier">map</span><span class="special">;</span>
10194     <span class="special">}</span>
10195 <span class="special">};</span>
10196
10197 <span class="comment">// Work-arounds for Microsoft Visual C++ 7.1</span>
10198 <span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span>
10199 <span class="preprocessor">#define</span> <span class="identifier">MapListOf</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">MapListOf</span><span class="special">(</span><span class="identifier">x</span><span class="special">)&gt;</span>
10200 <span class="preprocessor">#define</span> <span class="identifier">_value</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">x</span><span class="special">)&gt;</span>
10201 <span class="preprocessor">#endif</span>
10202
10203 <span class="comment">// The grammar for valid map-list expressions, and a</span>
10204 <span class="comment">// transform that populates the map.</span>
10205 <span class="keyword">struct</span> <span class="identifier">MapListOf</span>
10206   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
10207         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10208             <span class="comment">// map_list_of(a,b)</span>
10209             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span>
10210                 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">map_list_of_tag</span><span class="special">&gt;</span>
10211               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
10212               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
10213             <span class="special">&gt;</span>
10214           <span class="special">,</span> <span class="identifier">insert</span><span class="special">(</span>
10215                 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_data</span>
10216               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child1</span><span class="special">)</span>
10217               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child2</span><span class="special">)</span>
10218             <span class="special">)</span>
10219         <span class="special">&gt;</span>
10220       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10221             <span class="comment">// map_list_of(a,b)(c,d)...</span>
10222             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span>
10223                 <span class="identifier">MapListOf</span>
10224               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
10225               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
10226             <span class="special">&gt;</span>
10227           <span class="special">,</span> <span class="identifier">insert</span><span class="special">(</span>
10228                 <span class="identifier">MapListOf</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child0</span><span class="special">)</span>
10229               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child1</span><span class="special">)</span>
10230               <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_child2</span><span class="special">)</span>
10231             <span class="special">)</span>
10232         <span class="special">&gt;</span>
10233     <span class="special">&gt;</span>
10234 <span class="special">{};</span>
10235
10236 <span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span>
10237 <span class="preprocessor">#undef</span> <span class="identifier">MapListOf</span>
10238 <span class="preprocessor">#undef</span> <span class="identifier">_value</span>
10239 <span class="preprocessor">#endif</span>
10240
10241 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
10242 <span class="keyword">struct</span> <span class="identifier">map_list_of_expr</span><span class="special">;</span>
10243
10244 <span class="keyword">struct</span> <span class="identifier">map_list_of_dom</span>
10245   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special">&lt;</span><span class="identifier">map_list_of_expr</span><span class="special">&gt;,</span> <span class="identifier">MapListOf</span><span class="special">&gt;</span>
10246 <span class="special">{};</span>
10247
10248 <span class="comment">// An expression wrapper that provides a conversion to a</span>
10249 <span class="comment">// map that uses the MapListOf</span>
10250 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
10251 <span class="keyword">struct</span> <span class="identifier">map_list_of_expr</span>
10252 <span class="special">{</span>
10253     <span class="identifier">BOOST_PROTO_BASIC_EXTENDS</span><span class="special">(</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">map_list_of_expr</span><span class="special">,</span> <span class="identifier">map_list_of_dom</span><span class="special">)</span>
10254     <span class="identifier">BOOST_PROTO_EXTENDS_FUNCTION</span><span class="special">()</span>
10255
10256     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Value</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Cmp</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Al</span><span class="special">&gt;</span>
10257     <span class="keyword">operator</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">Key</span><span class="special">,</span> <span class="identifier">Value</span><span class="special">,</span> <span class="identifier">Cmp</span><span class="special">,</span> <span class="identifier">Al</span><span class="special">&gt;</span> <span class="special">()</span> <span class="keyword">const</span>
10258     <span class="special">{</span>
10259         <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">MapListOf</span><span class="special">&gt;));</span>
10260         <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">Key</span><span class="special">,</span> <span class="identifier">Value</span><span class="special">,</span> <span class="identifier">Cmp</span><span class="special">,</span> <span class="identifier">Al</span><span class="special">&gt;</span> <span class="identifier">map</span><span class="special">;</span>
10261         <span class="keyword">return</span> <span class="identifier">MapListOf</span><span class="special">()(*</span><span class="keyword">this</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">map</span><span class="special">);</span>
10262     <span class="special">}</span>
10263 <span class="special">};</span>
10264
10265 <span class="identifier">map_list_of_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">map_list_of_tag</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">map_list_of</span> <span class="special">=</span> <span class="special">{{{}}};</span>
10266
10267 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
10268 <span class="special">{</span>
10269     <span class="comment">// Initialize a map:</span>
10270     <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">op</span> <span class="special">=</span>
10271         <span class="identifier">map_list_of</span>
10272             <span class="special">(</span><span class="string">"&lt;"</span><span class="special">,</span> <span class="number">1</span><span class="special">)</span>
10273             <span class="special">(</span><span class="string">"&lt;="</span><span class="special">,</span><span class="number">2</span><span class="special">)</span>
10274             <span class="special">(</span><span class="string">"&gt;"</span><span class="special">,</span> <span class="number">3</span><span class="special">)</span>
10275             <span class="special">(</span><span class="string">"&gt;="</span><span class="special">,</span><span class="number">4</span><span class="special">)</span>
10276             <span class="special">(</span><span class="string">"="</span><span class="special">,</span> <span class="number">5</span><span class="special">)</span>
10277             <span class="special">(</span><span class="string">"&lt;&gt;"</span><span class="special">,</span><span class="number">6</span><span class="special">)</span>
10278         <span class="special">;</span>
10279
10280     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"&lt;\"  --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"&lt;"</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
10281     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"&lt;=\" --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"&lt;="</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
10282     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"&gt;\"  --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"&gt;"</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
10283     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"&gt;=\" --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"&gt;="</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
10284     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"=\"  --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"="</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
10285     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"\"&lt;&gt;\" --&gt; "</span> <span class="special">&lt;&lt;</span> <span class="identifier">op</span><span class="special">[</span><span class="string">"&lt;&gt;"</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
10286
10287     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
10288 <span class="special">}</span>
10289 </pre>
10290 <p>
10291         </p>
10292 </div>
10293 <div class="section">
10294 <div class="titlepage"><div><div><h4 class="title">
10295 <a name="boost_proto.users_guide.examples.future_group"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.future_group" title="Future Group: A More Advanced Transform">Future
10296         Group: A More Advanced Transform</a>
10297 </h4></div></div></div>
10298 <p>
10299           An advanced example of a Proto transform that implements Howard Hinnant's
10300           design for <span class="emphasis"><em>future groups</em></span> that block for all or some
10301           asynchronous operations to complete and returns their results in a tuple
10302           of the appropriate type.
10303         </p>
10304 <p>
10305 </p>
10306 <pre class="programlisting"><span class="comment">//  Copyright 2008 Eric Niebler. Distributed under the Boost</span>
10307 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
10308 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
10309 <span class="comment">//</span>
10310 <span class="comment">// This is an example of using Proto transforms to implement</span>
10311 <span class="comment">// Howard Hinnant's future group proposal.</span>
10312
10313 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10314 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">as_vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10315 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">joint_view</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10316 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">single_view</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10317 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10318 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10319 <span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
10320 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
10321 <span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span>
10322 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>
10323
10324 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">L</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">&gt;</span>
10325 <span class="keyword">struct</span> <span class="identifier">pick_left</span>
10326 <span class="special">{</span>
10327     <span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span><span class="identifier">L</span><span class="special">,</span> <span class="identifier">R</span><span class="special">&gt;));</span>
10328     <span class="keyword">typedef</span> <span class="identifier">L</span> <span class="identifier">type</span><span class="special">;</span>
10329 <span class="special">};</span>
10330
10331 <span class="comment">// Work-arounds for Microsoft Visual C++ 7.1</span>
10332 <span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span>
10333 <span class="preprocessor">#define</span> <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">x</span><span class="special">)&gt;</span>
10334 <span class="preprocessor">#endif</span>
10335
10336 <span class="comment">// Define the grammar of future group expression, as well as a</span>
10337 <span class="comment">// transform to turn them into a Fusion sequence of the correct</span>
10338 <span class="comment">// type.</span>
10339 <span class="keyword">struct</span> <span class="identifier">FutureGroup</span>
10340   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
10341         <span class="comment">// terminals become a single-element Fusion sequence</span>
10342         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10343             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
10344           <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">single_view</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">&gt;(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
10345         <span class="special">&gt;</span>
10346         <span class="comment">// (a &amp;&amp; b) becomes a concatenation of the sequence</span>
10347         <span class="comment">// from 'a' and the one from 'b':</span>
10348       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10349             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_and</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">,</span> <span class="identifier">FutureGroup</span><span class="special">&gt;</span>
10350           <span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">joint_view</span><span class="special">&lt;</span>
10351                 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">)</span> <span class="special">&gt;</span>
10352               <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">add_const</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)</span> <span class="special">&gt;</span>
10353             <span class="special">&gt;(</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span> <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">))</span>
10354         <span class="special">&gt;</span>
10355         <span class="comment">// (a || b) becomes the sequence for 'a', so long</span>
10356         <span class="comment">// as it is the same as the sequence for 'b'.</span>
10357       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10358             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">logical_or</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">,</span> <span class="identifier">FutureGroup</span><span class="special">&gt;</span>
10359           <span class="special">,</span> <span class="identifier">pick_left</span><span class="special">&lt;</span>
10360                 <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">)</span>
10361               <span class="special">,</span> <span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">)</span>
10362             <span class="special">&gt;(</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">))</span>
10363         <span class="special">&gt;</span>
10364     <span class="special">&gt;</span>
10365 <span class="special">{};</span>
10366
10367 <span class="preprocessor">#if</span> <span class="identifier">BOOST_WORKAROUND</span><span class="special">(</span><span class="identifier">BOOST_MSVC</span><span class="special">,</span> <span class="special">==</span> <span class="number">1310</span><span class="special">)</span>
10368 <span class="preprocessor">#undef</span> <span class="identifier">FutureGroup</span>
10369 <span class="preprocessor">#endif</span>
10370
10371 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">E</span><span class="special">&gt;</span>
10372 <span class="keyword">struct</span> <span class="identifier">future_expr</span><span class="special">;</span>
10373
10374 <span class="keyword">struct</span> <span class="identifier">future_dom</span>
10375   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">future_expr</span><span class="special">&gt;,</span> <span class="identifier">FutureGroup</span><span class="special">&gt;</span>
10376 <span class="special">{};</span>
10377
10378 <span class="comment">// Expressions in the future group domain have a .get()</span>
10379 <span class="comment">// member function that (ostensibly) blocks for the futures</span>
10380 <span class="comment">// to complete and returns the results in an appropriate</span>
10381 <span class="comment">// tuple.</span>
10382 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">E</span><span class="special">&gt;</span>
10383 <span class="keyword">struct</span> <span class="identifier">future_expr</span>
10384   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">future_expr</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">&gt;,</span> <span class="identifier">future_dom</span><span class="special">&gt;</span>
10385 <span class="special">{</span>
10386     <span class="keyword">explicit</span> <span class="identifier">future_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">e</span><span class="special">)</span>
10387       <span class="special">:</span> <span class="identifier">future_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span>
10388     <span class="special">{}</span>
10389
10390     <span class="keyword">typename</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">as_vector</span><span class="special">&lt;</span>
10391         <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">FutureGroup</span><span class="special">(</span><span class="identifier">E</span><span class="special">)&gt;::</span><span class="identifier">type</span>
10392     <span class="special">&gt;::</span><span class="identifier">type</span>
10393     <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span>
10394     <span class="special">{</span>
10395         <span class="keyword">return</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">as_vector</span><span class="special">(</span><span class="identifier">FutureGroup</span><span class="special">()(*</span><span class="keyword">this</span><span class="special">));</span>
10396     <span class="special">}</span>
10397 <span class="special">};</span>
10398
10399 <span class="comment">// The future&lt;&gt; type has an even simpler .get()</span>
10400 <span class="comment">// member function.</span>
10401 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
10402 <span class="keyword">struct</span> <span class="identifier">future</span>
10403   <span class="special">:</span> <span class="identifier">future_expr</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span>
10404 <span class="special">{</span>
10405     <span class="identifier">future</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">t</span> <span class="special">=</span> <span class="identifier">T</span><span class="special">())</span>
10406       <span class="special">:</span> <span class="identifier">future</span><span class="special">::</span><span class="identifier">proto_derived_expr</span><span class="special">(</span><span class="identifier">future</span><span class="special">::</span><span class="identifier">proto_base_expr</span><span class="special">::</span><span class="identifier">make</span><span class="special">(</span><span class="identifier">t</span><span class="special">))</span>
10407     <span class="special">{}</span>
10408
10409     <span class="identifier">T</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span>
10410     <span class="special">{</span>
10411         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">value</span><span class="special">(*</span><span class="keyword">this</span><span class="special">);</span>
10412     <span class="special">}</span>
10413 <span class="special">};</span>
10414
10415 <span class="comment">// TEST CASES</span>
10416 <span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{};</span>
10417 <span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{};</span>
10418 <span class="keyword">struct</span> <span class="identifier">C</span> <span class="special">{};</span>
10419
10420 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
10421 <span class="special">{</span>
10422     <span class="keyword">using</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">vector</span><span class="special">;</span>
10423     <span class="identifier">future</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span> <span class="identifier">a</span><span class="special">;</span>
10424     <span class="identifier">future</span><span class="special">&lt;</span><span class="identifier">B</span><span class="special">&gt;</span> <span class="identifier">b</span><span class="special">;</span>
10425     <span class="identifier">future</span><span class="special">&lt;</span><span class="identifier">C</span><span class="special">&gt;</span> <span class="identifier">c</span><span class="special">;</span>
10426     <span class="identifier">future</span><span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">ab</span><span class="special">;</span>
10427
10428     <span class="comment">// Verify that various future groups have the</span>
10429     <span class="comment">// correct return types.</span>
10430     <span class="identifier">A</span>                       <span class="identifier">t0</span> <span class="special">=</span> <span class="identifier">a</span><span class="special">.</span><span class="identifier">get</span><span class="special">();</span>
10431     <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">&gt;</span>         <span class="identifier">t1</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&amp;&amp;</span> <span class="identifier">b</span> <span class="special">&amp;&amp;</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span>
10432     <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">C</span><span class="special">&gt;</span>            <span class="identifier">t2</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">a</span> <span class="special">||</span> <span class="identifier">a</span><span class="special">)</span> <span class="special">&amp;&amp;</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span>
10433     <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">&gt;</span>         <span class="identifier">t3</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">a</span> <span class="special">&amp;&amp;</span> <span class="identifier">b</span> <span class="special">||</span> <span class="identifier">a</span> <span class="special">&amp;&amp;</span> <span class="identifier">b</span><span class="special">)</span> <span class="special">&amp;&amp;</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span>
10434     <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;,</span> <span class="identifier">C</span><span class="special">&gt;</span> <span class="identifier">t4</span> <span class="special">=</span> <span class="special">((</span><span class="identifier">ab</span> <span class="special">||</span> <span class="identifier">ab</span><span class="special">)</span> <span class="special">&amp;&amp;</span> <span class="identifier">c</span><span class="special">).</span><span class="identifier">get</span><span class="special">();</span>
10435
10436     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
10437 <span class="special">}</span>
10438 </pre>
10439 <p>
10440         </p>
10441 </div>
10442 <div class="section">
10443 <div class="titlepage"><div><div><h4 class="title">
10444 <a name="boost_proto.users_guide.examples.lambda"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.lambda" title="Lambda: A Simple Lambda Library with Proto">Lambda: A
10445         Simple Lambda Library with Proto</a>
10446 </h4></div></div></div>
10447 <p>
10448           This is an advanced example that shows how to implement a simple lambda
10449           EDSL with Proto, like the Boost.Lambda_library. It uses contexts, transforms
10450           and expression extension.
10451         </p>
10452 <p>
10453 </p>
10454 <pre class="programlisting"><span class="comment">///////////////////////////////////////////////////////////////////////////////</span>
10455 <span class="comment">// Copyright 2008 Eric Niebler. Distributed under the Boost</span>
10456 <span class="comment">// Software License, Version 1.0. (See accompanying file</span>
10457 <span class="comment">// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
10458 <span class="comment">//</span>
10459 <span class="comment">// This example builds a simple but functional lambda library using Proto.</span>
10460
10461 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
10462 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">algorithm</span><span class="special">&gt;</span>
10463 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10464 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">min_max</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10465 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">eval_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10466 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">identity</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10467 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">next_prior</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10468 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">tuple</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10469 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10470 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">ostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10471 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">iostream</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10472 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10473 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">context</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10474 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">transform</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10475 <span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
10476 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
10477 <span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span>
10478 <span class="keyword">using</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">;</span>
10479
10480 <span class="comment">// Forward declaration of the lambda expression wrapper</span>
10481 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
10482 <span class="keyword">struct</span> <span class="identifier">lambda</span><span class="special">;</span>
10483
10484 <span class="keyword">struct</span> <span class="identifier">lambda_domain</span>
10485   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">pod_generator</span><span class="special">&lt;</span><span class="identifier">lambda</span><span class="special">&gt;</span> <span class="special">&gt;</span>
10486 <span class="special">{};</span>
10487
10488 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span>
10489 <span class="keyword">struct</span> <span class="identifier">placeholder</span>
10490 <span class="special">{</span>
10491     <span class="keyword">typedef</span> <span class="identifier">I</span> <span class="identifier">arity</span><span class="special">;</span>
10492 <span class="special">};</span>
10493
10494 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
10495 <span class="keyword">struct</span> <span class="identifier">placeholder_arity</span>
10496 <span class="special">{</span>
10497     <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">T</span><span class="special">::</span><span class="identifier">arity</span> <span class="identifier">type</span><span class="special">;</span>
10498 <span class="special">};</span>
10499
10500 <span class="comment">// The lambda grammar, with the transforms for calculating the max arity</span>
10501 <span class="keyword">struct</span> <span class="identifier">lambda_arity</span>
10502   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
10503         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10504             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
10505           <span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">next</span><span class="special">&lt;</span><span class="identifier">placeholder_arity</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">&gt;</span> <span class="special">&gt;()</span>
10506         <span class="special">&gt;</span>
10507       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span>
10508           <span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;()</span>
10509         <span class="special">&gt;</span>
10510       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10511             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">nary_expr</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">vararg</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
10512           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">fold</span><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(),</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">max</span><span class="special">&lt;</span><span class="identifier">lambda_arity</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">&gt;()&gt;</span>
10513         <span class="special">&gt;</span>
10514     <span class="special">&gt;</span>
10515 <span class="special">{};</span>
10516
10517 <span class="comment">// The lambda context is the same as the default context</span>
10518 <span class="comment">// with the addition of special handling for lambda placeholders</span>
10519 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tuple</span><span class="special">&gt;</span>
10520 <span class="keyword">struct</span> <span class="identifier">lambda_context</span>
10521   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable_context</span><span class="special">&lt;</span><span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">Tuple</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&gt;</span>
10522 <span class="special">{</span>
10523     <span class="identifier">lambda_context</span><span class="special">(</span><span class="identifier">Tuple</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">args</span><span class="special">)</span>
10524       <span class="special">:</span> <span class="identifier">args_</span><span class="special">(</span><span class="identifier">args</span><span class="special">)</span>
10525     <span class="special">{}</span>
10526
10527     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
10528     <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>
10529
10530     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span>
10531     <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;)&gt;</span>
10532       <span class="special">:</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at</span><span class="special">&lt;</span><span class="identifier">Tuple</span><span class="special">,</span> <span class="identifier">I</span><span class="special">&gt;</span>
10533     <span class="special">{};</span>
10534
10535     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span>
10536     <span class="keyword">typename</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at</span><span class="special">&lt;</span><span class="identifier">Tuple</span><span class="special">,</span> <span class="identifier">I</span><span class="special">&gt;::</span><span class="identifier">type</span>
10537     <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">,</span> <span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
10538     <span class="special">{</span>
10539         <span class="keyword">return</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">at</span><span class="special">&lt;</span><span class="identifier">I</span><span class="special">&gt;(</span><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">args_</span><span class="special">);</span>
10540     <span class="special">}</span>
10541
10542     <span class="identifier">Tuple</span> <span class="identifier">args_</span><span class="special">;</span>
10543 <span class="special">};</span>
10544
10545 <span class="comment">// The lambda&lt;&gt; expression wrapper makes expressions polymorphic</span>
10546 <span class="comment">// function objects</span>
10547 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
10548 <span class="keyword">struct</span> <span class="identifier">lambda</span>
10549 <span class="special">{</span>
10550     <span class="identifier">BOOST_PROTO_BASIC_EXTENDS</span><span class="special">(</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">lambda</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;,</span> <span class="identifier">lambda_domain</span><span class="special">)</span>
10551     <span class="identifier">BOOST_PROTO_EXTENDS_ASSIGN</span><span class="special">()</span>
10552     <span class="identifier">BOOST_PROTO_EXTENDS_SUBSCRIPT</span><span class="special">()</span>
10553
10554     <span class="comment">// Calculate the arity of this lambda expression</span>
10555     <span class="keyword">static</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="identifier">arity</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">lambda_arity</span><span class="special">(</span><span class="identifier">T</span><span class="special">)&gt;::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span><span class="special">;</span>
10556
10557     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
10558     <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>
10559
10560     <span class="comment">// Define nested result&lt;&gt; specializations to calculate the return</span>
10561     <span class="comment">// type of this lambda expression. But be careful not to evaluate</span>
10562     <span class="comment">// the return type of the nullary function unless we have a nullary</span>
10563     <span class="comment">// lambda!</span>
10564     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">&gt;</span>
10565     <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">()&gt;</span>
10566       <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">eval_if_c</span><span class="special">&lt;</span>
10567             <span class="number">0</span> <span class="special">==</span> <span class="identifier">arity</span>
10568           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
10569           <span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">identity</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;</span>
10570         <span class="special">&gt;</span>
10571     <span class="special">{};</span>
10572
10573     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">&gt;</span>
10574     <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">A0</span><span class="special">)&gt;</span>
10575       <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
10576     <span class="special">{};</span>
10577
10578     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">&gt;</span>
10579     <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span> <span class="identifier">A1</span><span class="special">)&gt;</span>
10580       <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">eval</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span><span class="special">,</span> <span class="identifier">A1</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
10581     <span class="special">{};</span>
10582
10583     <span class="comment">// Define our operator () that evaluates the lambda expression.</span>
10584     <span class="keyword">typename</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">lambda</span><span class="special">()&gt;::</span><span class="identifier">type</span>
10585     <span class="keyword">operator</span> <span class="special">()()</span> <span class="keyword">const</span>
10586     <span class="special">{</span>
10587         <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span> <span class="identifier">args</span><span class="special">;</span>
10588         <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span> <span class="special">&gt;</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">args</span><span class="special">);</span>
10589         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
10590     <span class="special">}</span>
10591
10592     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">&gt;</span>
10593     <span class="keyword">typename</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">lambda</span><span class="special">(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;)&gt;::</span><span class="identifier">type</span>
10594     <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a0</span><span class="special">)</span> <span class="keyword">const</span>
10595     <span class="special">{</span>
10596         <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;&gt;</span> <span class="identifier">args</span><span class="special">(</span><span class="identifier">a0</span><span class="special">);</span>
10597         <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;&gt;</span> <span class="special">&gt;</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">args</span><span class="special">);</span>
10598         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
10599     <span class="special">}</span>
10600
10601     <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">&gt;</span>
10602     <span class="keyword">typename</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">lambda</span><span class="special">(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;)&gt;::</span><span class="identifier">type</span>
10603     <span class="keyword">operator</span> <span class="special">()(</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a1</span><span class="special">)</span> <span class="keyword">const</span>
10604     <span class="special">{</span>
10605         <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;&gt;</span> <span class="identifier">args</span><span class="special">(</span><span class="identifier">a0</span><span class="special">,</span> <span class="identifier">a1</span><span class="special">);</span>
10606         <span class="identifier">lambda_context</span><span class="special">&lt;</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A0</span> <span class="keyword">const</span> <span class="special">&amp;,</span> <span class="identifier">A1</span> <span class="keyword">const</span> <span class="special">&amp;&gt;</span> <span class="special">&gt;</span> <span class="identifier">ctx</span><span class="special">(</span><span class="identifier">args</span><span class="special">);</span>
10607         <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
10608     <span class="special">}</span>
10609 <span class="special">};</span>
10610
10611 <span class="comment">// Define some lambda placeholders</span>
10612 <span class="identifier">lambda</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{{}};</span>
10613 <span class="identifier">lambda</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{{}};</span>
10614
10615 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
10616 <span class="identifier">lambda</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">val</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">)</span>
10617 <span class="special">{</span>
10618     <span class="identifier">lambda</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">that</span> <span class="special">=</span> <span class="special">{{</span><span class="identifier">t</span><span class="special">}};</span>
10619     <span class="keyword">return</span> <span class="identifier">that</span><span class="special">;</span>
10620 <span class="special">}</span>
10621
10622 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
10623 <span class="identifier">lambda</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">var</span><span class="special">(</span><span class="identifier">T</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">)</span>
10624 <span class="special">{</span>
10625     <span class="identifier">lambda</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">that</span> <span class="special">=</span> <span class="special">{{</span><span class="identifier">t</span><span class="special">}};</span>
10626     <span class="keyword">return</span> <span class="identifier">that</span><span class="special">;</span>
10627 <span class="special">}</span>
10628
10629 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
10630 <span class="keyword">struct</span> <span class="identifier">construct_helper</span>
10631 <span class="special">{</span>
10632     <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">result_type</span><span class="special">;</span> <span class="comment">// for TR1 result_of</span>
10633
10634     <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span>
10635     <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">T</span><span class="special">();</span> <span class="special">}</span>
10636
10637     <span class="comment">// Generate BOOST_PROTO_MAX_ARITY overloads of the</span>
10638     <span class="comment">// following function call operator.</span>
10639 <span class="preprocessor">#define</span> <span class="identifier">BOOST_PROTO_LOCAL_MACRO</span><span class="special">(</span><span class="identifier">N</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">,</span> <span class="identifier">A_const_ref_a</span><span class="special">,</span> <span class="identifier">a</span><span class="special">)\</span>
10640     <span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">typename_A</span><span class="special">(</span><span class="identifier">N</span><span class="special">)&gt;</span>                                       <span class="special">\</span>
10641     <span class="identifier">T</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">A_const_ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">))</span> <span class="keyword">const</span>                          <span class="special">\</span>
10642     <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">a</span><span class="special">(</span><span class="identifier">N</span><span class="special">));</span> <span class="special">}</span>
10643 <span class="preprocessor">#define</span> <span class="identifier">BOOST_PROTO_LOCAL_a</span> <span class="identifier">BOOST_PROTO_a</span>
10644 <span class="preprocessor">#include</span> <span class="identifier">BOOST_PROTO_LOCAL_ITERATE</span><span class="special">()</span>
10645 <span class="special">};</span>
10646
10647 <span class="comment">// Generate BOOST_PROTO_MAX_ARITY-1 overloads of the</span>
10648 <span class="comment">// following construct() function template.</span>
10649 <span class="preprocessor">#define</span> <span class="identifier">M0</span><span class="special">(</span><span class="identifier">N</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">,</span> <span class="identifier">A_const_ref_a</span><span class="special">,</span> <span class="identifier">ref_a</span><span class="special">)</span>      <span class="special">\</span>
10650 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">typename_A</span><span class="special">(</span><span class="identifier">N</span><span class="special">)&gt;</span>                               <span class="special">\</span>
10651 <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>                             <span class="special">\</span>
10652     <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>                                          <span class="special">\</span>
10653   <span class="special">,</span> <span class="identifier">lambda_domain</span>                                                 <span class="special">\</span>
10654   <span class="special">,</span> <span class="identifier">construct_helper</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>                                           <span class="special">\</span>
10655   <span class="special">,</span> <span class="identifier">A_const_ref</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span>                                                <span class="special">\</span>
10656 <span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span>                                                     <span class="special">\</span>
10657 <span class="identifier">construct</span><span class="special">(</span><span class="identifier">A_const_ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">))</span>                                       <span class="special">\</span>
10658 <span class="special">{</span>                                                                 <span class="special">\</span>
10659     <span class="keyword">return</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special">&lt;</span>                                      <span class="special">\</span>
10660         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">function</span>                                      <span class="special">\</span>
10661       <span class="special">,</span> <span class="identifier">lambda_domain</span>                                             <span class="special">\</span>
10662     <span class="special">&gt;(</span>                                                            <span class="special">\</span>
10663         <span class="identifier">construct_helper</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;()</span>                                     <span class="special">\</span>
10664       <span class="special">,</span> <span class="identifier">ref_a</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span>                                                  <span class="special">\</span>
10665     <span class="special">);</span>                                                            <span class="special">\</span>
10666 <span class="special">}</span>
10667 <span class="identifier">BOOST_PROTO_REPEAT_FROM_TO</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="identifier">BOOST_PROTO_MAX_ARITY</span><span class="special">,</span> <span class="identifier">M0</span><span class="special">)</span>
10668 <span class="preprocessor">#undef</span> <span class="identifier">M0</span>
10669
10670 <span class="keyword">struct</span> <span class="identifier">S</span>
10671 <span class="special">{</span>
10672     <span class="identifier">S</span><span class="special">()</span> <span class="special">{}</span>
10673     <span class="identifier">S</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">c</span><span class="special">)</span>
10674     <span class="special">{</span>
10675         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"S("</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="string">","</span> <span class="special">&lt;&lt;</span> <span class="identifier">c</span> <span class="special">&lt;&lt;</span> <span class="string">")\n"</span><span class="special">;</span>
10676     <span class="special">}</span>
10677 <span class="special">};</span>
10678
10679 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
10680 <span class="special">{</span>
10681     <span class="comment">// Create some lambda objects and immediately</span>
10682     <span class="comment">// invoke them by applying their operator():</span>
10683     <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span><span class="special">)</span> <span class="special">/</span> <span class="number">4</span> <span class="special">)(</span><span class="number">42</span><span class="special">);</span>
10684     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 11</span>
10685
10686     <span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="special">(</span> <span class="special">(-(</span><span class="identifier">_1</span> <span class="special">+</span> <span class="number">2</span><span class="special">))</span> <span class="special">/</span> <span class="number">4</span> <span class="special">)(</span><span class="number">42</span><span class="special">);</span>
10687     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">j</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints -11</span>
10688
10689     <span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="special">(</span> <span class="special">(</span><span class="number">4</span> <span class="special">-</span> <span class="identifier">_2</span><span class="special">)</span> <span class="special">*</span> <span class="number">3</span> <span class="special">)(</span><span class="number">42</span><span class="special">,</span> <span class="number">3.14</span><span class="special">);</span>
10690     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">d</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 2.58</span>
10691
10692     <span class="comment">// check non-const ref terminals</span>
10693     <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">_1</span> <span class="special">&lt;&lt;</span> <span class="string">" -- "</span> <span class="special">&lt;&lt;</span> <span class="identifier">_2</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span><span class="special">)(</span><span class="number">42</span><span class="special">,</span> <span class="string">"Life, the Universe and Everything!"</span><span class="special">);</span>
10694     <span class="comment">// prints "42 -- Life, the Universe and Everything!"</span>
10695
10696     <span class="comment">// "Nullary" lambdas work too</span>
10697     <span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">val</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">+</span> <span class="identifier">val</span><span class="special">(</span><span class="number">2</span><span class="special">))();</span>
10698     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">k</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 3</span>
10699
10700     <span class="comment">// check array indexing for kicks</span>
10701     <span class="keyword">int</span> <span class="identifier">integers</span><span class="special">[</span><span class="number">5</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">};</span>
10702     <span class="special">(</span><span class="identifier">var</span><span class="special">(</span><span class="identifier">integers</span><span class="special">)[</span><span class="number">2</span><span class="special">]</span> <span class="special">=</span> <span class="number">2</span><span class="special">)();</span>
10703     <span class="special">(</span><span class="identifier">var</span><span class="special">(</span><span class="identifier">integers</span><span class="special">)[</span><span class="identifier">_1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">_1</span><span class="special">)(</span><span class="number">3</span><span class="special">);</span>
10704     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">integers</span><span class="special">[</span><span class="number">2</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 2</span>
10705     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">integers</span><span class="special">[</span><span class="number">3</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// prints 3</span>
10706
10707     <span class="comment">// Now use a lambda with an STL algorithm!</span>
10708     <span class="keyword">int</span> <span class="identifier">rgi</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">};</span>
10709     <span class="keyword">char</span> <span class="identifier">rgc</span><span class="special">[</span><span class="number">4</span><span class="special">]</span> <span class="special">=</span> <span class="special">{</span><span class="char">'a'</span><span class="special">,</span><span class="char">'b'</span><span class="special">,</span><span class="char">'c'</span><span class="special">,</span><span class="char">'d'</span><span class="special">};</span>
10710     <span class="identifier">S</span> <span class="identifier">rgs</span><span class="special">[</span><span class="number">4</span><span class="special">];</span>
10711
10712     <span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">rgi</span><span class="special">,</span> <span class="identifier">rgi</span><span class="special">+</span><span class="number">4</span><span class="special">,</span> <span class="identifier">rgc</span><span class="special">,</span> <span class="identifier">rgs</span><span class="special">,</span> <span class="identifier">construct</span><span class="special">&lt;</span><span class="identifier">S</span><span class="special">&gt;(</span><span class="identifier">_1</span><span class="special">,</span> <span class="identifier">_2</span><span class="special">));</span>
10713     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
10714 <span class="special">}</span>
10715 </pre>
10716 <p>
10717         </p>
10718 </div>
10719 <div class="section">
10720 <div class="titlepage"><div><div><h4 class="title">
10721 <a name="boost_proto.users_guide.examples.checked_calc"></a><a class="link" href="users_guide.html#boost_proto.users_guide.examples.checked_calc" title="Checked Calculator: A Simple Example of External Transforms">Checked
10722         Calculator: A Simple Example of External Transforms</a>
10723 </h4></div></div></div>
10724 <p>
10725           This is an advanced example that shows how to externally parameterize a
10726           grammar's transforms. It defines a calculator EDSL with a grammar that
10727           can perform either checked or unchecked arithmetic.
10728         </p>
10729 <p>
10730 </p>
10731 <pre class="programlisting"><span class="comment">//  Copyright 2011 Eric Niebler. Distributed under the Boost</span>
10732 <span class="comment">//  Software License, Version 1.0. (See accompanying file</span>
10733 <span class="comment">//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</span>
10734 <span class="comment">//</span>
10735 <span class="comment">// This is an example of how to specify a transform externally so</span>
10736 <span class="comment">// that a single grammar can be used to drive multiple differnt</span>
10737 <span class="comment">// calculations. In particular, it defines a calculator grammar</span>
10738 <span class="comment">// that computes the result of an expression with either checked</span>
10739 <span class="comment">// or non-checked division.</span>
10740
10741 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
10742 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10743 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="keyword">int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10744 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">next</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10745 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">mpl</span><span class="special">/</span><span class="identifier">min_max</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10746 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10747 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">generation</span><span class="special">/</span><span class="identifier">make_vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10748 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">proto</span><span class="special">/</span><span class="identifier">proto</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
10749 <span class="keyword">namespace</span> <span class="identifier">mpl</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">;</span>
10750 <span class="keyword">namespace</span> <span class="identifier">proto</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">;</span>
10751 <span class="keyword">namespace</span> <span class="identifier">fusion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">;</span>
10752
10753 <span class="comment">// The argument placeholder type</span>
10754 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">I</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">placeholder</span> <span class="special">:</span> <span class="identifier">I</span> <span class="special">{};</span>
10755
10756 <span class="comment">// Give each rule in the grammar a "name". This is so that we</span>
10757 <span class="comment">// can easily dispatch on it later.</span>
10758 <span class="keyword">struct</span> <span class="identifier">calc_grammar</span><span class="special">;</span>
10759 <span class="keyword">struct</span> <span class="identifier">divides_rule</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">divides</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span> <span class="special">{};</span>
10760
10761 <span class="comment">// Use external transforms in calc_gramar</span>
10762 <span class="keyword">struct</span> <span class="identifier">calc_grammar</span>
10763   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;</span>
10764         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10765             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">&gt;</span> <span class="special">&gt;</span>
10766             <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">functional</span><span class="special">::</span><span class="identifier">at</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_state</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span><span class="special">)</span>
10767         <span class="special">&gt;</span>
10768       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10769             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">convertible_to</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;</span>
10770           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_value</span>
10771         <span class="special">&gt;</span>
10772       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10773             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
10774           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
10775         <span class="special">&gt;</span>
10776       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10777             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">minus</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
10778           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
10779         <span class="special">&gt;</span>
10780       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10781             <span class="identifier">proto</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">,</span> <span class="identifier">calc_grammar</span><span class="special">&gt;</span>
10782           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span>
10783         <span class="special">&gt;</span>
10784         <span class="comment">// Note that we don't specify how division nodes are</span>
10785         <span class="comment">// handled here. Proto::external_transform is a placeholder</span>
10786         <span class="comment">// for an actual transform.</span>
10787       <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10788             <span class="identifier">divides_rule</span>
10789           <span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transform</span>
10790         <span class="special">&gt;</span>
10791     <span class="special">&gt;</span>
10792 <span class="special">{};</span>
10793
10794 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">calc_expr</span><span class="special">;</span>
10795 <span class="keyword">struct</span> <span class="identifier">calc_domain</span> <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">generator</span><span class="special">&lt;</span><span class="identifier">calc_expr</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">{};</span>
10796
10797 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span>
10798 <span class="keyword">struct</span> <span class="identifier">calc_expr</span>
10799   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">E</span><span class="special">&gt;,</span> <span class="identifier">calc_domain</span><span class="special">&gt;</span>
10800 <span class="special">{</span>
10801     <span class="identifier">calc_expr</span><span class="special">(</span><span class="identifier">E</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">e</span> <span class="special">=</span> <span class="identifier">E</span><span class="special">())</span> <span class="special">:</span> <span class="identifier">calc_expr</span><span class="special">::</span><span class="identifier">proto_extends</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span> <span class="special">{}</span>
10802 <span class="special">};</span>
10803
10804 <span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">_1</span><span class="special">;</span>
10805 <span class="identifier">calc_expr</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span> <span class="identifier">_2</span><span class="special">;</span>
10806
10807 <span class="comment">// Use proto::external_transforms to map from named grammar rules to</span>
10808 <span class="comment">// transforms.</span>
10809 <span class="keyword">struct</span> <span class="identifier">non_checked_division</span>
10810   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special">&lt;</span>
10811         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span> <span class="identifier">divides_rule</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special">&lt;</span><span class="identifier">calc_grammar</span><span class="special">&gt;</span> <span class="special">&gt;</span>
10812     <span class="special">&gt;</span>
10813 <span class="special">{};</span>
10814
10815 <span class="keyword">struct</span> <span class="identifier">division_by_zero</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="special">{};</span>
10816
10817 <span class="keyword">struct</span> <span class="identifier">do_checked_divide</span>
10818   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">callable</span>
10819 <span class="special">{</span>
10820     <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">result_type</span><span class="special">;</span>
10821     <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">int</span> <span class="identifier">left</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">right</span><span class="special">)</span> <span class="keyword">const</span>
10822     <span class="special">{</span>
10823         <span class="keyword">if</span> <span class="special">(</span><span class="identifier">right</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">throw</span> <span class="identifier">division_by_zero</span><span class="special">();</span>
10824         <span class="keyword">return</span> <span class="identifier">left</span> <span class="special">/</span> <span class="identifier">right</span><span class="special">;</span>
10825     <span class="special">}</span>
10826 <span class="special">};</span>
10827
10828 <span class="comment">// Use proto::external_transforms again, this time to map the divides_rule</span>
10829 <span class="comment">// to a transforms that performs checked division.</span>
10830 <span class="keyword">struct</span> <span class="identifier">checked_division</span>
10831   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special">&lt;</span>
10832         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span>
10833             <span class="identifier">divides_rule</span>
10834           <span class="special">,</span> <span class="identifier">do_checked_divide</span><span class="special">(</span><span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_left</span><span class="special">),</span> <span class="identifier">calc_grammar</span><span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_right</span><span class="special">))</span>
10835         <span class="special">&gt;</span>
10836     <span class="special">&gt;</span>
10837 <span class="special">{};</span>
10838
10839 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
10840 <span class="special">{</span>
10841     <span class="identifier">non_checked_division</span> <span class="identifier">non_checked</span><span class="special">;</span>
10842     <span class="keyword">int</span> <span class="identifier">result2</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">2</span><span class="special">),</span> <span class="identifier">non_checked</span><span class="special">);</span>
10843     <span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">result2</span> <span class="special">==</span> <span class="number">3</span><span class="special">);</span>
10844
10845     <span class="keyword">try</span>
10846     <span class="special">{</span>
10847         <span class="identifier">checked_division</span> <span class="identifier">checked</span><span class="special">;</span>
10848         <span class="comment">// This should throw</span>
10849         <span class="keyword">int</span> <span class="identifier">result3</span> <span class="special">=</span> <span class="identifier">calc_grammar</span><span class="special">()(</span><span class="identifier">_1</span> <span class="special">/</span> <span class="identifier">_2</span><span class="special">,</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">make_vector</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">0</span><span class="special">),</span> <span class="identifier">checked</span><span class="special">);</span>
10850         <span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span> <span class="comment">// shouldn't get here!</span>
10851     <span class="special">}</span>
10852     <span class="keyword">catch</span><span class="special">(</span><span class="identifier">division_by_zero</span><span class="special">)</span>
10853     <span class="special">{</span>
10854         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"caught division by zero!\n"</span><span class="special">;</span>
10855     <span class="special">}</span>
10856 <span class="special">}</span>
10857 </pre>
10858 <p>
10859         </p>
10860 </div>
10861 </div>
10862 <div class="section">
10863 <div class="titlepage"><div><div><h3 class="title">
10864 <a name="boost_proto.users_guide.resources"></a><a class="link" href="users_guide.html#boost_proto.users_guide.resources" title="Background and Resources">Background and Resources</a>
10865 </h3></div></div></div>
10866 <p>
10867         Proto was initially developed as part of <a href="../../../libs/xpressive/index.html" target="_top">Boost.Xpressive</a>
10868         to simplify the job of transforming an expression template into an executable
10869         finite state machine capable of matching a regular expression. Since then,
10870         Proto has found application in the redesigned and improved Spirit-2 and the
10871         related Karma library. As a result of these efforts, Proto evolved into a
10872         generic and abstract grammar and tree transformation framework applicable
10873         in a wide variety of EDSL scenarios.
10874       </p>
10875 <p>
10876         The grammar and tree transformation framework is modeled on Spirit's grammar
10877         and semantic action framework. The expression tree data structure is similar
10878         to Fusion data structures in many respects, and is interoperable with Fusion's
10879         iterators and algorithms.
10880       </p>
10881 <p>
10882         The syntax for the grammar-matching features of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;&gt;</span></code> is inspired by MPL's lambda expressions.
10883       </p>
10884 <p>
10885         The idea for using function types for Proto's composite transforms is inspired
10886         by Aleksey Gurtovoy's <a href="http://lists.boost.org/Archives/boost/2002/11/39718.php" target="_top">"round"
10887         lambda</a> notation.
10888       </p>
10889 <h5>
10890 <a name="boost_proto.users_guide.resources.h0"></a>
10891         <span class="phrase"><a name="boost_proto.users_guide.resources.references"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.resources.references">References</a>
10892       </h5>
10893 <div class="blockquote"><blockquote class="blockquote"><p>
10894           <a name="boost_proto.users_guide.resources.SYB"></a>Ren, D. and Erwig,
10895           M. 2006. A generic recursion toolbox for Haskell or: scrap your boilerplate
10896           systematically. In <span class="emphasis"><em>Proceedings of the 2006 ACM SIGPLAN Workshop
10897           on Haskell</em></span> (Portland, Oregon, USA, September 17 - 17, 2006).
10898           Haskell '06. ACM, New York, NY, 13-24. DOI=<a href="http://doi.acm.org/10.1145/1159842.1159845" target="_top">http://doi.acm.org/10.1145/1159842.1159845</a>
10899         </p></blockquote></div>
10900 <h5>
10901 <a name="boost_proto.users_guide.resources.h1"></a>
10902         <span class="phrase"><a name="boost_proto.users_guide.resources.further_reading"></a></span><a class="link" href="users_guide.html#boost_proto.users_guide.resources.further_reading">Further
10903         Reading</a>
10904       </h5>
10905 <p>
10906         A technical paper about an earlier version of Proto was accepted into the
10907         <a href="http://lcsd.cs.tamu.edu/2007/" target="_top">ACM SIGPLAN Symposium on Library-Centric
10908         Software Design LCSD'07</a>, and can be found at <a href="http://lcsd.cs.tamu.edu/2007/final/1/1_Paper.pdf" target="_top">http://lcsd.cs.tamu.edu/2007/final/1/1_Paper.pdf</a>.
10909         The tree transforms described in that paper differ from what exists today.
10910       </p>
10911 </div>
10912 <div class="section">
10913 <div class="titlepage"><div><div><h3 class="title">
10914 <a name="boost_proto.users_guide.glossary"></a><a class="link" href="users_guide.html#boost_proto.users_guide.glossary" title="Glossary">Glossary</a>
10915 </h3></div></div></div>
10916 <div class="variablelist">
10917 <p class="title"><b></b></p>
10918 <dl class="variablelist">
10919 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.callable_transform"></a> callable transform</span></dt>
10920 <dd><p>
10921               A transform of the form <code class="computeroutput"><span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">,...)</span></code> (i.e., a function type) where
10922               <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">&gt;::</span><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">true</span></code>.
10923               <code class="computeroutput"><span class="identifier">R</span></code> is treated as a polymorphic
10924               function object and the arguments are treated as transforms that yield
10925               the arguments to the function object.
10926             </p></dd>
10927 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.context"></a> context</span></dt>
10928 <dd><p>
10929               In Proto, the term <span class="emphasis"><em>context</em></span> refers to an object
10930               that can be passed, along with an expression to evaluate, to the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">eval</span><span class="special">()</span></code>
10931               function. The context determines how the expression is evaluated. All
10932               context structs define a nested <code class="computeroutput"><span class="identifier">eval</span><span class="special">&lt;&gt;</span></code> template that, when instantiated
10933               with a node tag type (e.g., <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code>),
10934               is a binary polymorphic function object that accepts an expression
10935               of that type and the context object. In this way, contexts associate
10936               behaviors with expression nodes.
10937             </p></dd>
10938 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.domain"></a> domain</span></dt>
10939 <dd><p>
10940               In Proto, the term <span class="emphasis"><em>domain</em></span> refers to a type that
10941               associates expressions within that domain with a <span class="emphasis"><em>generator</em></span>
10942               for that domain and optionally a <span class="emphasis"><em>grammar</em></span> for the
10943               domain. Domains are used primarily to imbue expressions within that
10944               domain with additional members and to restrict Proto's operator overloads
10945               such that expressions not conforming to the domain's grammar are never
10946               created. Domains are empty structs that inherit from <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">domain</span><span class="special">&lt;&gt;</span></code>.
10947             </p></dd>
10948 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.dsl"></a> domain-specific language</span></dt>
10949 <dd><p>
10950               A programming language that targets a particular problem space by providing
10951               programming idioms, abstractions and constructs that match the constructs
10952               within that problem space.
10953             </p></dd>
10954 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.edsl"></a> embedded domain-specific language</span></dt>
10955 <dd><p>
10956               A domain-specific language implemented as a library. The language in
10957               which the library is written is called the "host" language,
10958               and the language implemented by the library is called the "embedded"
10959               language.
10960             </p></dd>
10961 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.expression"></a> expression</span></dt>
10962 <dd><p>
10963               In Proto, an <span class="emphasis"><em>expression</em></span> is a heterogeneous tree
10964               where each node is either an instantiation of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">expr</span><span class="special">&lt;&gt;</span></code>, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">basic_expr</span><span class="special">&lt;&gt;</span></code> or some type that is an extension
10965               (via <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special">&lt;&gt;</span></code>
10966               or <code class="computeroutput"><span class="identifier">BOOST_PROTO_EXTENDS</span><span class="special">()</span></code>) of such an instantiation.
10967             </p></dd>
10968 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.expression_template"></a> expression template</span></dt>
10969 <dd><p>
10970               A C++ technique using templates and operator overloading to cause expressions
10971               to build trees that represent the expression for lazy evaluation later,
10972               rather than evaluating the expression eagerly. Some C++ libraries use
10973               expression templates to build embedded domain-specific languages.
10974             </p></dd>
10975 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.generator"></a> generator</span></dt>
10976 <dd><p>
10977               In Proto, a <span class="emphasis"><em>generator</em></span> is a unary polymorphic function
10978               object that you specify when defining a <span class="emphasis"><em>domain</em></span>.
10979               After constructing a new expression, Proto passes the expression to
10980               your domain's generator for further processing. Often, the generator
10981               wraps the expression in an extension wrapper that adds additional members
10982               to it.
10983             </p></dd>
10984 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.grammar"></a> grammar</span></dt>
10985 <dd><p>
10986               In Proto, a <span class="emphasis"><em>grammar</em></span> is a type that describes a
10987               subset of Proto expression types. Expressions in a domain must conform
10988               to that domain's grammar. The <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special">&lt;&gt;</span></code> metafunction evaluates whether
10989               an expression type matches a grammar. Grammars are either primitives
10990               such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span></code>, composites such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;&gt;</span></code>,
10991               control structures such as <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">or_</span><span class="special">&lt;&gt;</span></code>, or some type derived from
10992               a grammar.
10993             </p></dd>
10994 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.object_transform"></a> object transform</span></dt>
10995 <dd><p>
10996               A transform of the form <code class="computeroutput"><span class="identifier">R</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span><span class="identifier">A1</span><span class="special">,...)</span></code> (i.e., a function type) where
10997               <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">is_callable</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">&gt;::</span><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">false</span></code>.
10998               <code class="computeroutput"><span class="identifier">R</span></code> is treated as the
10999               type of an object to construct and the arguments are treated as transforms
11000               that yield the parameters to the constructor.
11001             </p></dd>
11002 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.polymorphic_function_object"></a> polymorphic function object</span></dt>
11003 <dd><p>
11004               An instance of a class type with an overloaded function call operator
11005               and a nested <code class="computeroutput"><span class="identifier">result_type</span></code>
11006               typedef or <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;&gt;</span></code>
11007               template for calculating the return type of the function call operator.
11008             </p></dd>
11009 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.primitive_transform"></a> primitive transform</span></dt>
11010 <dd><p>
11011               A type that defines a kind of polymorphic function object that takes
11012               three arguments: expression, state, and data. Primitive transforms
11013               can be used to compose callable transforms and object transforms.
11014             </p></dd>
11015 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.subdomain"></a> sub-domain</span></dt>
11016 <dd><p>
11017               A sub-domain is a domain that declares another domain as its super-domain.
11018               Expressions in sub-domains can be combined with expressions in the
11019               super-domain, and the resulting expression is in the super-domain.
11020             </p></dd>
11021 <dt><span class="term"> <a name="boost_proto.users_guide.glossary.transform"></a> transform</span></dt>
11022 <dd><p>
11023               Transforms are used to manipulate expression trees. They come in three
11024               flavors: primitive transforms, callable transforms, or object transforms.
11025               A transform <code class="computeroutput"><em class="replaceable"><code>T</code></em></code> can be made into
11026               a ternary polymorphic function object with <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;&gt;</span></code>, as in <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <em class="replaceable"><code>T</code></em><span class="special">&gt;</span></code>.
11027               Such a function object accepts <span class="emphasis"><em>expression</em></span>, <span class="emphasis"><em>state</em></span>,
11028               and <span class="emphasis"><em>data</em></span> parameters, and computes a result from
11029               them.
11030             </p></dd>
11031 </dl>
11032 </div>
11033 </div>
11034 <div class="footnotes">
11035 <br><hr style="width:100; text-align:left;margin-left: 0">
11036 <div id="ftn.boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0" class="footnote"><p><a href="#boost_proto.users_guide.front_end.customizing_expressions_in_your_domain.per_domain_as_child.f0" class="para"><sup class="para">[34] </sup></a>
11037               It's not always possible to hold something by value. By default, <code class="computeroutput"><a class="link" href="../boost/proto/as_expr.html" title="Function as_expr">proto::as_expr()</a></code> makes an exception
11038               for functions, abstract types, and iostreams (types derived from <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span></code>). These objects are held
11039               by reference. All others are held by value, even arrays.
11040             </p></div>
11041 <div id="ftn.boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0" class="footnote"><p><a href="#boost_proto.users_guide.back_end.expression_transformation.example__calculator_arity.f0" class="para"><sup class="para">[35] </sup></a>
11042               This error message was generated with Microsoft Visual C++ 9.0. Different
11043               compilers will emit different messages with varying degrees of readability.
11044             </p></div>
11045 </div>
11046 </div>
11047 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
11048 <td align="left"></td>
11049 <td align="right"><div class="copyright-footer">Copyright &#169; 2008 Eric Niebler<p>
11050         Distributed under the Boost Software License, Version 1.0. (See accompanying
11051         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
11052       </p>
11053 </div></td>
11054 </tr></table>
11055 <hr>
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