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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="phoenix.inside.actor"></a><a class="link" href="actor.html" title="Actors in Detail">Actors in Detail</a>
  28 </h3></div></div></div>
  29 <h5>
  30 <a name="phoenix.inside.actor.h0"></a>
  31         <span class="phrase"><a name="phoenix.inside.actor.actor"></a></span><a class="link" href="actor.html#phoenix.inside.actor.actor">Actor</a>
  32       </h5>
  33 <p>
  34         The main concept is the <code class="computeroutput"><span class="identifier">Actor</span></code>.
  35         An <code class="computeroutput"><span class="identifier">Actor</span></code> is a model of the
  36         <a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/functional/concepts/poly.html" target="_top">Polymorphic
  37         Function Object</a> concept (that can accept 0 to N arguments (where
  38         N is a predefined maximum).
  39       </p>
  40 <p>
  41         An <code class="computeroutput"><span class="identifier">Actor</span></code> contains a valid
  42         Phoenix Expression, a call to one of the function call operator overloads,
  43         starts the evaluation process.
  44       </p>
  45 <div class="note"><table border="0" summary="Note">
  46 <tr>
  47 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
  48 <th align="left">Note</th>
  49 </tr>
  50 <tr><td align="left" valign="top"><p>
  51           You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>,
  52           the predefined maximum arity an actor can take. By default, <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code> is set to 10.
  53         </p></td></tr>
  54 </table></div>
  55 <p>
  56         The <code class="computeroutput"><span class="identifier">actor</span></code> template class
  57         models the <code class="computeroutput"><span class="identifier">Actor</span></code> concept:
  58       </p>
  59 <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>
  60 <span class="keyword">struct</span> <span class="identifier">actor</span>
  61 <span class="special">{</span>
  62     <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>
  63     <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>
  64
  65     <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;::</span><span class="identifier">type</span>
  66     <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span><span class="special">;</span>
  67
  68     <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">&gt;</span>
  69     <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">T0</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span>
  70     <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T0</span><span class="special">&amp;</span> <span class="identifier">_0</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
  71
  72     <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">&gt;</span>
  73     <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">T0</span> <span class="keyword">const</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span>
  74     <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T0</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">_0</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
  75
  76     <span class="comment">//...</span>
  77 <span class="special">};</span>
  78 </pre>
  79 <div class="table">
  80 <a name="phoenix.inside.actor.t0"></a><p class="title"><b>Table&#160;1.9.&#160;Actor Concept Requirements</b></p>
  81 <div class="table-contents"><table class="table" summary="Actor Concept Requirements">
  82 <colgroup>
  83 <col>
  84 <col>
  85 </colgroup>
  86 <thead><tr>
  87 <th>
  88                 <p>
  89                   Expression
  90                 </p>
  91               </th>
  92 <th>
  93                 <p>
  94                   Semantics
  95                 </p>
  96               </th>
  97 </tr></thead>
  98 <tbody>
  99 <tr>
 100 <td>
 101                 <p>
 102                   <code class="computeroutput"><span class="identifier">actor</span><span class="special">(</span><span class="identifier">arg0</span><span class="special">,</span>
 103                   <span class="identifier">arg1</span><span class="special">,</span>
 104                   <span class="special">...,</span> <span class="identifier">argN</span><span class="special">)</span></code>
 105                 </p>
 106               </td>
 107 <td>
 108                 <p>
 109                   Function call operators to start the evaluation
 110                 </p>
 111               </td>
 112 </tr>
 113 <tr>
 114 <td>
 115                 <p>
 116                   <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">&lt;</span><span class="identifier">Actor</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">&gt;(</span><span class="identifier">Arg0</span><span class="special">,</span>
 117                   <span class="identifier">Arg1</span><span class="special">,</span>
 118                   <span class="special">...,</span> <span class="identifier">ArgN</span><span class="special">)&gt;::</span><span class="identifier">type</span></code>
 119                 </p>
 120               </td>
 121 <td>
 122                 <p>
 123                   Result of the evaluation
 124                 </p>
 125               </td>
 126 </tr>
 127 <tr>
 128 <td>
 129                 <p>
 130                   <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span>
 131                   <span class="identifier">Arg0</span><span class="special">,</span>
 132                   <span class="identifier">Arg1</span><span class="special">,</span>
 133                   <span class="special">...,</span> <span class="identifier">ArgN</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
 134                 </p>
 135               </td>
 136 <td>
 137                 <p>
 138                   Result of the evaluation
 139                 </p>
 140               </td>
 141 </tr>
 142 </tbody>
 143 </table></div>
 144 </div>
 145 <br class="table-break"><h5>
 146 <a name="phoenix.inside.actor.h1"></a>
 147         <span class="phrase"><a name="phoenix.inside.actor.function_call_operators"></a></span><a class="link" href="actor.html#phoenix.inside.actor.function_call_operators">Function
 148         Call Operators</a>
 149       </h5>
 150 <p>
 151         There are 2*N function call operators for 0 to N arguments (N == <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>). The actor class accepts
 152         the arguments and forwards the arguments to the default evaluation action.
 153       </p>
 154 <p>
 155         Additionally, there exist function call operators accepting permutations
 156         of const and non-const references. These operators are created for all N
 157         &lt;= <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_PERFECT_FORWARD_LIMIT</span></code>
 158         (which defaults to 3).
 159       </p>
 160 <div class="note"><table border="0" summary="Note">
 161 <tr>
 162 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
 163 <th align="left">Note</th>
 164 </tr>
 165 <tr><td align="left" valign="top">
 166 <p>
 167           <span class="bold"><strong>Forwarding Function Problem</strong></span>
 168         </p>
 169 <p>
 170           There is a known issue with current C++ called the "<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm" target="_top">Forwarding
 171           Function Problem</a>". The problem is that given an arbitrary
 172           function <code class="computeroutput"><span class="identifier">F</span></code>, using current
 173           C++ language rules, one cannot create a forwarding function <code class="computeroutput"><span class="identifier">FF</span></code> that transparently assumes the arguments
 174           of <code class="computeroutput"><span class="identifier">F</span></code>.
 175         </p>
 176 </td></tr>
 177 </table></div>
 178 <h5>
 179 <a name="phoenix.inside.actor.h2"></a>
 180         <span class="phrase"><a name="phoenix.inside.actor.context"></a></span><a class="link" href="actor.html#phoenix.inside.actor.context">Context</a>
 181       </h5>
 182 <p>
 183         On an actor function call, before calling the evaluation function, the actor
 184         created a <span class="bold"><strong>context</strong></span>. This context consists
 185         of an <code class="computeroutput"><span class="identifier">Environment</span></code> and an
 186         <code class="computeroutput"><span class="identifier">Action</span></code> part. These contain
 187         all information necessary to evaluate the given expression.
 188       </p>
 189 <div class="table">
 190 <a name="phoenix.inside.actor.t1"></a><p class="title"><b>Table&#160;1.10.&#160;Context Concept Requirements</b></p>
 191 <div class="table-contents"><table class="table" summary="Context Concept Requirements">
 192 <colgroup>
 193 <col>
 194 <col>
 195 </colgroup>
 196 <thead><tr>
 197 <th>
 198                 <p>
 199                   Expression
 200                 </p>
 201               </th>
 202 <th>
 203                 <p>
 204                   Semantics
 205                 </p>
 206               </th>
 207 </tr></thead>
 208 <tbody>
 209 <tr>
 210 <td>
 211                 <p>
 212                   <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">context</span><span class="special">&lt;</span><span class="identifier">Env</span><span class="special">,</span>
 213                   <span class="identifier">Actions</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
 214                 </p>
 215               </td>
 216 <td>
 217                 <p>
 218                   Type of a Context
 219                 </p>
 220               </td>
 221 </tr>
 222 <tr>
 223 <td>
 224                 <p>
 225                   <code class="computeroutput"><span class="identifier">context</span><span class="special">(</span><span class="identifier">e</span><span class="special">,</span>
 226                   <span class="identifier">a</span><span class="special">)</span></code>
 227                 </p>
 228               </td>
 229 <td>
 230                 <p>
 231                   A Context containing environment <code class="computeroutput"><span class="identifier">e</span></code>
 232                   and actions <code class="computeroutput"><span class="identifier">a</span></code>
 233                 </p>
 234               </td>
 235 </tr>
 236 <tr>
 237 <td>
 238                 <p>
 239                   <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">env</span><span class="special">&lt;</span><span class="identifier">Context</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
 240                 </p>
 241               </td>
 242 <td>
 243                 <p>
 244                   Type of the contained Environment
 245                 </p>
 246               </td>
 247 </tr>
 248 <tr>
 249 <td>
 250                 <p>
 251                   <code class="computeroutput"><span class="identifier">env</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span></code>
 252                 </p>
 253               </td>
 254 <td>
 255                 <p>
 256                   The environment
 257                 </p>
 258               </td>
 259 </tr>
 260 <tr>
 261 <td>
 262                 <p>
 263                   <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actions</span><span class="special">&lt;</span><span class="identifier">Context</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
 264                 </p>
 265               </td>
 266 <td>
 267                 <p>
 268                   Type of the contained Actions
 269                 </p>
 270               </td>
 271 </tr>
 272 <tr>
 273 <td>
 274                 <p>
 275                   <code class="computeroutput"><span class="identifier">actions</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span></code>
 276                 </p>
 277               </td>
 278 <td>
 279                 <p>
 280                   The actions
 281                 </p>
 282               </td>
 283 </tr>
 284 </tbody>
 285 </table></div>
 286 </div>
 287 <br class="table-break"><h5>
 288 <a name="phoenix.inside.actor.h3"></a>
 289         <span class="phrase"><a name="phoenix.inside.actor.environment"></a></span><a class="link" href="actor.html#phoenix.inside.actor.environment">Environment</a>
 290       </h5>
 291 <p>
 292         The Environment is a model of <a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/sequence/concepts/random_access_sequence.html" target="_top">Random
 293         Access Sequence</a>.
 294       </p>
 295 <p>
 296         The arguments passed to the actor's function call operator are collected
 297         inside the Environment:
 298       </p>
 299 <p>
 300         <span class="inlinemediaobject"><img src="../../images/funnel_in.png" alt="funnel_in"></span>
 301       </p>
 302 <p>
 303         Other parts of the library (e.g. the scope module) extends the <code class="computeroutput"><span class="identifier">Environment</span></code> concept to hold other information
 304         such as local variables, etc.
 305       </p>
 306 <h5>
 307 <a name="phoenix.inside.actor.h4"></a>
 308         <span class="phrase"><a name="phoenix.inside.actor.actions"></a></span><a class="link" href="actor.html#phoenix.inside.actor.actions">Actions</a>
 309       </h5>
 310 <p>
 311         Actions is the part of Phoenix which are responsible for giving the actual
 312         expressions a specific behaviour. During the traversal of the Phoenix Expression
 313         Tree these actions are called whenever a specified rule in the grammar matches.
 314       </p>
 315 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">actions</span>
 316 <span class="special">{</span>
 317     <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Rule</span><span class="special">&gt;</span>
 318     <span class="keyword">struct</span> <span class="identifier">when</span><span class="special">;</span>
 319 <span class="special">};</span>
 320 </pre>
 321 <p>
 322         The nested <code class="computeroutput"><span class="identifier">when</span></code> template
 323         is required to be <a href="http://www.boost.org/doc/libs/release/doc/html/PrimitiveTransform.html" target="_top">Proto
 324         Primitive Transform</a>. No worries, you don't have to learn <a href="http://www.boost.org/doc/libs/release/libs/proto/index.html" target="_top">Boost.Proto</a>
 325         just yet! Phoenix provides some wrappers to let you define simple actions
 326         without the need to dive deep into proto.
 327       </p>
 328 <p>
 329         Phoenix ships with a predefined <code class="computeroutput"><span class="identifier">default_actions</span></code>
 330         class that evaluates the expressions with C++ semantics:
 331       </p>
 332 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">default_actions</span>
 333 <span class="special">{</span>
 334     <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Rule</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Dummy</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
 335     <span class="keyword">struct</span> <span class="identifier">when</span>
 336         <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">meta_grammar</span><span class="special">&gt;</span>
 337     <span class="special">{};</span>
 338 <span class="special">};</span>
 339 </pre>
 340 <p>
 341         For more information on how to use the default_actions class and how to attach
 342         custom actions to the evaluation process, see <a class="link" href="actions.html" title="More on Actions">more
 343         on actions</a>.
 344       </p>
 345 <h5>
 346 <a name="phoenix.inside.actor.h5"></a>
 347         <span class="phrase"><a name="phoenix.inside.actor.evaluation"></a></span><a class="link" href="actor.html#phoenix.inside.actor.evaluation">Evaluation</a>
 348       </h5>
 349 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">evaluator</span>
 350 <span class="special">{</span>
 351     <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>
 352     <span class="emphasis"><em>unspecified</em></span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&amp;,</span> <span class="identifier">Context</span> <span class="special">&amp;);</span>
 353 <span class="special">};</span>
 354
 355 <span class="identifier">evaluator</span> <span class="keyword">const</span> <span class="identifier">eval</span> <span class="special">=</span> <span class="special">{};</span>
 356 </pre>
 357 <p>
 358         The evaluation of a Phoenix expression is started by a call to the function
 359         call operator of <code class="computeroutput"><span class="identifier">evaluator</span></code>.
 360       </p>
 361 <p>
 362         The evaluator is called by the <code class="computeroutput"><span class="identifier">actor</span></code>
 363         function operator overloads after the context is built up. For reference,
 364         here is a typical <code class="computeroutput"><span class="identifier">actor</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code>
 365         that accepts two arguments:
 366       </p>
 367 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">&gt;</span>
 368 <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">T0</span> <span class="special">&amp;,</span> <span class="identifier">T1</span> <span class="special">&amp;&gt;::</span><span class="identifier">type</span>
 369 <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T0</span> <span class="special">&amp;</span><span class="identifier">t0</span><span class="special">,</span> <span class="identifier">T1</span> <span class="special">&amp;</span><span class="identifier">t1</span><span class="special">)</span> <span class="keyword">const</span>
 370 <span class="special">{</span>
 371     <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">vector2</span><span class="special">&lt;</span><span class="identifier">T0</span> <span class="special">&amp;,</span> <span class="identifier">T1</span> <span class="special">&amp;&gt;</span> <span class="identifier">env</span><span class="special">(</span><span class="identifier">t0</span><span class="special">,</span> <span class="identifier">t1</span><span class="special">);</span>
 372
 373     <span class="keyword">return</span> <span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">context</span><span class="special">(</span><span class="identifier">env</span><span class="special">,</span> <span class="identifier">default_actions</span><span class="special">()));</span>
 374 <span class="special">}</span>
 375 </pre>
 376 <h5>
 377 <a name="phoenix.inside.actor.h6"></a>
 378         <span class="phrase"><a name="phoenix.inside.actor.result_of__actor"></a></span><a class="link" href="actor.html#phoenix.inside.actor.result_of__actor">result_of::actor</a>
 379       </h5>
 380 <p>
 381         For reasons of symmetry to the family of <code class="computeroutput"><span class="identifier">actor</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> there is a special metafunction usable
 382         for actor result type calculation named <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span></code>.
 383         This metafunction allows us to directly specify the types of the parameters
 384         to be passed to the <code class="computeroutput"><span class="identifier">actor</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> function. Here's a typical <code class="computeroutput"><span class="identifier">actor_result</span></code> that accepts two arguments:
 385       </p>
 386 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">result_of</span>
 387 <span class="special">{</span>
 388     <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">T0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">&gt;</span>
 389     <span class="keyword">struct</span> <span class="identifier">actor</span>
 390     <span class="special">{</span>
 391         <span class="keyword">typedef</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">vector2</span><span class="special">&lt;</span><span class="identifier">T0</span><span class="special">,</span> <span class="identifier">T1</span><span class="special">&gt;</span>                                           <span class="identifier">env_tpe</span><span class="special">;</span>
 392         <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">context</span><span class="special">&lt;</span><span class="identifier">env_type</span><span class="special">,</span> <span class="identifier">default_actions</span><span class="special">&gt;::</span><span class="identifier">type</span>      <span class="identifier">ctx_type</span>
 393         <span class="keyword">typedef</span> <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">evaluator</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span><span class="special">&amp;,</span> <span class="identifier">ctx_type</span><span class="special">)&gt;::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
 394     <span class="special">};</span>
 395 <span class="special">}</span>
 396 </pre>
 397 </div>
 398 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 399 <td align="left"></td>
 400 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010, 2014 Joel de Guzman, Dan Marsden, Thomas Heller, John
 401       Fletcher<p>
 402         Distributed under the Boost Software License, Version 1.0. (See accompanying
 403         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>)
 404       </p>
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