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  24 </div>
  25 <div class="section">
  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="boost_typeerasure.any"></a><a class="link" href="any.html" title="Using Any">Using Any</a>
  28 </h2></div></div></div>
  29 <div class="toc"><dl class="toc">
  30 <dt><span class="section"><a href="any.html#boost_typeerasure.any.construction">Construction</a></span></dt>
  31 <dt><span class="section"><a href="any.html#boost_typeerasure.any.conversions">Conversions</a></span></dt>
  32 <dt><span class="section"><a href="any.html#boost_typeerasure.any.references">References</a></span></dt>
  33 <dt><span class="section"><a href="any.html#boost_typeerasure.any.limit">Syntax Limitations</a></span></dt>
  34 </dl></div>
  35 <div class="section">
  36 <div class="titlepage"><div><div><h3 class="title">
  37 <a name="boost_typeerasure.any.construction"></a><a class="link" href="any.html#boost_typeerasure.any.construction" title="Construction">Construction</a>
  38 </h3></div></div></div>
  39 <p>
  40         (For the source of the examples in this section see <a href="../../../libs/type_erasure/example/construction.cpp" target="_top">construction.cpp</a>)
  41       </p>
  42 <p>
  43         The library provides the <code class="computeroutput"><a class="link" href="../boost/type_erasure/constructible.html" title="Struct template constructible">constructible</a></code>
  44         concept to allow an <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
  45         to capture constructors. The single template argument should be a function
  46         signature. The return type must be a placeholder specifying the type to be
  47         constructed. The arguments are the arguments of the constructor.
  48       </p>
  49 <p>
  50 </p>
  51 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
  52     <span class="identifier">copy_constructible</span><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">&gt;,</span>
  53     <span class="identifier">copy_constructible</span><span class="special">&lt;</span><span class="identifier">_b</span><span class="special">&gt;,</span>
  54     <span class="identifier">copy_constructible</span><span class="special">&lt;</span><span class="identifier">_c</span><span class="special">&gt;,</span>
  55     <span class="identifier">constructible</span><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">_b</span><span class="special">&amp;,</span> <span class="keyword">const</span> <span class="identifier">_c</span><span class="special">&amp;)&gt;</span>
  56 <span class="special">&gt;</span> <span class="identifier">construct</span><span class="special">;</span>
  57
  58 <span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span>
  59     <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">_a</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="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;,</span>
  60     <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">_b</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>
  61     <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">_c</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span>
  62 <span class="special">&gt;</span> <span class="identifier">types</span><span class="special">;</span>
  63
  64 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">,</span> <span class="identifier">_b</span><span class="special">&gt;</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">(</span><span class="number">10</span><span class="special">),</span> <span class="identifier">make_binding</span><span class="special">&lt;</span><span class="identifier">types</span><span class="special">&gt;());</span>
  65 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">,</span> <span class="identifier">_c</span><span class="special">&gt;</span> <span class="identifier">val</span><span class="special">(</span><span class="number">2.5</span><span class="special">,</span> <span class="identifier">make_binding</span><span class="special">&lt;</span><span class="identifier">types</span><span class="special">&gt;());</span>
  66 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">&gt;</span> <span class="identifier">v</span><span class="special">(</span><span class="identifier">size</span><span class="special">,</span> <span class="identifier">val</span><span class="special">);</span>
  67 <span class="comment">// v holds std::vector&lt;double&gt;(10, 2.5);</span>
  68 </pre>
  69 <p>
  70       </p>
  71 <p>
  72         Now, suppose that we want a default constructor? We can't have the default
  73         constructor of <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
  74         call the default constructor of the contained type, because it would have
  75         no way of knowing what the contained type is. So, we'll need to pass the
  76         placeholder binding information explicitly.
  77       </p>
  78 <p>
  79 </p>
  80 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
  81     <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
  82     <span class="identifier">constructible</span><span class="special">&lt;</span><span class="identifier">_self</span><span class="special">()&gt;</span>
  83 <span class="special">&gt;</span> <span class="identifier">construct</span><span class="special">;</span>
  84
  85 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">(</span><span class="string">"Test"</span><span class="special">));</span>
  86 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">binding_of</span><span class="special">(</span><span class="identifier">x</span><span class="special">));</span> <span class="comment">// y == ""</span>
  87 </pre>
  88 <p>
  89       </p>
  90 <p>
  91         This method is not restricted to the default constructor. If the constructor
  92         takes arguments, they can be passed after the bindings.
  93       </p>
  94 <p>
  95 </p>
  96 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
  97     <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
  98     <span class="identifier">constructible</span><span class="special">&lt;</span><span class="identifier">_self</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">,</span> <span class="keyword">char</span><span class="special">)&gt;</span>
  99 <span class="special">&gt;</span> <span class="identifier">construct</span><span class="special">;</span>
 100
 101 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">(</span><span class="string">"Test"</span><span class="special">));</span>
 102 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">construct</span><span class="special">&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">binding_of</span><span class="special">(</span><span class="identifier">x</span><span class="special">),</span> <span class="number">5</span><span class="special">,</span> <span class="char">'A'</span><span class="special">);</span>
 103 </pre>
 104 <p>
 105       </p>
 106 </div>
 107 <div class="section">
 108 <div class="titlepage"><div><div><h3 class="title">
 109 <a name="boost_typeerasure.any.conversions"></a><a class="link" href="any.html#boost_typeerasure.any.conversions" title="Conversions">Conversions</a>
 110 </h3></div></div></div>
 111 <p>
 112         (For the source of the examples in this section see <a href="../../../libs/type_erasure/example/convert.cpp" target="_top">convert.cpp</a>)
 113       </p>
 114 <p>
 115         An <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> can be converted
 116         to another <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> as long
 117         as the conversion is an "upcast."
 118       </p>
 119 <p>
 120 </p>
 121 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">any</span><span class="special">&lt;</span>
 122     <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
 123         <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
 124         <span class="identifier">typeid_</span><span class="special">&lt;&gt;,</span>
 125         <span class="identifier">ostreamable</span><span class="special">&lt;&gt;</span>
 126     <span class="special">&gt;</span>
 127 <span class="special">&gt;</span> <span class="identifier">any_printable</span><span class="special">;</span>
 128 <span class="keyword">typedef</span> <span class="identifier">any</span><span class="special">&lt;</span>
 129     <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
 130         <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
 131         <span class="identifier">typeid_</span><span class="special">&lt;&gt;</span>
 132     <span class="special">&gt;</span>
 133 <span class="special">&gt;</span> <span class="identifier">common_any</span><span class="special">;</span>
 134 <span class="identifier">any_printable</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
 135 <span class="identifier">common_any</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
 136 </pre>
 137 <p>
 138       </p>
 139 <p>
 140         This conversion is okay because the requirements of <code class="computeroutput"><span class="identifier">common_any</span></code>
 141         are a subset of the requirements of <code class="computeroutput"><span class="identifier">any_printable</span></code>.
 142         Conversion in the other direction is illegal.
 143       </p>
 144 <p>
 145 </p>
 146 <pre class="programlisting"><span class="identifier">common_any</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
 147 <span class="identifier">any_printable</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// error</span>
 148 </pre>
 149 <p>
 150       </p>
 151 </div>
 152 <div class="section">
 153 <div class="titlepage"><div><div><h3 class="title">
 154 <a name="boost_typeerasure.any.references"></a><a class="link" href="any.html#boost_typeerasure.any.references" title="References">References</a>
 155 </h3></div></div></div>
 156 <p>
 157         (For the source of the examples in this section see <a href="../../../libs/type_erasure/example/references.cpp" target="_top">references.cpp</a>)
 158       </p>
 159 <p>
 160         To capture by reference, we simply add a reference to the <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>.
 161       </p>
 162 <p>
 163 </p>
 164 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
 165 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">typeid_</span><span class="special">&lt;&gt;,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
 166 <span class="identifier">any_cast</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">=</span> <span class="number">5</span><span class="special">;</span> <span class="comment">// now i is 5</span>
 167 </pre>
 168 <p>
 169       </p>
 170 <div class="note"><table border="0" summary="Note">
 171 <tr>
 172 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
 173 <th align="left">Note</th>
 174 </tr>
 175 <tr><td align="left" valign="top"><p>
 176           <code class="computeroutput"><span class="identifier">_self</span></code> is the default <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>, so it is
 177           easiest to use <code class="computeroutput"><span class="identifier">_self</span><span class="special">&amp;</span></code>.
 178           We could use another <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>
 179           instead. <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code><code class="computeroutput"><span class="special">&lt;</span></code><code class="computeroutput"><a class="link" href="../boost/type_erasure/typeid_.html" title="Struct template typeid_">typeid_</a></code><code class="computeroutput"><span class="special">&lt;</span><span class="identifier">_a</span><span class="special">&gt;,</span> <span class="identifier">_a</span><span class="special">&amp;&gt;</span></code> has exactly the same behavior.
 180         </p></td></tr>
 181 </table></div>
 182 <p>
 183         References cannot be rebound. Just like a built-in C++ reference, once you've
 184         initialized it you can't change it to point to something else.
 185 </p>
 186 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">j</span><span class="special">;</span>
 187 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">typeid_</span><span class="special">&lt;&gt;,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">i</span><span class="special">),</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">j</span><span class="special">);</span>
 188 <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// error</span>
 189 </pre>
 190 <p>
 191       </p>
 192 <div class="note"><table border="0" summary="Note">
 193 <tr>
 194 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
 195 <th align="left">Note</th>
 196 </tr>
 197 <tr><td align="left" valign="top"><p>
 198           As with any other operation, <code class="computeroutput"><span class="identifier">x</span>
 199           <span class="special">=</span> <span class="identifier">y</span></code>
 200           for references acts on <code class="computeroutput"><span class="identifier">i</span></code>
 201           and <code class="computeroutput"><span class="identifier">j</span></code>. Assignment like
 202           this is legal if <code class="computeroutput"><a class="link" href="../boost/type_erasure/assignable.html" title="Struct template assignable">assignable</a></code><code class="computeroutput"><span class="special">&lt;&gt;</span></code> is in the Concept, but <code class="computeroutput"><span class="identifier">x</span></code> would still hold a reference to <code class="computeroutput"><span class="identifier">i</span></code>.
 203         </p></td></tr>
 204 </table></div>
 205 <p>
 206         A reference can be bound to another <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>.
 207       </p>
 208 <p>
 209 </p>
 210 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
 211     <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
 212     <span class="identifier">incrementable</span><span class="special">&lt;&gt;</span>
 213 <span class="special">&gt;</span> <span class="identifier">requirements</span><span class="special">;</span>
 214
 215 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
 216 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
 217 <span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// x is now 11</span>
 218 </pre>
 219 <p>
 220       </p>
 221 <p>
 222         If a reference is used after the underlying object goes out of scope or is
 223         reset, the behavior is undefined.
 224       </p>
 225 <p>
 226 </p>
 227 <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span>
 228     <span class="identifier">copy_constructible</span><span class="special">&lt;&gt;,</span>
 229     <span class="identifier">incrementable</span><span class="special">&lt;&gt;,</span>
 230     <span class="identifier">relaxed</span>
 231 <span class="special">&gt;</span> <span class="identifier">requirements</span><span class="special">;</span>
 232 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
 233 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
 234 <span class="identifier">x</span> <span class="special">=</span> <span class="number">1.0</span><span class="special">;</span>
 235 <span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// undefined behavior.</span>
 236 </pre>
 237 <p>
 238       </p>
 239 <p>
 240         This only applies when a reference is constructed from a value. If a reference
 241         is constructed from another reference, the new reference does not depend
 242         on the old one.
 243       </p>
 244 <p>
 245 </p>
 246 <pre class="programlisting"><span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
 247 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special">&lt;</span><span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="special">&gt;</span> <span class="identifier">p</span><span class="special">(</span>
 248     <span class="keyword">new</span> <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;(</span><span class="identifier">x</span><span class="special">));</span>
 249 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">y</span><span class="special">(*</span><span class="identifier">p</span><span class="special">);</span> <span class="comment">// equivalent to y(x);</span>
 250 <span class="identifier">p</span><span class="special">.</span><span class="identifier">reset</span><span class="special">();</span>
 251 <span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// okay</span>
 252 </pre>
 253 <p>
 254       </p>
 255 <p>
 256         Both const and non-const references are supported.
 257       </p>
 258 <p>
 259 </p>
 260 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
 261 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">incrementable</span><span class="special">&lt;&gt;,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
 262 <span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">incrementable</span><span class="special">&lt;&gt;,</span> <span class="keyword">const</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
 263 </pre>
 264 <p>
 265       </p>
 266 <p>
 267         A reference to non-const can be converted to a reference to const, but not
 268         the other way around. Naturally, we can't apply mutating operations to a
 269         const reference.
 270       </p>
 271 <pre class="programlisting"><span class="identifier">any</span><span class="special">&lt;</span><span class="identifier">incrementable</span><span class="special">&lt;&gt;,</span> <span class="identifier">_self</span><span class="special">&amp;&gt;</span> <span class="identifier">z</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span> <span class="comment">// error</span>
 272 <span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// error</span>
 273 </pre>
 274 </div>
 275 <div class="section">
 276 <div class="titlepage"><div><div><h3 class="title">
 277 <a name="boost_typeerasure.any.limit"></a><a class="link" href="any.html#boost_typeerasure.any.limit" title="Syntax Limitations">Syntax Limitations</a>
 278 </h3></div></div></div>
 279 <p>
 280         In most cases using an any has the same syntax as using the underlying object.
 281         However, there are a few cases where this is not possible to implement. An
 282         <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> reference is proxy
 283         and cannot be used in contexts where a real reference is required. In particular,
 284         <code class="computeroutput"><a class="link" href="../boost/type_erasure/forward_iterator.html" title="Struct template forward_iterator">forward_iterator</a></code>
 285         does not create a conforming ForwardIterator (unless the value_type is fixed.)
 286         Another difference is that all operations which do not take at least one
 287         <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> argument have to
 288         be passed the type information explicitly. Static member functions and constructors
 289         can fall in this category. All this means that generic algorithms might not
 290         work when applied to <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
 291         arguments.
 292       </p>
 293 </div>
 294 </div>
 295 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 296 <td align="left"></td>
 297 <td align="right"><div class="copyright-footer">Copyright &#169; 2011-2013 Steven Watanabe<p>
 298         Distributed under the Boost Software License, Version 1.0. (See accompanying
 299         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>)
 300       </p>
 301 </div></td>
 302 </tr></table>
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