ceph/src/boost/libs/mpl/doc/refmanual/if.html

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  14 <div class="section" id="if">
  15 <h1><a class="toc-backref" href="./type-selection.html#id1520">if_</a></h1>
  16 <div class="section" id="id859">
  17 <h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
  18 <pre class="literal-block">
  19 template&lt;
  20       typename C
  21     , typename T1
  22     , typename T2
  23     &gt;
  24 struct <a href="./if.html" class="identifier">if_</a>
  25 {
  26     typedef <em>unspecified</em> type;
  27 };
  28 </pre>
  29 </div>
  30 <div class="section" id="id860">
  31 <h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
  32 <p>Returns one of its two arguments, <tt class="literal"><span class="pre">T1</span></tt> or <tt class="literal"><span class="pre">T2</span></tt>, depending on the value <tt class="literal"><span class="pre">C</span></tt>.</p>
  33 </div>
  34 <div class="section" id="id861">
  35 <h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
  36 <pre class="literal-block">
  37 #include &lt;<a href="../../../../boost/mpl/if.hpp" class="header">boost/mpl/if.hpp</a>&gt;
  38 </pre>
  39 </div>
  40 <div class="section" id="id862">
  41 <h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
  42 <table border="1" class="docutils table">
  43 <colgroup>
  44 <col width="15%" />
  45 <col width="36%" />
  46 <col width="48%" />
  47 </colgroup>
  48 <thead valign="bottom">
  49 <tr><th class="head">Parameter</th>
  50 <th class="head">Requirement</th>
  51 <th class="head">Description</th>
  52 </tr>
  53 </thead>
  54 <tbody valign="top">
  55 <tr><td><tt class="literal"><span class="pre">C</span></tt></td>
  56 <td><a class="reference internal" href="./integral-constant.html">Integral Constant</a></td>
  57 <td>A selection condition.</td>
  58 </tr>
  59 <tr><td><tt class="literal"><span class="pre">T1</span></tt>, <tt class="literal"><span class="pre">T2</span></tt></td>
  60 <td>Any type</td>
  61 <td>Types to select from.</td>
  62 </tr>
  63 </tbody>
  64 </table>
  65 </div>
  66 <div class="section" id="id863">
  67 <h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
  68 <p>For any <a class="reference internal" href="./integral-constant.html">Integral Constant</a> <tt class="literal"><span class="pre">c</span></tt> and arbitrary types <tt class="literal"><span class="pre">t1</span></tt>, <tt class="literal"><span class="pre">t2</span></tt>:</p>
  69 <pre class="literal-block">
  70 typedef <a href="./if.html" class="identifier">if_</a>&lt;c,t1,t2&gt;::type t;
  71 </pre>
  72 <table class="docutils field-list" frame="void" rules="none">
  73 <col class="field-name" />
  74 <col class="field-body" />
  75 <tbody valign="top">
  76 <tr class="field"><th class="field-name">Return type:</th><td class="field-body">Any type.</td>
  77 </tr>
  78 <tr class="field"><th class="field-name">Semantics:</th><td class="field-body">If <tt class="literal"><span class="pre">c::value</span> <span class="pre">==</span> <span class="pre">true</span></tt>, <tt class="literal"><span class="pre">t</span></tt> is identical to <tt class="literal"><span class="pre">t1</span></tt>; otherwise <tt class="literal"><span class="pre">t</span></tt> is
  79 identical to <tt class="literal"><span class="pre">t2</span></tt>.</td>
  80 </tr>
  81 </tbody>
  82 </table>
  83 </div>
  84 <div class="section" id="id864">
  85 <h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
  86 <pre class="literal-block">
  87 typedef <a href="./if.html" class="identifier">if_</a>&lt;<a href="./bool.html" class="identifier">true_</a>,char,long&gt;::type t1;
  88 typedef <a href="./if.html" class="identifier">if_</a>&lt;<a href="./bool.html" class="identifier">false_</a>,char,long&gt;::type t2;
  89
  90 <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt;t1, char&gt; ));
  91 <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt;t2, long&gt; ));
  92 </pre>
  93 <pre class="literal-block">
  94 // allocates space for an object of class T on heap or &quot;inplace&quot;
  95 // depending on its <a href="./size.html" class="identifier">size</a>
  96 template&lt; typename T &gt; struct lightweight
  97 {
  98     // ...
  99     typedef typename <a href="./if.html" class="identifier">if_</a>&lt;
 100           <a href="./less-equal.html" class="identifier">less_equal</a>&lt; <a href="./sizeof.html" class="identifier">sizeof_</a>&lt;T&gt;, <a href="./sizeof.html" class="identifier">sizeof_</a>&lt;T*&gt; &gt;
 101         , inplace_storage&lt;T&gt;
 102         , heap_storage&lt;T&gt;
 103         &gt;::type impl_t;
 104
 105     impl_t impl;
 106 };
 107 </pre>
 108 </div>
 109 <div class="section" id="id865">
 110 <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
 111 <p><a class="reference internal" href="./metafunctions.html">Metafunctions</a>, <a class="reference internal" href="./integral-constant.html">Integral Constant</a>, <a class="reference internal" href="./if-c.html">if_c</a>, <a class="reference internal" href="./eval-if.html">eval_if</a></p>
 112 <!-- Metafunctions/Type Selection//if_c |20 -->
 113 </div>
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