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

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  12 <td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./metafunctions.html" class="navigation-link">Metafunctions</a> / <a href="./miscellaneous.html" class="navigation-link">Miscellaneous</a> / <a href="./identity.html" class="navigation-link">identity</a></td>
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  14 <div class="section" id="identity">
  15 <h1><a class="toc-backref" href="./miscellaneous.html#id1564">identity</a></h1>
  16 <div class="section" id="id1146">
  17 <h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
  18 <pre class="literal-block">
  19 template&lt;
  20       typename X
  21     &gt;
  22 struct <a href="./identity.html" class="identifier">identity</a>
  23 {
  24     typedef X type;
  25 };
  26 </pre>
  27 </div>
  28 <div class="section" id="id1147">
  29 <h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
  30 <p>The <a class="reference external" href="http://mathworld.wolfram.com/IdentityFunction.html" target="_top">identity</a> metafunction. Returns <tt class="literal"><span class="pre">X</span></tt> unchanged.</p>
  31 </div>
  32 <div class="section" id="id1149">
  33 <h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
  34 <pre class="literal-block">
  35 #include &lt;<a href="../../../../boost/mpl/identity.hpp" class="header">boost/mpl/identity.hpp</a>&gt;
  36 </pre>
  37 </div>
  38 <div class="section" id="id1150">
  39 <h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3>
  40 <p><a class="reference internal" href="./metafunction.html">Metafunction</a></p>
  41 </div>
  42 <div class="section" id="id1151">
  43 <h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
  44 <table border="1" class="docutils table">
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  46 <col width="22%" />
  47 <col width="28%" />
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  50 <thead valign="bottom">
  51 <tr><th class="head">Parameter</th>
  52 <th class="head">Requirement</th>
  53 <th class="head">Description</th>
  54 </tr>
  55 </thead>
  56 <tbody valign="top">
  57 <tr><td><tt class="literal"><span class="pre">X</span></tt></td>
  58 <td>Any type</td>
  59 <td>An argument to be returned.</td>
  60 </tr>
  61 </tbody>
  62 </table>
  63 </div>
  64 <div class="section" id="id1152">
  65 <h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
  66 <p>For an arbitrary type <tt class="literal"><span class="pre">x</span></tt>:</p>
  67 <pre class="literal-block">
  68 typedef <a href="./identity.html" class="identifier">identity</a>&lt;x&gt;::type r;
  69 </pre>
  70 <table class="docutils field-list" frame="void" rules="none">
  71 <col class="field-name" />
  72 <col class="field-body" />
  73 <tbody valign="top">
  74 <tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p>
  75 </td>
  76 </tr>
  77 <tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
  78 <pre class="literal-block">
  79 typedef x r;
  80 </pre>
  81 </td>
  82 </tr>
  83 <tr class="field"><th class="field-name">Postcondition:</th><td class="field-body"><p class="first last"><tt class="literal"><span class="pre">is_same&lt;r,x&gt;::value</span> <span class="pre">==</span> <span class="pre">true</span></tt>.</p>
  84 </td>
  85 </tr>
  86 </tbody>
  87 </table>
  88 </div>
  89 <div class="section" id="id1153">
  90 <h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
  91 <pre class="literal-block">
  92 typedef <a href="./apply.html" class="identifier">apply</a>&lt; <a href="./identity.html" class="identifier">identity</a>&lt;<a href="./placeholders.html" class="identifier">_1</a>&gt;, char &gt;::type t1;
  93 typedef <a href="./apply.html" class="identifier">apply</a>&lt; <a href="./identity.html" class="identifier">identity</a>&lt;<a href="./placeholders.html" class="identifier">_2</a>&gt;, char,int &gt;::type t2;
  94
  95 <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t1, char &gt; ));
  96 <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t2, int > ));
  97 </pre>
  98 </div>
  99 <div class="section" id="id1154">
 100 <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
 101 <p><a class="reference internal" href="./metafunctions.html">Metafunctions</a>, <a class="reference internal" href="./placeholders.html">Placeholders</a>, <a class="reference internal" href="./trivial.html#label-metafunctions-trivial">Trivial Metafunctions</a>, <a class="reference internal" href="./always.html">always</a>, <a class="reference internal" href="./apply.html">apply</a></p>
 102 <!-- Metafunctions/Miscellaneous//always |20 -->
 103 </div>
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