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7<title>The MPL Reference Manual: identity</title>
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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>
13</tr></table><div class="header-separator"></div>
14<div class="section" id="identity">
15<h1><a class="toc-backref" href="./miscellaneous.html#id488" name="identity">identity</a></h1>
16<div class="section" id="identity-synopsis">
17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
18<pre class="literal-block">
19template&lt; 
20      typename X
21    &gt;
22struct <a href="./identity.html" class="identifier">identity</a>
23{
24    typedef X type;
25};
26</pre>
27</div>
28<div class="section" id="identity-description">
29<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
30<p>The <a class="reference" 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="identity-header">
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="identity-model-of">
39<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3>
40<p><a class="reference" href="./metafunction.html">Metafunction</a></p>
41</div>
42<div class="section" id="identity-parameters">
43<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
44<table border="1" class="table">
45<colgroup>
46<col width="22%" />
47<col width="28%" />
48<col width="51%" />
49</colgroup>
50<thead valign="bottom">
51<tr><th>Parameter</th>
52<th>Requirement</th>
53<th>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="identity-expression-semantics">
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">
68typedef <a href="./identity.html" class="identifier">identity</a>&lt;x&gt;::type r;
69</pre>
70<table class="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">A type.</td>
75</tr>
76<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
77<pre class="last literal-block">
78typedef x r;
79</pre>
80</td>
81</tr>
82<tr class="field"><th class="field-name">Postcondition:</th><td class="field-body"><tt class="literal"><span class="pre">is_same&lt;r,x&gt;::value</span> <span class="pre">==</span> <span class="pre">true</span></tt>.</td>
83</tr>
84</tbody>
85</table>
86</div>
87<div class="section" id="identity-example">
88<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
89<pre class="literal-block">
90typedef <a href="./apply.html" class="identifier">apply</a>&lt; <a href="./identity.html" class="identifier">identity</a>&lt;_1&gt;, char &gt;::type t1;
91typedef <a href="./apply.html" class="identifier">apply</a>&lt; <a href="./identity.html" class="identifier">identity</a>&lt;_2&gt;, char,int &gt;::type t2;
92
93<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t1, char &gt; ));
94<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t2, int > ));
95</pre>
96</div>
97<div class="section" id="identity-see-also">
98<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
99<p><a class="reference" href="./metafunctions.html">Metafunctions</a>, <a class="reference" href="./placeholders.html">Placeholders</a>, <a class="reference" href="./trivial.html">Trivial Metafunctions</a>, <a class="refentry reference" href="./always.html"><tt class="refentry literal"><span class="pre">always</span></tt></a>, <a class="refentry reference" href="./apply.html"><tt class="refentry literal"><span class="pre">apply</span></tt></a></p>
100<!-- modtime: November 10, 2004 04:40:31 +0000 -->
101<!-- Metafunctions/Miscellaneous//always |20 -->
102</div>
103</div>
104
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