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<div class="section" id="identity">
<h1><a class="toc-backref" href="./miscellaneous.html#id1564">identity</a></h1>
<div class="section" id="id1146">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename X
    &gt;
struct <a href="./identity.html" class="identifier">identity</a>
{
    typedef X type;
};
</pre>
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<div class="section" id="id1147">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<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>
</div>
<div class="section" id="id1149">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/identity.hpp" class="header">boost/mpl/identity.hpp</a>&gt;
</pre>
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<div class="section" id="id1150">
<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3>
<p><a class="reference internal" href="./metafunction.html">Metafunction</a></p>
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<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
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<th class="head">Requirement</th>
<th class="head">Description</th>
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<tr><td><tt class="literal"><span class="pre">X</span></tt></td>
<td>Any type</td>
<td>An argument to be returned.</td>
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<div class="section" id="id1152">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<p>For an arbitrary type <tt class="literal"><span class="pre">x</span></tt>:</p>
<pre class="literal-block">
typedef <a href="./identity.html" class="identifier">identity</a>&lt;x&gt;::type r;
</pre>
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<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p>
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<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
<pre class="literal-block">
typedef x r;
</pre>
</td>
</tr>
<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>
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<div class="section" id="id1153">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
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;
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;

<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t1, char &gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t2, int > ));
</pre>
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<div class="section" id="id1154">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<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>
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