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<div class="section" id="key-type">
<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1439">key_type</a></h1>
<div class="section" id="id300">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename Sequence
    , typename X
    &gt;
struct <a href="./key-type.html" class="identifier">key_type</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
</div>
<div class="section" id="id301">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>Returns the <a class="reference internal" href="./associative-sequence.html#key-part">key</a> that would be used to identify <tt class="literal"><span class="pre">X</span></tt> in <tt class="literal"><span class="pre">Sequence</span></tt>.</p>
</div>
<div class="section" id="id302">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/key_type.hpp" class="header">boost/mpl/key_type.hpp</a>&gt;
</pre>
</div>
<div class="section" id="id303">
<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3>
<p><a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a></p>
</div>
<div class="section" id="id304">
<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
<table border="1" class="docutils table">
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<col width="17%" />
<col width="30%" />
<col width="53%" />
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<thead valign="bottom">
<tr><th class="head">Parameter</th>
<th class="head">Requirement</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">Sequence</span></tt></td>
<td><a class="reference internal" href="./associative-sequence.html">Associative Sequence</a></td>
<td>A sequence to query.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">X</span></tt></td>
<td>Any type</td>
<td>The type to get the <a class="reference internal" href="./associative-sequence.html#key-part">key</a> for.</td>
</tr>
</tbody>
</table>
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<div class="section" id="id305">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<p>For any <a class="reference internal" href="./associative-sequence.html">Associative Sequence</a> <tt class="literal"><span class="pre">s</span></tt>, iterators <tt class="literal"><span class="pre">pos1</span></tt> and <tt class="literal"><span class="pre">pos2</span></tt> in <tt class="literal"><span class="pre">s</span></tt>, and an
artibrary type <tt class="literal"><span class="pre">x</span></tt>:</p>
<pre class="literal-block">
typedef <a href="./key-type.html" class="identifier">key_type</a>&lt;s,x&gt;::type k;
</pre>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Return type:</th><td class="field-body">A type.</td>
</tr>
<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><tt class="literal"><span class="pre">x</span></tt> can be put in <tt class="literal"><span class="pre">s</span></tt>.</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">k</span></tt> is the <a class="reference internal" href="./associative-sequence.html#key-part">key</a> that would be used to identify <tt class="literal"><span class="pre">x</span></tt> in <tt class="literal"><span class="pre">s</span></tt>.</td>
</tr>
<tr class="field"><th class="field-name">Postcondition:</th><td class="field-body">If <tt class="literal"><span class="pre"><a href="./key-type.html" class="identifier">key_type</a>&lt;</span> <span class="pre">s,<a href="./deref.html" class="identifier">deref</a>&lt;pos1&gt;::type</span> <span class="pre">&gt;::type</span></tt> is identical to
<tt class="literal"><span class="pre"><a href="./key-type.html" class="identifier">key_type</a>&lt;</span> <span class="pre">s,<a href="./deref.html" class="identifier">deref</a>&lt;pos2&gt;::type</span> <span class="pre">&gt;::type</span></tt> then <tt class="literal"><span class="pre">pos1</span></tt> is identical to <tt class="literal"><span class="pre">pos2</span></tt>.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id306">
<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
<p>Amortized constant time.</p>
</div>
<div class="section" id="id307">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
typedef <a href="./key-type.html" class="identifier">key_type</a>&lt; <a href="./map.html" class="identifier">map</a>&lt;&gt;,<a href="./pair.html" class="identifier">pair</a>&lt;int,unsigned&gt; &gt;::type k1;
typedef <a href="./key-type.html" class="identifier">key_type</a>&lt; <a href="./set.html" class="identifier">set</a>&lt;&gt;,<a href="./pair.html" class="identifier">pair</a>&lt;int,unsigned&gt; &gt;::type k2;

<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; k1,int &gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; k2,<a href="./pair.html" class="identifier">pair</a>&lt;int,unsigned&gt; > ));
</pre>
</div>
<div class="section" id="id308">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./associative-sequence.html">Associative Sequence</a>, <a class="reference internal" href="./value-type.html">value_type</a>, <a class="reference internal" href="./has-key.html">has_key</a>, <a class="reference internal" href="./set.html">set</a>, <a class="reference internal" href="./map.html">map</a></p>
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