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<div class="section" id="has-key">
<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1435">has_key</a></h1>
<div class="section" id="id265">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename Sequence
    , typename Key
    &gt;
struct <a href="./has-key.html" class="identifier">has_key</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
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<div class="section" id="id266">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>Returns a true-valued <a class="reference internal" href="./integral-constant.html">Integral Constant</a> if <tt class="literal"><span class="pre">Sequence</span></tt> contains an element
with key <tt class="literal"><span class="pre">Key</span></tt>.</p>
</div>
<div class="section" id="id267">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/has_key.hpp" class="header">boost/mpl/has_key.hpp</a>&gt;
</pre>
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<div class="section" id="id268">
<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>
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<div class="section" id="id269">
<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
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<tr><th class="head">Parameter</th>
<th class="head">Requirement</th>
<th class="head">Description</th>
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<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">Key</span></tt></td>
<td>Any type</td>
<td>The queried key.</td>
</tr>
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<div class="section" id="id270">
<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>, and arbitrary type <tt class="literal"><span class="pre">key</span></tt>:</p>
<pre class="literal-block">
typedef <a href="./has-key.html" class="identifier">has_key</a>&lt;s,key&gt;::type c;
</pre>
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<tr class="field"><th class="field-name">Return type:</th><td class="field-body">Boolean <a class="reference internal" href="./integral-constant.html">Integral Constant</a>.</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">c::value</span> <span class="pre">==</span> <span class="pre">true</span></tt> if <tt class="literal"><span class="pre">key</span></tt> is in <tt class="literal"><span class="pre">s</span></tt>'s set of keys; otherwise
<tt class="literal"><span class="pre">c::value</span> <span class="pre">==</span> <span class="pre">false</span></tt>.</td>
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<div class="section" id="id271">
<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
<p>Amortized constant time.</p>
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<div class="section" id="id272">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
typedef <a href="./map.html" class="identifier">map</a>&lt; <a href="./pair.html" class="identifier">pair</a>&lt;int,unsigned&gt;, <a href="./pair.html" class="identifier">pair</a>&lt;char,long&gt; &gt; m;
<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>(( <a href="./has-key.html" class="identifier">has_key</a>&lt;m,long&gt; ));

typedef <a href="./insert.html" class="identifier">insert</a>&lt; m, <a href="./pair.html" class="identifier">pair</a>&lt;long,unsigned long&gt; &gt; m1;
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( <a href="./has-key.html" class="identifier">has_key</a>&lt;m1,long&gt; ));
</pre>
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<div class="section" id="id273">
<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="./count.html">count</a>, <a class="reference internal" href="./insert.html">insert</a>, <a class="reference internal" href="./erase-key.html">erase_key</a></p>
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