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<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./sequences.html" class="navigation-link">Sequences</a> / <a href="./intrinsic-metafunctions.html" class="navigation-link">Intrinsic Metafunctions</a> / <a href="./front.html" class="navigation-link">front</a></td>
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<div class="section" id="front">
<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1434">front</a></h1>
<div class="section" id="id256">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename Sequence
    &gt;
struct <a href="./front.html" class="identifier">front</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
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<div class="section" id="id257">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>Returns the first element in the sequence.</p>
</div>
<div class="section" id="id258">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/front.hpp" class="header">boost/mpl/front.hpp</a>&gt;
</pre>
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<div class="section" id="id259">
<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="id260">
<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>
</tr>
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<tr><td><tt class="literal"><span class="pre">Sequence</span></tt></td>
<td><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a></td>
<td>A sequence to be examined.</td>
</tr>
</tbody>
</table>
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<div class="section" id="id261">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<p>For any <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">s</span></tt>:</p>
<pre class="literal-block">
typedef <a href="./front.html" class="identifier">front</a>&lt;s&gt;::type t;
</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">Precondition:</th><td class="field-body"><p class="first"><tt class="literal"><span class="pre"><a href="./empty.html" class="identifier">empty</a>&lt;s&gt;::value</span> <span class="pre">==</span> <span class="pre">false</span></tt>.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
<pre class="last literal-block">
typedef <a href="./deref.html" class="identifier">deref</a>&lt; <a href="./begin.html" class="identifier">begin</a>&lt;s&gt;::type &gt;::type t;
</pre>
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</table>
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<div class="section" id="id262">
<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
<p>Amortized constant time.</p>
</div>
<div class="section" id="id263">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
typedef <a href="./list.html" class="identifier">list</a>&lt;long&gt;::type types1;
typedef <a href="./list.html" class="identifier">list</a>&lt;int,long&gt;::type types2;
typedef <a href="./list.html" class="identifier">list</a>&lt;char,int,long&gt;::type types3;

<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; <a href="./front.html" class="identifier">front</a>&lt;types1&gt;::type, long &gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; <a href="./front.html" class="identifier">front</a>&lt;types2&gt;::type, int&gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; <a href="./front.html" class="identifier">front</a>&lt;types3&gt;::type, char&gt; ));
</pre>
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<div class="section" id="id264">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, <a class="reference internal" href="./back.html">back</a>, <a class="reference internal" href="./push-front.html">push_front</a>, <a class="reference internal" href="./begin.html">begin</a>, <a class="reference internal" href="./deref.html">deref</a>, <a class="reference internal" href="./at.html">at</a></p>
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