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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="./begin.html" class="navigation-link">begin</a></td>
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<div class="section" id="begin">
<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1428">begin</a></h1>
<div class="section" id="id202">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename X
    &gt;
struct <a href="./begin.html" class="identifier">begin</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
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<div class="section" id="id203">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>Returns an iterator that points to the first element of the sequence. If
the argument is not a <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, returns <a class="reference internal" href="./void.html">void_</a>.</p>
</div>
<div class="section" id="id204">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/begin_end.hpp" class="header">boost/mpl/begin_end.hpp</a>&gt;
</pre>
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<div class="section" id="id205">
<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="id206">
<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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<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">X</span></tt></td>
<td>Any type</td>
<td>A type whose begin iterator, if any, will be
returned.</td>
</tr>
</tbody>
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</div>
<div class="section" id="id207">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<p>For any arbitrary type <tt class="literal"><span class="pre">x</span></tt>:</p>
<pre class="literal-block">
typedef <a href="./begin.html" class="identifier">begin</a>&lt;x&gt;::type first;
</pre>
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<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><a class="reference internal" href="./forward-iterator.html">Forward Iterator</a> or <a class="reference internal" href="./void.html">void_</a>.</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body">If <tt class="literal"><span class="pre">x</span></tt> is a <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, <tt class="literal"><span class="pre">first</span></tt> is an iterator pointing to the
first element of <tt class="literal"><span class="pre">s</span></tt>; otherwise <tt class="literal"><span class="pre">first</span></tt> is <a class="reference internal" href="./void.html">void_</a>.</td>
</tr>
<tr class="field"><th class="field-name">Postcondition:</th><td class="field-body">If <tt class="literal"><span class="pre">first</span></tt> is an iterator, it is either dereferenceable or past-the-end; it
is past-the-end if and only if <tt class="literal"><span class="pre"><a href="./size.html" class="identifier">size</a>&lt;x&gt;::value</span> <span class="pre">==</span> <span class="pre">0</span></tt>.</td>
</tr>
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<div class="section" id="id208">
<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
<p>Amortized constant time.</p>
</div>
<div class="section" id="id209">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
typedef <a href="./vector.html" class="identifier">vector</a>&lt; unsigned char,unsigned short,
    unsigned int,unsigned long &gt; unsigned_types;

typedef <a href="./begin.html" class="identifier">begin</a>&lt;unsigned_types&gt;::type iter;
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; <a href="./deref.html" class="identifier">deref</a>&lt;iter&gt;::type, unsigned char &gt; ));

<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; <a href="./begin.html" class="identifier">begin</a>&lt;int&gt;::type, <a href="./void.html" class="identifier">void_</a> > ));
</pre>
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<div class="section" id="id210">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./iterators.html">Iterators</a>, <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, <a class="reference internal" href="./end.html">end</a>, <a class="reference internal" href="./size.html">size</a>, <a class="reference internal" href="./empty.html">empty</a></p>
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