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4<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
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9<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
10<title>The MPL Reference Manual: begin</title>
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13<body class="docframe refmanual">
14<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./back.html" class="navigation-link">Prev</a>&nbsp;<a href="./clear.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./back.html" class="navigation-link">Back</a>&nbsp;<a href="./clear.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./intrinsic-metafunctions.html" class="navigation-link">Up</a>&nbsp;<a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td>
15<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>
16</tr></table><div class="header-separator"></div>
17<div class="section" id="begin">
18<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id356" name="begin">begin</a></h1>
19<div class="section" id="begin-synopsis">
20<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
21<pre class="literal-block">
22template&lt;
23      typename X
24    &gt;
25struct <a href="./begin.html" class="identifier">begin</a>
26{
27    typedef <em>unspecified</em> type;
28};
29</pre>
30</div>
31<div class="section" id="begin-description">
32<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
33<p>Returns an iterator that points to the first element of the sequence. If
34the argument is not a <a class="reference" href="./forward-sequence.html">Forward Sequence</a>, returns <a class="refentry reference" href="./void.html"><tt class="refentry literal"><span class="pre">void_</span></tt></a>.</p>
35</div>
36<div class="section" id="begin-header">
37<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
38<pre class="literal-block">
39#include &lt;<a href="../../../../boost/mpl/begin_end.hpp" class="header">boost/mpl/begin_end.hpp</a>&gt;
40</pre>
41</div>
42<div class="section" id="begin-model-of">
43<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3>
44<p><a class="reference" href="./tag-dispatched.html">Tag Dispatched Metafunction</a></p>
45</div>
46<div class="section" id="begin-parameters">
47<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
48<table border="1" class="table">
49<colgroup>
50<col width="18%" />
51<col width="22%" />
52<col width="60%" />
53</colgroup>
54<thead valign="bottom">
55<tr><th>Parameter</th>
56<th>Requirement</th>
57<th>Description</th>
58</tr>
59</thead>
60<tbody valign="top">
61<tr><td><tt class="literal"><span class="pre">X</span></tt></td>
62<td>Any type</td>
63<td>A type whose begin iterator, if any, will be
64returned.</td>
65</tr>
66</tbody>
67</table>
68</div>
69<div class="section" id="begin-expression-semantics">
70<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
71<p>For any arbitrary type <tt class="literal"><span class="pre">x</span></tt>:</p>
72<pre class="literal-block">
73typedef <a href="./begin.html" class="identifier">begin</a>&lt;x&gt;::type first;
74</pre>
75<table class="field-list" frame="void" rules="none">
76<col class="field-name" />
77<col class="field-body" />
78<tbody valign="top">
79<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><a class="reference" href="./forward-iterator.html">Forward Iterator</a> or <a class="refentry reference" href="./void.html"><tt class="refentry literal"><span class="pre">void_</span></tt></a>.</td>
80</tr>
81<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" href="./forward-sequence.html">Forward Sequence</a>, <tt class="literal"><span class="pre">first</span></tt> is an iterator pointing to the
82first element of <tt class="literal"><span class="pre">s</span></tt>; otherwise <tt class="literal"><span class="pre">first</span></tt> is <a class="refentry reference" href="./void.html"><tt class="refentry literal"><span class="pre">void_</span></tt></a>.</td>
83</tr>
84<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
85is 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>
86</tr>
87</tbody>
88</table>
89</div>
90<div class="section" id="begin-complexity">
91<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
92<p>Amortized constant time.</p>
93</div>
94<div class="section" id="begin-example">
95<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
96<pre class="literal-block">
97typedef <a href="./vector.html" class="identifier">vector</a>&lt; unsigned char,unsigned short,
98    unsigned int,unsigned long &gt; unsigned_types;
99
100typedef <a href="./begin.html" class="identifier">begin</a>&lt;unsigned_types&gt;::type iter;
101<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; ));
102
103<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> > ));
104</pre>
105</div>
106<div class="section" id="begin-see-also">
107<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
108<p><a class="reference" href="./iterators.html">Iterators</a>, <a class="reference" href="./forward-sequence.html">Forward Sequence</a>, <a class="refentry reference" href="./end.html"><tt class="refentry literal"><span class="pre">end</span></tt></a>, <a class="refentry reference" href="./size.html"><tt class="refentry literal"><span class="pre">size</span></tt></a>, <a class="refentry reference" href="./empty.html"><tt class="refentry literal"><span class="pre">empty</span></tt></a></p>
109<!-- modtime: November 13, 2004 01:25:11 +0000 -->
110<!-- Sequences/Intrinsic Metafunctions//clear -->
111</div>
112</div>
113
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