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6<meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" />
7<title>The MPL Reference Manual: front</title>
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10<body class="docframe refmanual">
11<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./erase-key.html" class="navigation-link">Prev</a>&nbsp;<a href="./has-key.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./erase-key.html" class="navigation-link">Back</a>&nbsp;<a href="./has-key.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>
12<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>
13</tr></table><div class="header-separator"></div>
14<div class="section" id="front">
15<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1434">front</a></h1>
16<div class="section" id="id256">
17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
18<pre class="literal-block">
19template&lt;
20 typename Sequence
21 &gt;
22struct <a href="./front.html" class="identifier">front</a>
23{
24 typedef <em>unspecified</em> type;
25};
26</pre>
27</div>
28<div class="section" id="id257">
29<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
30<p>Returns the first element in the sequence.</p>
31</div>
32<div class="section" id="id258">
33<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
34<pre class="literal-block">
35#include &lt;<a href="../../../../boost/mpl/front.hpp" class="header">boost/mpl/front.hpp</a>&gt;
36</pre>
37</div>
38<div class="section" id="id259">
39<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3>
40<p><a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a></p>
41</div>
42<div class="section" id="id260">
43<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
44<table border="1" class="docutils table">
45<colgroup>
46<col width="18%" />
47<col width="27%" />
48<col width="55%" />
49</colgroup>
50<thead valign="bottom">
51<tr><th class="head">Parameter</th>
52<th class="head">Requirement</th>
53<th class="head">Description</th>
54</tr>
55</thead>
56<tbody valign="top">
57<tr><td><tt class="literal"><span class="pre">Sequence</span></tt></td>
58<td><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a></td>
59<td>A sequence to be examined.</td>
60</tr>
61</tbody>
62</table>
63</div>
64<div class="section" id="id261">
65<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
66<p>For any <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">s</span></tt>:</p>
67<pre class="literal-block">
68typedef <a href="./front.html" class="identifier">front</a>&lt;s&gt;::type t;
69</pre>
70<table class="docutils field-list" frame="void" rules="none">
71<col class="field-name" />
72<col class="field-body" />
73<tbody valign="top">
74<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p>
75</td>
76</tr>
77<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>
78</td>
79</tr>
80<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
81<pre class="last literal-block">
82typedef <a href="./deref.html" class="identifier">deref</a>&lt; <a href="./begin.html" class="identifier">begin</a>&lt;s&gt;::type &gt;::type t;
83</pre>
84</td>
85</tr>
86</tbody>
87</table>
88</div>
89<div class="section" id="id262">
90<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
91<p>Amortized constant time.</p>
92</div>
93<div class="section" id="id263">
94<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
95<pre class="literal-block">
96typedef <a href="./list.html" class="identifier">list</a>&lt;long&gt;::type types1;
97typedef <a href="./list.html" class="identifier">list</a>&lt;int,long&gt;::type types2;
98typedef <a href="./list.html" class="identifier">list</a>&lt;char,int,long&gt;::type types3;
99
100<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; ));
101<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; ));
102<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; ));
103</pre>
104</div>
105<div class="section" id="id264">
106<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
107<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>
108<!-- Sequences/Intrinsic Metafunctions//has_key -->
109</div>
110</div>
111
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114<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div>
115Distributed under the Boost Software License, Version 1.0. (See accompanying
116file LICENSE_1_0.txt or copy at <a class="reference external" href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</div></td></tr></table></body>
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