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7<title>The MPL Reference Manual: value_type</title>
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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="./value-type.html" class="navigation-link">value_type</a></td>
13</tr></table><div class="header-separator"></div>
14<div class="section" id="value-type">
15<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1447">value_type</a></h1>
16<div class="section" id="id371">
17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
18<pre class="literal-block">
19template&lt;
20 typename Sequence
21 , typename X
22 &gt;
23struct <a href="./value-type.html" class="identifier">value_type</a>
24{
25 typedef <em>unspecified</em> type;
26};
27</pre>
28</div>
29<div class="section" id="id372">
30<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
31<p>Returns the <a class="reference internal" href="./associative-sequence.html#value-part">value</a> that would be used for element <tt class="literal"><span class="pre">X</span></tt> in <tt class="literal"><span class="pre">Sequence</span></tt>.</p>
32</div>
33<div class="section" id="id373">
34<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
35<pre class="literal-block">
36#include &lt;<a href="../../../../boost/mpl/value_type.hpp" class="header">boost/mpl/value_type.hpp</a>&gt;
37</pre>
38</div>
39<div class="section" id="id374">
40<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3>
41<p><a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a></p>
42</div>
43<div class="section" id="id375">
44<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
45<table border="1" class="docutils table">
46<colgroup>
47<col width="17%" />
48<col width="30%" />
49<col width="53%" />
50</colgroup>
51<thead valign="bottom">
52<tr><th class="head">Parameter</th>
53<th class="head">Requirement</th>
54<th class="head">Description</th>
55</tr>
56</thead>
57<tbody valign="top">
58<tr><td><tt class="literal"><span class="pre">Sequence</span></tt></td>
59<td><a class="reference internal" href="./associative-sequence.html">Associative Sequence</a></td>
60<td>A sequence to query.</td>
61</tr>
62<tr><td><tt class="literal"><span class="pre">X</span></tt></td>
63<td>Any type</td>
64<td>The type to get the <a class="reference internal" href="./associative-sequence.html#value-part">value</a> for.</td>
65</tr>
66</tbody>
67</table>
68</div>
69<div class="section" id="id376">
70<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
71<p>For any <a class="reference internal" href="./associative-sequence.html">Associative Sequence</a> <tt class="literal"><span class="pre">s</span></tt>, and an artibrary type <tt class="literal"><span class="pre">x</span></tt>:</p>
72<pre class="literal-block">
73typedef <a href="./value-type.html" class="identifier">value_type</a>&lt;s,x&gt;::type v;
74</pre>
75<table class="docutils 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"><p class="first">A type.</p>
80</td>
81</tr>
82<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><p class="first"><tt class="literal"><span class="pre">x</span></tt> can be put in <tt class="literal"><span class="pre">s</span></tt>.</p>
83</td>
84</tr>
85<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first"><tt class="literal"><span class="pre">v</span></tt> is the <a class="reference internal" href="./associative-sequence.html#value-part">value</a> that would be used for <tt class="literal"><span class="pre">x</span></tt> in <tt class="literal"><span class="pre">s</span></tt>.</p>
86</td>
87</tr>
88<tr class="field"><th class="field-name">Postcondition:</th><td class="field-body"><p class="first">If
89.. parsed-literal:</p>
90<pre class="literal-block">
91<a href="./has-key.html" class="identifier">has_key</a>&lt; s,<a href="./key-type.html" class="identifier">key_type</a>&lt;s,x&gt;::type &gt;::type
92</pre>
93<p>then
94.. parsed-literal:</p>
95<pre class="literal-block">
96<a href="./at.html" class="identifier">at</a>&lt; s,<a href="./key-type.html" class="identifier">key_type</a>&lt;s,x&gt;::type &gt;::type
97</pre>
98<p class="last">is identical to <tt class="literal"><span class="pre"><a href="./value-type.html" class="identifier">value_type</a>&lt;s,x&gt;::type</span></tt>.</p>
99</td>
100</tr>
101</tbody>
102</table>
103</div>
104<div class="section" id="id377">
105<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
106<p>Amortized constant time.</p>
107</div>
108<div class="section" id="id378">
109<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
110<pre class="literal-block">
111typedef <a href="./value-type.html" class="identifier">value_type</a>&lt; <a href="./map.html" class="identifier">map</a>&lt;&gt;,<a href="./pair.html" class="identifier">pair</a>&lt;int,unsigned&gt; &gt;::type v1;
112typedef <a href="./value-type.html" class="identifier">value_type</a>&lt; <a href="./set.html" class="identifier">set</a>&lt;&gt;,<a href="./pair.html" class="identifier">pair</a>&lt;int,unsigned&gt; &gt;::type v2;
113
114<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; v1,unsigned &gt; ));
115<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; v2,<a href="./pair.html" class="identifier">pair</a>&lt;int,unsigned&gt; > ));
116</pre>
117</div>
118<div class="section" id="id379">
119<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
120<p><a class="reference internal" href="./associative-sequence.html">Associative Sequence</a>, <a class="reference internal" href="./key-type.html">key_type</a>, <a class="reference internal" href="./at.html">at</a>, <a class="reference internal" href="./set.html">set</a>, <a class="reference internal" href="./map.html">map</a></p>
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126<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div>
127Distributed under the Boost Software License, Version 1.0. (See accompanying
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