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6<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
7<title>The MPL Reference Manual: at</title>
8<link rel="stylesheet" href="../style.css" type="text/css" />
9</head>
10<body class="docframe refmanual">
11<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./intrinsic-metafunctions.html" class="navigation-link">Prev</a>&nbsp;<a href="./at-c.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group">Back&nbsp;<a href="./at-c.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="./at.html" class="navigation-link">at</a></td>
13</tr></table><div class="header-separator"></div>
14<div class="section" id="at">
15<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id353" name="at">at</a></h1>
16<div class="section" id="at-synopsis">
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 N
22    &gt;
23struct <a href="./at.html" class="identifier">at</a>
24{
25    typedef <em>unspecified</em> type;
26};
27
28template&lt;
29      typename AssocSeq
30    , typename Key
31    , typename Default = <em>unspecified</em>
32    &gt;
33struct <a href="./at.html" class="identifier">at</a>
34{
35    typedef <em>unspecified</em> type;
36};
37</pre>
38</div>
39<div class="section" id="at-description">
40<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
41<p><tt class="literal"><span class="pre"><a href="./at.html" class="identifier">at</a></span></tt> is an <a class="reference" href="./terminology.html#overloaded-name">overloaded name</a>:</p>
42<ul class="simple">
43<li><tt class="literal"><span class="pre"><a href="./at.html" class="identifier">at</a>&lt;Sequence,N&gt;</span></tt> returns the <tt class="literal"><span class="pre">N</span></tt>-th element from the beginning of the
44<a class="reference" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">Sequence</span></tt>.</li>
45<li><tt class="literal"><span class="pre"><a href="./at.html" class="identifier">at</a>&lt;AssocSeq,Key,Default&gt;</span></tt> returns the first element associated with <tt class="literal"><span class="pre">Key</span></tt> 
46in the <a class="reference" href="./associative-sequence.html">Associative Sequence</a> <tt class="literal"><span class="pre">AssocSeq</span></tt>, or <tt class="literal"><span class="pre">Default</span></tt> if no such element
47exists.</li>
48</ul>
49</div>
50<div class="section" id="at-header">
51<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
52<pre class="literal-block">
53#include &lt;<a href="../../../../boost/mpl/at.hpp" class="header">boost/mpl/at.hpp</a>&gt;
54</pre>
55</div>
56<div class="section" id="at-model-of">
57<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3>
58<p><a class="reference" href="./tag-dispatched.html">Tag Dispatched Metafunction</a></p>
59</div>
60<div class="section" id="at-parameters">
61<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
62<table border="1" class="table">
63<colgroup>
64<col width="17%" />
65<col width="30%" />
66<col width="53%" />
67</colgroup>
68<thead valign="bottom">
69<tr><th>Parameter</th>
70<th>Requirement</th>
71<th>Description</th>
72</tr>
73</thead>
74<tbody valign="top">
75<tr><td><tt class="literal"><span class="pre">Sequence</span></tt></td>
76<td><a class="reference" href="./forward-sequence.html">Forward Sequence</a></td>
77<td>A sequence to be examined.</td>
78</tr>
79<tr><td><tt class="literal"><span class="pre">AssocSeq</span></tt></td>
80<td><a class="reference" href="./associative-sequence.html">Associative Sequence</a></td>
81<td>A sequence to be examined.</td>
82</tr>
83<tr><td><tt class="literal"><span class="pre">N</span></tt></td>
84<td><a class="reference" href="./integral-constant.html">Integral Constant</a></td>
85<td>An offset from the beginning of the sequence
86specifying the element to be retrieved.</td>
87</tr>
88<tr><td><tt class="literal"><span class="pre">Key</span></tt></td>
89<td>Any type</td>
90<td>A key for the element to be retrieved.</td>
91</tr>
92<tr><td><tt class="literal"><span class="pre">Default</span></tt></td>
93<td>Any type</td>
94<td>A default value to return if the element is
95not found.</td>
96</tr>
97</tbody>
98</table>
99</div>
100<div class="section" id="at-expression-semantics">
101<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
102<div class="expression-semantics compound">
103<p class="compound-first">For any <a class="reference" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">s</span></tt>, and <a class="reference" href="./integral-constant.html">Integral Constant</a> <tt class="literal"><span class="pre">n</span></tt>:</p>
104<pre class="compound-middle literal-block">
105typedef <a href="./at.html" class="identifier">at</a>&lt;s,n&gt;::type t;
106</pre>
107<table class="compound-last field-list" frame="void" rules="none">
108<col class="field-name" />
109<col class="field-body" />
110<tbody valign="top">
111<tr class="field"><th class="field-name">Return type:</th><td class="field-body">A type.</td>
112</tr>
113<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><tt class="literal"><span class="pre">0</span> <span class="pre">&lt;=</span> <span class="pre">n::value</span> <span class="pre">&lt;</span> <span class="pre"><a href="./size.html" class="identifier">size</a>&lt;s&gt;::value</span></tt>.</td>
114</tr>
115<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
116<pre class="last literal-block">
117typedef <a href="./deref.html" class="identifier">deref</a>&lt; <a href="./advance.html" class="identifier">advance</a>&lt; <a href="./begin.html" class="identifier">begin</a>&lt;s&gt;::type,n &gt;::type &gt;::type t;
118</pre>
119</td>
120</tr>
121</tbody>
122</table>
123</div>
124<div class="expression-semantics compound">
125<p class="compound-first">For any <a class="reference" href="./associative-sequence.html">Associative Sequence</a> <tt class="literal"><span class="pre">s</span></tt>, and arbitrary types <tt class="literal"><span class="pre">key</span></tt> and <tt class="literal"><span class="pre">x</span></tt>:</p>
126<pre class="compound-middle literal-block">
127typedef <a href="./at.html" class="identifier">at</a>&lt;s,key,x&gt;::type t;
128</pre>
129<table class="compound-middle field-list" frame="void" rules="none">
130<col class="field-name" />
131<col class="field-body" />
132<tbody valign="top">
133<tr class="field"><th class="field-name">Return type:</th><td class="field-body">A type.</td>
134</tr>
135<tr class="field"><th class="field-name">Semantics:</th><td class="field-body">If <tt class="literal"><span class="pre"><a href="./has-key.html" class="identifier">has_key</a>&lt;s,key&gt;::value</span> <span class="pre">==</span> <span class="pre">true</span></tt>, <tt class="literal"><span class="pre">t</span></tt> is the value type associated with <tt class="literal"><span class="pre">key</span></tt>;
136otherwise <tt class="literal"><span class="pre">t</span></tt> is identical to <tt class="literal"><span class="pre">x</span></tt>.</td>
137</tr>
138</tbody>
139</table>
140<!-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -->
141<pre class="compound-middle literal-block">
142typedef <a href="./at.html" class="identifier">at</a>&lt;s,key&gt;::type t;
143</pre>
144<table class="compound-last field-list" frame="void" rules="none">
145<col class="field-name" />
146<col class="field-body" />
147<tbody valign="top">
148<tr class="field"><th class="field-name">Return type:</th><td class="field-body">A type.</td>
149</tr>
150<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
151<pre class="last literal-block">
152typedef <a href="./at.html" class="identifier">at</a>&lt;s,key,<a href="./void.html" class="identifier">void_</a>&gt;::type t;
153</pre>
154</td>
155</tr>
156</tbody>
157</table>
158</div>
159</div>
160<div class="section" id="complexity">
161<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
162<table border="1" class="table">
163<colgroup>
164<col width="47%" />
165<col width="53%" />
166</colgroup>
167<thead valign="bottom">
168<tr><th>Sequence archetype</th>
169<th>Complexity</th>
170</tr>
171</thead>
172<tbody valign="top">
173<tr><td><a class="reference" href="./forward-sequence.html">Forward Sequence</a></td>
174<td>Linear.</td>
175</tr>
176<tr><td><a class="reference" href="./random-access-sequence.html">Random Access Sequence</a></td>
177<td>Amortized constant time.</td>
178</tr>
179<tr><td><a class="reference" href="./associative-sequence.html">Associative Sequence</a></td>
180<td>Amortized constant time.</td>
181</tr>
182</tbody>
183</table>
184</div>
185<div class="section" id="at-example">
186<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
187<pre class="literal-block">
188typedef <a href="./range-c.html" class="identifier">range_c</a>&lt;long,10,50&gt; range;
189<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./at.html" class="identifier">at</a>&lt; range, <a href="./int.html" class="identifier">int_</a>&lt;0&gt; &gt;::value), ==, 10 );
190<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./at.html" class="identifier">at</a>&lt; range, <a href="./int.html" class="identifier">int_</a>&lt;10&gt; &gt;::value), ==, 20 );
191<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./at.html" class="identifier">at</a>&lt; range, <a href="./int.html" class="identifier">int_</a>&lt;40&gt; &gt;::value), ==, 50 );
192</pre>
193<pre class="literal-block">
194typedef <a href="./set.html" class="identifier">set</a>&lt; int const,long*,double &gt; s;
195
196<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; <a href="./at.html" class="identifier">at</a>&lt;s,char&gt;::type, <a href="./void.html" class="identifier">void_</a> &gt; ));
197<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; <a href="./at.html" class="identifier">at</a>&lt;s,int&gt;::type, int > ));
198</pre>
199</div>
200<div class="section" id="at-see-also">
201<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
202<p><a class="reference" href="./forward-sequence.html">Forward Sequence</a>, <a class="reference" href="./random-access-sequence.html">Random Access Sequence</a>, <a class="reference" href="./associative-sequence.html">Associative Sequence</a>, <a class="refentry reference" href="./at-c.html"><tt class="refentry literal"><span class="pre">at_c</span></tt></a>, <a class="refentry reference" href="./front.html"><tt class="refentry literal"><span class="pre">front</span></tt></a>, <a class="refentry reference" href="./back.html"><tt class="refentry literal"><span class="pre">back</span></tt></a></p>
203<!-- modtime: November 13, 2004 01:23:02 +0000 -->
204<!-- Sequences/Intrinsic Metafunctions//at_c -->
205</div>
206</div>
207
208<div class="footer-separator"></div>
209<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./intrinsic-metafunctions.html" class="navigation-link">Prev</a>&nbsp;<a href="./at-c.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group">Back&nbsp;<a href="./at-c.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>
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