1<?xml version="1.0" encoding="utf-8" ?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 4<head> 5<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 6<meta name="generator" content="Docutils 0.7: 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> <a href="./at-c.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./at-c.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./intrinsic-metafunctions.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </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#id1425">at</a></h1> 16<div class="section" id="id177"> 17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> 18<pre class="literal-block"> 19template< 20 typename Sequence 21 , typename N 22 > 23struct <a href="./at.html" class="identifier">at</a> 24{ 25 typedef <em>unspecified</em> type; 26}; 27 28template< 29 typename AssocSeq 30 , typename Key 31 , typename Default = <em>unspecified</em> 32 > 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="id178"> 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 internal" 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><Sequence,N></span></tt> returns the <tt class="literal"><span class="pre">N</span></tt>-th element from the beginning of the 44<a class="reference internal" 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><AssocSeq,Key,Default></span></tt> returns the first element associated with <tt class="literal"><span class="pre">Key</span></tt> 46in the <a class="reference internal" 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="id179"> 51<h3><a class="subsection-title" href="#header" name="header">Header</a></h3> 52<pre class="literal-block"> 53#include <<a href="../../../../boost/mpl/at.hpp" class="header">boost/mpl/at.hpp</a>> 54</pre> 55</div> 56<div class="section" id="id180"> 57<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3> 58<p><a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a></p> 59</div> 60<div class="section" id="id181"> 61<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> 62<table border="1" class="docutils table"> 63<colgroup> 64<col width="17%" /> 65<col width="30%" /> 66<col width="53%" /> 67</colgroup> 68<thead valign="bottom"> 69<tr><th class="head">Parameter</th> 70<th class="head">Requirement</th> 71<th class="head">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 internal" 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 internal" 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 internal" 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="id182"> 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 internal" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">s</span></tt>, and <a class="reference internal" 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><s,n>::type t; 106</pre> 107<table class="compound-last docutils 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"><p class="first">A type.</p> 112</td> 113</tr> 114<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><p class="first"><tt class="literal"><span class="pre">0</span> <span class="pre"><=</span> <span class="pre">n::value</span> <span class="pre"><</span> <span class="pre"><a href="./size.html" class="identifier">size</a><s>::value</span></tt>.</p> 115</td> 116</tr> 117<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p> 118<pre class="last literal-block"> 119typedef <a href="./deref.html" class="identifier">deref</a>< <a href="./advance.html" class="identifier">advance</a>< <a href="./begin.html" class="identifier">begin</a><s>::type,n >::type >::type t; 120</pre> 121</td> 122</tr> 123</tbody> 124</table> 125</div> 126<div class="expression-semantics compound"> 127<p class="compound-first">For any <a class="reference internal" 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> 128<pre class="compound-middle literal-block"> 129typedef <a href="./at.html" class="identifier">at</a><s,key,x>::type t; 130</pre> 131<table class="compound-middle docutils field-list" frame="void" rules="none"> 132<col class="field-name" /> 133<col class="field-body" /> 134<tbody valign="top"> 135<tr class="field"><th class="field-name">Return type:</th><td class="field-body">A type.</td> 136</tr> 137<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><s,key>::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>; 138otherwise <tt class="literal"><span class="pre">t</span></tt> is identical to <tt class="literal"><span class="pre">x</span></tt>.</td> 139</tr> 140</tbody> 141</table> 142<!-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ --> 143<pre class="compound-middle literal-block"> 144typedef <a href="./at.html" class="identifier">at</a><s,key>::type t; 145</pre> 146<table class="compound-last docutils field-list" frame="void" rules="none"> 147<col class="field-name" /> 148<col class="field-body" /> 149<tbody valign="top"> 150<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p> 151</td> 152</tr> 153<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p> 154<pre class="last literal-block"> 155typedef <a href="./at.html" class="identifier">at</a><s,key,<a href="./void.html" class="identifier">void_</a>>::type t; 156</pre> 157</td> 158</tr> 159</tbody> 160</table> 161</div> 162</div> 163<div class="section" id="complexity"> 164<h3>Complexity</h3> 165<table border="1" class="docutils table"> 166<colgroup> 167<col width="47%" /> 168<col width="53%" /> 169</colgroup> 170<thead valign="bottom"> 171<tr><th class="head">Sequence archetype</th> 172<th class="head">Complexity</th> 173</tr> 174</thead> 175<tbody valign="top"> 176<tr><td><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a></td> 177<td>Linear.</td> 178</tr> 179<tr><td><a class="reference internal" href="./random-access-sequence.html">Random Access Sequence</a></td> 180<td>Amortized constant time.</td> 181</tr> 182<tr><td><a class="reference internal" href="./associative-sequence.html">Associative Sequence</a></td> 183<td>Amortized constant time.</td> 184</tr> 185</tbody> 186</table> 187</div> 188<div class="section" id="id183"> 189<h3><a class="subsection-title" href="#example" name="example">Example</a></h3> 190<pre class="literal-block"> 191typedef <a href="./range-c.html" class="identifier">range_c</a><long,10,50> range; 192<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./at.html" class="identifier">at</a>< range, <a href="./int.html" class="identifier">int_</a><0> >::value), ==, 10 ); 193<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./at.html" class="identifier">at</a>< range, <a href="./int.html" class="identifier">int_</a><10> >::value), ==, 20 ); 194<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./at.html" class="identifier">at</a>< range, <a href="./int.html" class="identifier">int_</a><40> >::value), ==, 50 ); 195</pre> 196<pre class="literal-block"> 197typedef <a href="./set.html" class="identifier">set</a>< int const,long*,double > s; 198 199<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< <a href="./at.html" class="identifier">at</a><s,char>::type, <a href="./void.html" class="identifier">void_</a> > )); 200<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< <a href="./at.html" class="identifier">at</a><s,int>::type, int > )); 201</pre> 202</div> 203<div class="section" id="id184"> 204<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> 205<p><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, <a class="reference internal" href="./random-access-sequence.html">Random Access Sequence</a>, <a class="reference internal" href="./associative-sequence.html">Associative Sequence</a>, <a class="reference internal" href="./at-c.html">at_c</a>, <a class="reference internal" href="./front.html">front</a>, <a class="reference internal" href="./back.html">back</a></p> 206<!-- Sequences/Intrinsic Metafunctions//at_c --> 207</div> 208</div> 209 210<div class="footer-separator"></div> 211<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> <a href="./at-c.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./at-c.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./intrinsic-metafunctions.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td> 212<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> 213Distributed under the Boost Software License, Version 1.0. (See accompanying 214file 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> 215</html> 216