1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>at_key</title> 5<link rel="stylesheet" href="../../../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../../../index.html" title="Chapter 1. Fusion 2.2"> 8<link rel="up" href="../metafunctions.html" title="Metafunctions"> 9<link rel="prev" href="has_key.html" title="has_key"> 10<link rel="next" href="value_at_key.html" title="value_at_key"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="has_key.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../metafunctions.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="value_at_key.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h5 class="title"> 27<a name="fusion.sequence.intrinsic.metafunctions.at_key"></a><a class="link" href="at_key.html" title="at_key">at_key</a> 28</h5></div></div></div> 29<h6> 30<a name="fusion.sequence.intrinsic.metafunctions.at_key.h0"></a> 31 <span class="phrase"><a name="fusion.sequence.intrinsic.metafunctions.at_key.description"></a></span><a class="link" href="at_key.html#fusion.sequence.intrinsic.metafunctions.at_key.description">Description</a> 32 </h6> 33<p> 34 Returns the result type of <a class="link" href="../functions/at_key.html" title="at_key"><code class="computeroutput"><span class="identifier">at_key</span></code></a><a href="#ftn.fusion.sequence.intrinsic.metafunctions.at_key.f0" class="footnote" name="fusion.sequence.intrinsic.metafunctions.at_key.f0"><sup class="footnote">[9]</sup></a>. 35 </p> 36<h6> 37<a name="fusion.sequence.intrinsic.metafunctions.at_key.h1"></a> 38 <span class="phrase"><a name="fusion.sequence.intrinsic.metafunctions.at_key.synopsis"></a></span><a class="link" href="at_key.html#fusion.sequence.intrinsic.metafunctions.at_key.synopsis">Synopsis</a> 39 </h6> 40<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> 41 <span class="keyword">typename</span> <span class="identifier">Seq</span><span class="special">,</span> 42 <span class="keyword">typename</span> <span class="identifier">Key</span><span class="special">></span> 43<span class="keyword">struct</span> <span class="identifier">at_key</span> 44<span class="special">{</span> 45 <span class="keyword">typedef</span> <span class="emphasis"><em>unspecified</em></span> <span class="identifier">type</span><span class="special">;</span> 46<span class="special">};</span> 47</pre> 48<div class="table"> 49<a name="fusion.sequence.intrinsic.metafunctions.at_key.t0"></a><p class="title"><b>Table 1.33. Parameters</b></p> 50<div class="table-contents"><table class="table" summary="Parameters"> 51<colgroup> 52<col> 53<col> 54<col> 55</colgroup> 56<thead><tr> 57<th> 58 <p> 59 Parameter 60 </p> 61 </th> 62<th> 63 <p> 64 Requirement 65 </p> 66 </th> 67<th> 68 <p> 69 Description 70 </p> 71 </th> 72</tr></thead> 73<tbody> 74<tr> 75<td> 76 <p> 77 <code class="computeroutput"><span class="identifier">Seq</span></code> 78 </p> 79 </td> 80<td> 81 <p> 82 A model of <a class="link" href="../../concepts/associative_sequence.html" title="Associative Sequence">Associative 83 Sequence</a> 84 </p> 85 </td> 86<td> 87 <p> 88 Argument sequence 89 </p> 90 </td> 91</tr> 92<tr> 93<td> 94 <p> 95 <code class="computeroutput"><span class="identifier">Key</span></code> 96 </p> 97 </td> 98<td> 99 <p> 100 Any type 101 </p> 102 </td> 103<td> 104 <p> 105 Key type 106 </p> 107 </td> 108</tr> 109</tbody> 110</table></div> 111</div> 112<br class="table-break"><h6> 113<a name="fusion.sequence.intrinsic.metafunctions.at_key.h2"></a> 114 <span class="phrase"><a name="fusion.sequence.intrinsic.metafunctions.at_key.expression_semantics"></a></span><a class="link" href="at_key.html#fusion.sequence.intrinsic.metafunctions.at_key.expression_semantics">Expression 115 Semantics</a> 116 </h6> 117<pre class="programlisting"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at_key</span><span class="special"><</span><span class="identifier">Seq</span><span class="special">,</span> <span class="identifier">Key</span><span class="special">>::</span><span class="identifier">type</span> 118</pre> 119<p> 120 <span class="bold"><strong>Return type</strong></span>: Any type. 121 </p> 122<p> 123 <span class="bold"><strong>Precondition</strong></span>: <code class="computeroutput"><span class="identifier">has_key</span><span class="special"><</span><span class="identifier">Seq</span><span class="special">,</span> <span class="identifier">Key</span><span class="special">>::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">value</span> <span class="special">==</span> <span class="keyword">true</span></code> 124 (where <code class="computeroutput"><span class="identifier">Seq</span></code> is not <a class="link" href="../../concepts/unbounded_sequence.html" title="Unbounded Sequence">Unbounded Sequence</a>) 125 </p> 126<p> 127 <span class="bold"><strong>Semantics</strong></span>: Returns the result of using 128 <a class="link" href="../functions/at_key.html" title="at_key"><code class="computeroutput"><span class="identifier">at_key</span></code></a> to access the element 129 with key type <code class="computeroutput"><span class="identifier">Key</span></code> in 130 <code class="computeroutput"><span class="identifier">Seq</span></code>. 131 </p> 132<h6> 133<a name="fusion.sequence.intrinsic.metafunctions.at_key.h3"></a> 134 <span class="phrase"><a name="fusion.sequence.intrinsic.metafunctions.at_key.header"></a></span><a class="link" href="at_key.html#fusion.sequence.intrinsic.metafunctions.at_key.header">Header</a> 135 </h6> 136<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">sequence</span><span class="special">/</span><span class="identifier">intrinsic</span><span class="special">/</span><span class="identifier">at_key</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 137<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">at_key</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 138</pre> 139<h6> 140<a name="fusion.sequence.intrinsic.metafunctions.at_key.h4"></a> 141 <span class="phrase"><a name="fusion.sequence.intrinsic.metafunctions.at_key.example"></a></span><a class="link" href="at_key.html#fusion.sequence.intrinsic.metafunctions.at_key.example">Example</a> 142 </h6> 143<pre class="programlisting"><span class="keyword">typedef</span> <a class="link" href="../../../container/map.html" title="map"><code class="computeroutput"><span class="identifier">map</span></code></a><span class="special"><</span><a class="link" href="../../../support/pair.html" title="pair"><code class="computeroutput"><span class="identifier">pair</span></code></a><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">char</span><span class="special">>,</span> <a class="link" href="../../../support/pair.html" title="pair"><code class="computeroutput"><span class="identifier">pair</span></code></a><span class="special"><</span><span class="keyword">char</span><span class="special">,</span> <span class="keyword">char</span><span class="special">>,</span> <a class="link" href="../../../support/pair.html" title="pair"><code class="computeroutput"><span class="identifier">pair</span></code></a><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">char</span><span class="special">></span> <span class="special">></span> <span class="identifier">mymap</span><span class="special">;</span> 144<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span><a class="link" href="at_key.html" title="at_key"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at_key</span></code></a><span class="special"><</span><span class="identifier">mymap</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span><span class="special">,</span> <span class="keyword">char</span><span class="special">&>));</span> 145</pre> 146<div class="footnotes"> 147<br><hr style="width:100; text-align:left;margin-left: 0"> 148<div id="ftn.fusion.sequence.intrinsic.metafunctions.at_key.f0" class="footnote"><p><a href="#fusion.sequence.intrinsic.metafunctions.at_key.f0" class="para"><sup class="para">[9] </sup></a> 149 <a class="link" href="at_key.html" title="at_key"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">at_key</span></code></a> reflects the actual 150 return type of the function <a class="link" href="../functions/at_key.html" title="at_key"><code class="computeroutput"><span class="identifier">at_key</span></code></a>. <a class="link" href="../../../sequence.html" title="Sequence">Sequence</a>(s) 151 typically return references to its elements via the <a class="link" href="../functions/at_key.html" title="at_key"><code class="computeroutput"><span class="identifier">at_key</span></code></a> function. If you want 152 to get the actual element type, use <a class="link" href="value_at_key.html" title="value_at_key"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">value_at_key</span></code></a> 153 </p></div> 154</div> 155</div> 156<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 157<td align="left"></td> 158<td align="right"><div class="copyright-footer">Copyright © 2001-2006, 2011, 2012 Joel de Guzman, 159 Dan Marsden, Tobias Schwinger<p> 160 Distributed under the Boost Software License, Version 1.0. (See accompanying 161 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 162 </p> 163</div></td> 164</tr></table> 165<hr> 166<div class="spirit-nav"> 167<a accesskey="p" href="has_key.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../metafunctions.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="value_at_key.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a> 168</div> 169</body> 170</html> 171
