ceph/src/boost/libs/mpl/doc/refmanual/at.html

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   7 <title>The MPL Reference Manual: at</title>
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  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>
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  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">
  19 template&lt;
  20       typename Sequence
  21     , typename N
  22     &gt;
  23 struct <a href="./at.html" class="identifier">at</a>
  24 {
  25     typedef <em>unspecified</em> type;
  26 };
  27
  28 template&lt;
  29       typename AssocSeq
  30     , typename Key
  31     , typename Default = <em>unspecified</em>
  32     &gt;
  33 struct <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>&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 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>&lt;AssocSeq,Key,Default&gt;</span></tt> returns the first element associated with <tt class="literal"><span class="pre">Key</span></tt>
  46 in 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
  47 exists.</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 &lt;<a href="../../../../boost/mpl/at.hpp" class="header">boost/mpl/at.hpp</a>&gt;
  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
  86 specifying 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
  95 not 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">
 105 typedef <a href="./at.html" class="identifier">at</a>&lt;s,n&gt;::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">&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>.</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">
 119 typedef <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;
 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">
 129 typedef <a href="./at.html" class="identifier">at</a>&lt;s,key,x&gt;::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>&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>;
 138 otherwise <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">
 144 typedef <a href="./at.html" class="identifier">at</a>&lt;s,key&gt;::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">
 155 typedef <a href="./at.html" class="identifier">at</a>&lt;s,key,<a href="./void.html" class="identifier">void_</a>&gt;::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">
 191 typedef <a href="./range-c.html" class="identifier">range_c</a>&lt;long,10,50&gt; range;
 192 <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 );
 193 <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 );
 194 <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 );
 195 </pre>
 196 <pre class="literal-block">
 197 typedef <a href="./set.html" class="identifier">set</a>&lt; int const,long*,double &gt; s;
 198
 199 <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; ));
 200 <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 > ));
 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>
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 212 <td><div class="copyright-footer"><div class="copyright">Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams</div>
 213 Distributed under the Boost Software License, Version 1.0. (See accompanying
 214 file 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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