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| <title>The MPL Reference Manual: at</title> |
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| <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> |
| <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> |
| </tr></table><div class="header-separator"></div> |
| <div class="section" id="at"> |
| <h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1425">at</a></h1> |
| <div class="section" id="id177"> |
| <h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> |
| <pre class="literal-block"> |
| template< |
| typename Sequence |
| , typename N |
| > |
| struct <a href="./at.html" class="identifier">at</a> |
| { |
| typedef <em>unspecified</em> type; |
| }; |
| |
| template< |
| typename AssocSeq |
| , typename Key |
| , typename Default = <em>unspecified</em> |
| > |
| struct <a href="./at.html" class="identifier">at</a> |
| { |
| typedef <em>unspecified</em> type; |
| }; |
| </pre> |
| </div> |
| <div class="section" id="id178"> |
| <h3><a class="subsection-title" href="#description" name="description">Description</a></h3> |
| <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> |
| <ul class="simple"> |
| <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 |
| <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">Sequence</span></tt>.</li> |
| <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> |
| 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 |
| exists.</li> |
| </ul> |
| </div> |
| <div class="section" id="id179"> |
| <h3><a class="subsection-title" href="#header" name="header">Header</a></h3> |
| <pre class="literal-block"> |
| #include <<a href="../../../../boost/mpl/at.hpp" class="header">boost/mpl/at.hpp</a>> |
| </pre> |
| </div> |
| <div class="section" id="id180"> |
| <h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3> |
| <p><a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a></p> |
| </div> |
| <div class="section" id="id181"> |
| <h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> |
| <table border="1" class="docutils table"> |
| <colgroup> |
| <col width="17%" /> |
| <col width="30%" /> |
| <col width="53%" /> |
| </colgroup> |
| <thead valign="bottom"> |
| <tr><th class="head">Parameter</th> |
| <th class="head">Requirement</th> |
| <th class="head">Description</th> |
| </tr> |
| </thead> |
| <tbody valign="top"> |
| <tr><td><tt class="literal"><span class="pre">Sequence</span></tt></td> |
| <td><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a></td> |
| <td>A sequence to be examined.</td> |
| </tr> |
| <tr><td><tt class="literal"><span class="pre">AssocSeq</span></tt></td> |
| <td><a class="reference internal" href="./associative-sequence.html">Associative Sequence</a></td> |
| <td>A sequence to be examined.</td> |
| </tr> |
| <tr><td><tt class="literal"><span class="pre">N</span></tt></td> |
| <td><a class="reference internal" href="./integral-constant.html">Integral Constant</a></td> |
| <td>An offset from the beginning of the sequence |
| specifying the element to be retrieved.</td> |
| </tr> |
| <tr><td><tt class="literal"><span class="pre">Key</span></tt></td> |
| <td>Any type</td> |
| <td>A key for the element to be retrieved.</td> |
| </tr> |
| <tr><td><tt class="literal"><span class="pre">Default</span></tt></td> |
| <td>Any type</td> |
| <td>A default value to return if the element is |
| not found.</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <div class="section" id="id182"> |
| <h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> |
| <div class="expression-semantics compound"> |
| <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> |
| <pre class="compound-middle literal-block"> |
| typedef <a href="./at.html" class="identifier">at</a><s,n>::type t; |
| </pre> |
| <table class="compound-last docutils field-list" frame="void" rules="none"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p> |
| </td> |
| </tr> |
| <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> |
| </td> |
| </tr> |
| <tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p> |
| <pre class="last literal-block"> |
| typedef <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; |
| </pre> |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <div class="expression-semantics compound"> |
| <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> |
| <pre class="compound-middle literal-block"> |
| typedef <a href="./at.html" class="identifier">at</a><s,key,x>::type t; |
| </pre> |
| <table class="compound-middle docutils field-list" frame="void" rules="none"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <tr class="field"><th class="field-name">Return type:</th><td class="field-body">A type.</td> |
| </tr> |
| <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>; |
| otherwise <tt class="literal"><span class="pre">t</span></tt> is identical to <tt class="literal"><span class="pre">x</span></tt>.</td> |
| </tr> |
| </tbody> |
| </table> |
| <!-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ --> |
| <pre class="compound-middle literal-block"> |
| typedef <a href="./at.html" class="identifier">at</a><s,key>::type t; |
| </pre> |
| <table class="compound-last docutils field-list" frame="void" rules="none"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p> |
| </td> |
| </tr> |
| <tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p> |
| <pre class="last literal-block"> |
| typedef <a href="./at.html" class="identifier">at</a><s,key,<a href="./void.html" class="identifier">void_</a>>::type t; |
| </pre> |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <div class="section" id="complexity"> |
| <h3>Complexity</h3> |
| <table border="1" class="docutils table"> |
| <colgroup> |
| <col width="47%" /> |
| <col width="53%" /> |
| </colgroup> |
| <thead valign="bottom"> |
| <tr><th class="head">Sequence archetype</th> |
| <th class="head">Complexity</th> |
| </tr> |
| </thead> |
| <tbody valign="top"> |
| <tr><td><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a></td> |
| <td>Linear.</td> |
| </tr> |
| <tr><td><a class="reference internal" href="./random-access-sequence.html">Random Access Sequence</a></td> |
| <td>Amortized constant time.</td> |
| </tr> |
| <tr><td><a class="reference internal" href="./associative-sequence.html">Associative Sequence</a></td> |
| <td>Amortized constant time.</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <div class="section" id="id183"> |
| <h3><a class="subsection-title" href="#example" name="example">Example</a></h3> |
| <pre class="literal-block"> |
| typedef <a href="./range-c.html" class="identifier">range_c</a><long,10,50> range; |
| <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 ); |
| <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 ); |
| <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 ); |
| </pre> |
| <pre class="literal-block"> |
| typedef <a href="./set.html" class="identifier">set</a>< int const,long*,double > s; |
| |
| <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> > )); |
| <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< <a href="./at.html" class="identifier">at</a><s,int>::type, int > )); |
| </pre> |
| </div> |
| <div class="section" id="id184"> |
| <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> |
| <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> |
| <!-- Sequences/Intrinsic Metafunctions//at_c --> |
| </div> |
| </div> |
| |
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| <td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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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