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59 lines
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59 lines
2.5 KiB
HTML
<!doctype html public "-//ietf//dtd html//en">
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<html><head><title>boost::mpl::at</title>
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<link rel="stylesheet" href="../mpl_wiki.css">
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</head><body bgcolor="white">
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<h1><a href="../Table_of_Contents.html"><img src="../mpl_logo.jpg" alt="[Home]" border=0 align="right"></a>at</h1><h3>Synopsis</h3>
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<p>
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<pre>
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template<
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typename Sequence
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, typename N
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>
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struct at
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{
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typedef <em>unspecified</em> type;
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};
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</pre>
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<p>
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<h3>Description</h3>
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<p>
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Returns a type identical to the <code>N</code>-th element from the beginning of the sequence.
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<p>
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<h3>Definition</h3>
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<p>
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<pre>
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#include "<a href="../../../../../boost/mpl/at.hpp">boost/mpl/at.hpp</a>"
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</pre>
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<p>
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<h3>Parameters</h3>
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<table border="1">
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<tr><th> Parameter </th><th> Requirement </th><th> Description </th></tr>
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<tr><td><code>Sequence</code></td><td>A model of <a href="../Forward_Sequence.html">Forward Sequence</a></td><td>A sequence to be examined. </td></tr>
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<tr><td><code>N</code></td><td>A model of <a href="../Integral_Constant.html">Integral Constant</a></td><td>The offset from the beginning of the sequence that specifies the element to be retrieved. </td></tr>
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</table>
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<p>
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<h3>Expression semantics</h3>
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<p>
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<table border="1">
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<tr><th> Expression </th><th> Expression type </th><th> Precondition </th><th> Semantics </th><th> Postcondition </th></tr>
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<tr><td><code>typedef at<Sequence,N>::type t;</code></td><td>A type</td><td><code>0 <= N::value < size<Sequence>::type::value</code></td><td>Equivalent to <code>typedef advance< begin<Sequence>::type,N >::type::type t;</code></td><td></td></tr>
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</table>
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<p>
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<h3>Complexity</h3>
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<p>
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Depends on the implementation of the particular sequence it is applied to. Linear in the worst case, or amortized constant time.
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<p>
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<h3>Example</h3>
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<p>
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<pre>
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typedef <a href="../Reference/range_c.html">range_c</a><long,10,50> range;
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BOOST_STATIC_ASSERT(at< range, int_<0> >::type::value == 10);
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BOOST_STATIC_ASSERT(at< range, int_<10> >::type::value == 20);
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BOOST_STATIC_ASSERT(at< range, int_<40> >::type::value == 50);
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</pre>
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<p>
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<h3>See also</h3>
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<p>
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<a href="../Forward_Sequence.html">Forward Sequence</a>, <code><a href="../Reference/at_c.html">at_c</a></code>, <code><a href="../Reference/front.html">front</a></code>, <code><a href="../Reference/back.html">back</a></code>
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<p><hr>
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<a href="../Table_of_Contents.html">Table of Contents</a><br>Last edited March 12, 2003 6:26 am</body></html> |