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70 lines
3.8 KiB
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70 lines
3.8 KiB
HTML
<!doctype html public "-//ietf//dtd html//en">
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<html><head><title>boost::mpl::copy_if</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>copy_if</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 State
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, typename BinaryOp
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, typename Pred
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>
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struct copy_if
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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 the result of the successive application of <code>BinaryOp</code> to the result of the previous <code>BinaryOp</code> invocation (<code>State</code> if it's the first call) and every element in the range <code>[begin<Sequence>::type,end<Sequence>::type)</code> that satisfies the predicate <code>Pred</code>, in the linear order. A typical application for <code>copy_if</code> is to conditionally copy the content of one sequence into another - see the example below.
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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/copy_if.hpp">boost/mpl/copy_if.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="../Sequence.html">Sequence</a></td><td>A sequence to iterate. </td></tr>
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<tr><td><code>State</code></td><td>A type</td><td>The initial state for the first <code>BinaryOp</code> application. </td></tr>
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<tr><td><code>BinaryOp</code></td><td>A model of [Lambda Function]</td><td>The operation to be executed on forward traversal. </td></tr>
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<tr><td><code>Pred</code></td><td>An unary Predicate [Lambda Expression]</td><td>The copying condition. </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 copy_if<Sequence,T,Op,Pred>::type s;</code></td><td>A type</td><td></td><td>Equivalent to <code>typedef lambda<Op>::type op; typedef lambda<Pred>::type pred; typedef fold< Sequence,T,if_< apply<pred,_2>, apply<op,_1,_2>, _1 > >::type s;</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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Linear. Exactly <code><a href="../Reference/size.html">size</a><Sequence>::type::value</code> applications of <code>Pred</code>, and at most <code><a href="../Reference/size.html">size</a><Sequence>::type::value</code> applications of <code>BinaryOp</code>.
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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/list_c.html">list_c</a><int,0,1,2,3,4,5,6,7,8,9>::type numbers;
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typedef <a href="../Reference/list_c.html">list_c</a><int,0,1,2,3,4>::type answer;
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typedef copy_if<
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numbers
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, <a href="../Reference/vector_c.html">vector_c</a><int>
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, <a href="../Reference/push_back.html">push_back</a><_1,_2>
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, <a href="../Reference/less.html">less</a><_1,int_<5> >
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>::type result;
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<p>
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BOOST_STATIC_ASSERT(<a href="../Reference/size.html">size</a><result>::value == 5);
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BOOST_STATIC_ASSERT((<a href="../Reference/equal.html">equal</a><result,answer>::type::value));
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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="../Algorithms.html">Algorithms</a>, <code><a href="../Reference/copy.html">copy</a></code>, <code><a href="../Reference/copy_backward_if.html">copy_backward_if</a></code>, <code><a href="../Reference/copy_backward.html">copy_backward</a></code>, <code><a href="../Reference/fold.html">fold</a></code>, <code><a href="../Reference/iter_fold.html">iter_fold</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 6:30 am</body></html> |