boost_mpl/doc/ref/Reference/copy_if.html

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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>
<p>
<pre>
template&lt;
      typename Sequence
    , typename State
    , typename BinaryOp
    , typename Pred
    &gt;
struct copy_if
{
    typedef <em>unspecified</em> type;
};
</pre>
<p>
<h3>Description</h3>
<p>
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&lt;Sequence&gt;::type,end&lt;Sequence&gt;::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.
<p>
<h3>Definition</h3>
<p>
<pre>
#include "<a href="../../../../../boost/mpl/copy_if.hpp">boost/mpl/copy_if.hpp</a>"
</pre>
<p>
<h3>Parameters</h3>
<table border="1">
<tr><th>&nbsp;Parameter&nbsp;</th><th>&nbsp;Requirement&nbsp;</th><th>&nbsp;Description &nbsp;</th></tr>
<tr><td><code>Sequence</code></td><td>A model of <a href="../Sequence.html">Sequence</a></td><td>A sequence to iterate. </td></tr>
<tr><td><code>State</code></td><td>A type</td><td>The initial state for the first <code>BinaryOp</code> application. </td></tr>
<tr><td><code>BinaryOp</code></td><td>A model of [Lambda Function]</td><td>The operation to be executed on forward traversal. </td></tr>
<tr><td><code>Pred</code></td><td>An unary Predicate [Lambda Expression]</td><td>The copying condition. </td></tr>
</table>
<p>
<h3>Expression semantics</h3>
<p>
<table border="1">
<tr><th>&nbsp;Expression&nbsp;</th><th>&nbsp;Expression&nbsp;type&nbsp;</th><th>&nbsp;Precondition&nbsp;</th><th>&nbsp;Semantics&nbsp;</th><th>&nbsp;Postcondition&nbsp;</th></tr>
<tr><td><code>typedef copy_if&lt;Sequence,T,Op,Pred&gt;::type s;</code></td><td>A type</td><td></td><td>Equivalent to <code>typedef lambda&lt;Op&gt;::type op; typedef lambda&lt;Pred&gt;::type pred; typedef fold&lt; Sequence,T,if_&lt; apply&lt;pred,_2&gt;, apply&lt;op,_1,_2&gt;, _1 &gt; &gt;::type s;</code>.</td><td></td></tr>
</table>
<p>
<h3>Complexity</h3>
<p>
Linear. Exactly <code><a href="../Reference/size.html">size</a>&lt;Sequence&gt;::type::value</code> applications of <code>Pred</code>, and at most <code><a href="../Reference/size.html">size</a>&lt;Sequence&gt;::type::value</code> applications of <code>BinaryOp</code>. 
<p>
<h3>Example</h3>
<p>
<pre>
typedef <a href="../Reference/list_c.html">list_c</a>&lt;int,0,1,2,3,4,5,6,7,8,9&gt;::type numbers;
typedef <a href="../Reference/list_c.html">list_c</a>&lt;int,0,1,2,3,4&gt;::type answer;
typedef copy_if&lt;
      numbers
    , <a href="../Reference/vector_c.html">vector_c</a>&lt;int&gt;
    , <a href="../Reference/push_back.html">push_back</a>&lt;_1,_2&gt;
    , <a href="../Reference/less.html">less</a>&lt;_1,int_&lt;5&gt; &gt;
    &gt;::type result;
<p>
BOOST_STATIC_ASSERT(<a href="../Reference/size.html">size</a>&lt;result&gt;::value == 5);
BOOST_STATIC_ASSERT((<a href="../Reference/equal.html">equal</a>&lt;result,answer&gt;::type::value));
</pre>
<p>
<h3>See also</h3>
<p>
<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>
<p><hr>
<a href="../Table_of_Contents.html">Table of Contents</a><br>Last edited March 12, 2003 6:30 am</body></html>