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<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="fusion.algorithm.transformation.functions.filter_if"></a><a class="link" href="filter_if.html" title="filter_if">filter_if</a>
</h5></div></div></div>
<h6>
<a name="fusion.algorithm.transformation.functions.filter_if.h0"></a>
<span><a name="fusion.algorithm.transformation.functions.filter_if.description"></a></span><a class="link" href="filter_if.html#fusion.algorithm.transformation.functions.filter_if.description">Description</a>
</h6>
<p>
For a given sequence, <a class="link" href="filter_if.html" title="filter_if"><code class="computeroutput"><span class="identifier">filter_if</span></code></a> returns a new sequences
containing only the elements with types for which a given <a href="http://www.boost.org/libs/mpl/doc/refmanual/lambda-expression.html" target="_top">MPL
Lambda Expression</a> evaluates to <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code>.
</p>
<h6>
<a name="fusion.algorithm.transformation.functions.filter_if.h1"></a>
<span><a name="fusion.algorithm.transformation.functions.filter_if.synopsis"></a></span><a class="link" href="filter_if.html#fusion.algorithm.transformation.functions.filter_if.synopsis">Synopsis</a>
</h6>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span>
<span class="keyword">typename</span> <span class="identifier">Pred</span><span class="special">,</span>
<span class="keyword">typename</span> <span class="identifier">Sequence</span>
<span class="special">></span>
<span class="keyword">typename</span> <a class="link" href="../metafunctions/filter_if.html" title="filter_if"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">filter_if</span></code></a><span class="special"><</span><span class="identifier">Sequence</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">Pred</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">filter_if</span><span class="special">(</span><span class="identifier">Sequence</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">seq</span><span class="special">);</span>
</pre>
<div class="table">
<a name="fusion.algorithm.transformation.functions.filter_if.t0"></a><p class="title"><b>Table 1.65. Parameters</b></p>
<div class="table-contents"><table class="table" summary="Parameters">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Parameter
</p>
</th>
<th>
<p>
Requirement
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">seq</span></code>
</p>
</td>
<td>
<p>
A model of <a class="link" href="../../../sequence/concepts/forward_sequence.html" title="Forward Sequence">Forward
Sequence</a>
</p>
</td>
<td>
<p>
Operation's argument
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Pred</span></code>
</p>
</td>
<td>
<p>
A unary <a href="http://www.boost.org/libs/mpl/doc/refmanual/lambda-expression.html" target="_top">MPL
Lambda Expression</a>
</p>
</td>
<td>
<p>
The predicate to filter by
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h6>
<a name="fusion.algorithm.transformation.functions.filter_if.h2"></a>
<span><a name="fusion.algorithm.transformation.functions.filter_if.expression_semantics"></a></span><a class="link" href="filter_if.html#fusion.algorithm.transformation.functions.filter_if.expression_semantics">Expression
Semantics</a>
</h6>
<pre class="programlisting"><a class="link" href="filter_if.html" title="filter_if"><code class="computeroutput"><span class="identifier">filter_if</span></code></a><span class="special"><</span><span class="identifier">Pred</span><span class="special">>(</span><span class="identifier">seq</span><span class="special">);</span>
</pre>
<p>
<span class="bold"><strong>Return type</strong></span>:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
A model of <a class="link" href="../../../sequence/concepts/forward_sequence.html" title="Forward Sequence">Forward
Sequence</a>.
</li>
<li class="listitem">
A model of <a class="link" href="../../../sequence/concepts/associative_sequence.html" title="Associative Sequence">Associative
Sequence</a> if <code class="computeroutput"><span class="identifier">seq</span></code>
implements the <a class="link" href="../../../sequence/concepts/associative_sequence.html" title="Associative Sequence">Associative
Sequence</a> model.
</li>
</ul></div>
<p>
<span class="bold"><strong>Semantics</strong></span>: Returns a sequence containing
all the elements of <code class="computeroutput"><span class="identifier">seq</span></code>
with types for which <code class="computeroutput"><span class="identifier">Pred</span></code>
evaluates to <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code>. The order of the retained elements
is the same as in the original sequence.
</p>
<h6>
<a name="fusion.algorithm.transformation.functions.filter_if.h3"></a>
<span><a name="fusion.algorithm.transformation.functions.filter_if.complexity"></a></span><a class="link" href="filter_if.html#fusion.algorithm.transformation.functions.filter_if.complexity">Complexity</a>
</h6>
<p>
Constant. Returns a view which is lazily evaluated.
</p>
<h6>
<a name="fusion.algorithm.transformation.functions.filter_if.h4"></a>
<span><a name="fusion.algorithm.transformation.functions.filter_if.header"></a></span><a class="link" href="filter_if.html#fusion.algorithm.transformation.functions.filter_if.header">Header</a>
</h6>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">algorithm</span><span class="special">/</span><span class="identifier">transformation</span><span class="special">/</span><span class="identifier">filter_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">filter_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<h6>
<a name="fusion.algorithm.transformation.functions.filter_if.h5"></a>
<span><a name="fusion.algorithm.transformation.functions.filter_if.example"></a></span><a class="link" href="filter_if.html#fusion.algorithm.transformation.functions.filter_if.example">Example</a>
</h6>
<pre class="programlisting"><span class="keyword">const</span> <a class="link" href="../../../container/vector.html" title="vector"><code class="computeroutput"><span class="identifier">vector</span></code></a><span class="special"><</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">vec</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3.0</span><span class="special">,</span><span class="number">4.0</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><a class="link" href="filter_if.html" title="filter_if"><code class="computeroutput"><span class="identifier">filter_if</span></code></a><span class="special"><</span><span class="identifier">is_integral</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">_</span><span class="special">></span> <span class="special">>(</span><span class="identifier">vec</span><span class="special">)</span> <span class="special">==</span> <a class="link" href="../../../container/generation/functions/make_vector.html" title="make_vector"><code class="computeroutput"><span class="identifier">make_vector</span></code></a><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">));</span>
</pre>
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