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<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./algorithms.html" class="navigation-link">Algorithms</a> / <a href="./iteration-algorithms.html" class="navigation-link">Iteration Algorithms</a> / <a href="./reverse-fold.html" class="navigation-link">reverse_fold</a></td>
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<div class="section" id="reverse-fold">
<h1><a class="toc-backref" href="./iteration-algorithms.html#id1471">reverse_fold</a></h1>
<div class="section" id="id509">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename Sequence
    , typename State
    , typename BackwardOp
    , typename ForwardOp = <a href="./placeholders.html" class="identifier">_1</a>
    &gt;
struct <a href="./reverse-fold.html" class="identifier">reverse_fold</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
</div>
<div class="section" id="id510">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>Returns the result of the successive application of binary <tt class="literal"><span class="pre">BackwardOp</span></tt> to the
result of the previous <tt class="literal"><span class="pre">BackwardOp</span></tt> invocation (<tt class="literal"><span class="pre">State</span></tt> if it's the first call)
and every element in the range [<tt class="literal"><span class="pre"><a href="./begin.html" class="identifier">begin</a>&lt;Sequence&gt;::type</span></tt>, <tt class="literal"><span class="pre"><a href="./end.html" class="identifier">end</a>&lt;Sequence&gt;::type</span></tt>) in
reverse order. If <tt class="literal"><span class="pre">ForwardOp</span></tt> is provided, then it is applied on forward
traversal to form the result that is passed to the first <tt class="literal"><span class="pre">BackwardOp</span></tt> call.</p>
</div>
<div class="section" id="id511">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/reverse_fold.hpp" class="header">boost/mpl/reverse_fold.hpp</a>&gt;
</pre>
</div>
<div class="section" id="id512">
<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
</div>
<div class="section" id="id513">
<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
<table border="1" class="docutils table">
<colgroup>
<col width="16%" />
<col width="33%" />
<col width="51%" />
</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 iterate.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">State</span></tt></td>
<td>Any type</td>
<td>The initial state for the first <tt class="literal"><span class="pre">BackwardOp</span></tt>
/ <tt class="literal"><span class="pre">ForwardOp</span></tt> application.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">BackwardOp</span></tt></td>
<td>Binary <a class="reference internal" href="./lambda-expression.html">Lambda Expression</a></td>
<td>The operation to be executed on backward
traversal.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">ForwardOp</span></tt></td>
<td>Binary <a class="reference internal" href="./lambda-expression.html">Lambda Expression</a></td>
<td>The operation to be executed on forward
traversal.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id514">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<p>For any <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">s</span></tt>, binary <a class="reference internal" href="./lambda-expression.html">Lambda Expression</a> <tt class="literal"><span class="pre">backward_op</span></tt> and <tt class="literal"><span class="pre">forward_op</span></tt>,
and arbitrary type <tt class="literal"><span class="pre">state</span></tt>:</p>
<pre class="literal-block">
typedef <a href="./reverse-fold.html" class="identifier">reverse_fold</a>&lt; s,state,backward_op &gt;::type t;
</pre>
<table class="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="./lambda.html" class="identifier">lambda</a>&lt;backward_op&gt;::type op;
typedef <a href="./reverse-iter-fold.html" class="identifier">reverse_iter_fold</a>&lt;
      s
    , state
    , apply_wrap2&lt; op, <a href="./placeholders.html" class="identifier">_1</a>, <a href="./deref.html" class="identifier">deref</a>&lt;<a href="./placeholders.html" class="identifier">_2</a>&gt; &gt;
    &gt;::type t;
</pre>
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</tr>
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<!-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -->
<pre class="literal-block">
typedef <a href="./reverse-fold.html" class="identifier">reverse_fold</a>&lt; s,state,backward_op,forward_op &gt;::type t;
</pre>
<table class="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="./reverse-fold.html" class="identifier">reverse_fold</a>&lt;
      Sequence
    , <a href="./fold.html" class="identifier">fold</a>&lt;s,state,forward_op&gt;::type
    , backward_op
    &gt;::type t;
</pre>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id515">
<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
<p>Linear. Exactly <tt class="literal"><span class="pre"><a href="./size.html" class="identifier">size</a>&lt;s&gt;::value</span></tt> applications of <tt class="literal"><span class="pre">backward_op</span></tt> and <tt class="literal"><span class="pre">forward_op</span></tt>.</p>
</div>
<div class="section" id="id516">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<p>Remove non-negative elements from a sequence <a class="footnote-reference" href="#reverse-fold-note" id="id517">[2]</a>.</p>
<pre class="literal-block">
typedef <a href="./list-c.html" class="identifier">list_c</a>&lt;int,5,-1,0,-7,-2,0,-5,4&gt; numbers;
typedef <a href="./list-c.html" class="identifier">list_c</a>&lt;int,-1,-7,-2,-5&gt; negatives;
typedef <a href="./reverse-fold.html" class="identifier">reverse_fold</a>&lt;
      numbers
    , <a href="./list-c.html" class="identifier">list_c</a>&lt;int&gt;
    , <a href="./if.html" class="identifier">if_</a>&lt; <a href="./less.html" class="identifier">less</a>&lt; <a href="./placeholders.html" class="identifier">_2</a>,<a href="./int.html" class="identifier">int_</a>&lt;0&gt; &gt;, <a href="./push-front.html" class="identifier">push_front</a>&lt;<a href="./placeholders.html" class="identifier">_1</a>,<a href="./placeholders.html" class="identifier">_2</a>,&gt;, <a href="./placeholders.html" class="identifier">_1</a> &gt;
    &gt;::type result;

<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( <a href="./equal.html" class="identifier">equal</a>&lt; negatives,result > ));
</pre>
<table class="docutils footnote" frame="void" id="reverse-fold-note" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id517">[2]</a></td><td>See <tt class="literal"><span class="pre"><a href="./remove-if.html" class="identifier">remove_if</a></span></tt> for a more compact way to do this.</td></tr>
</tbody>
</table>
</div>
<div class="section" id="id518">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./algorithms.html">Algorithms</a>, <a class="reference internal" href="./fold.html">fold</a>, <a class="reference internal" href="./reverse-iter-fold.html">reverse_iter_fold</a>, <a class="reference internal" href="./iter-fold.html">iter_fold</a></p>
<!-- Algorithms/Iteration Algorithms//reverse_iter_fold -->
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