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<div class="refentry" title="Struct template reverse_fold_tree">
<a name="boost.proto.reverse_fold_tree"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2><span class="refentrytitle">Struct template reverse_fold_tree</span></h2>
<p>boost::proto::reverse_fold_tree — A <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a> that recursively applies the
<code class="computeroutput"><a class="link" href="reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold<></a></code> transform to
sub-trees that all share a common tag type.</p>
</div>
<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="emphasis"><em>// In header: <<a class="link" href="../../proto/reference.html#header.boost.proto.transform.fold_tree_hpp" title="Header <boost/proto/transform/fold_tree.hpp>">boost/proto/transform/fold_tree.hpp</a>>
</em></span><span class="bold"><strong>template</strong></span><<span class="bold"><strong>typename</strong></span> Sequence, <span class="bold"><strong>typename</strong></span> State0, <span class="bold"><strong>typename</strong></span> Fun>
<span class="bold"><strong>struct</strong></span> <a class="link" href="reverse_fold_tree.html" title="Struct template reverse_fold_tree">reverse_fold_tree</a> :
<span class="bold"><strong></strong></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a>< reverse_fold_tree<Sequence, State0, Fun> >
{
<span class="emphasis"><em>// member classes/structs/unions</em></span>
<span class="bold"><strong>template</strong></span><<span class="bold"><strong>typename</strong></span> Expr, <span class="bold"><strong>typename</strong></span> State, <span class="bold"><strong>typename</strong></span> Data>
<span class="bold"><strong>struct</strong></span> <a class="link" href="reverse_fold_tree/impl.html" title="Struct template impl">impl</a> : <span class="bold"><strong></strong></span>
<a class="link" href="reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold</a><Sequence, State0, recurse_if_<typename Expr::proto_tag, Fun> >
::template impl<Expr, State, Data>
{
};
};</pre></div>
<div class="refsect1" title="Description">
<a name="id1670834"></a><h2>Description</h2>
<p>
<code class="computeroutput">proto::reverse_fold_tree<></code> is useful for flattening trees
into lists; for example, you might use <code class="computeroutput">proto::reverse_fold_tree<></code>
to flatten an expression tree like <code class="computeroutput">a | b | c</code> into a Fusion list like
<code class="computeroutput">cons(a, cons(b, cons(c)))</code>.
</p>
<p>
<code class="computeroutput">proto::reverse_fold_tree<></code> is easily understood in terms of
a <code class="computeroutput">recurse_if_<></code> helper, defined as follows:
</p>
<pre class="programlisting"> template<typename Tag, typename Fun>
struct recurse_if_ :
<a class="link" href="if_.html" title="Struct template if_">proto::if_</a><
// If the current node has type type "Tag" ...
boost::is_same<<a class="link" href="tag_of.html" title="Struct template tag_of">proto::tag_of</a><<a class="link" href="_.html" title="Struct _">proto::_</a>>, Tag>(),
// ... recurse, otherwise ...
<a class="link" href="reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold</a><<a class="link" href="_.html" title="Struct _">proto::_</a>, <a class="link" href="_state.html" title="Struct _state">proto::_state</a>, recurse_if_<Tag, Fun> >,
// ... apply the Fun transform.
Fun
>
{};</pre>
<p>
</p>
<p>
With <code class="computeroutput">recurse_if_<></code> as defined above,
<code class="computeroutput">proto::reverse_fold_tree<Sequence, State0, Fun>()(expr, state, data)</code>
is equivalent to:
</p>
<pre class="programlisting"><a class="link" href="reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold</a><
Sequence,
State0,
recurse_if_<typename Expr::proto_tag, Fun>
>()(expr, state, data).</pre>
<p>
It has the effect of folding a tree back-to-front, recursing into child nodes that share a
tag type with the parent node.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2008 Eric Niebler<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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