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<a name="boost.proto.reverse_fold_tree"></a><div class="titlepage"></div>
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<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&lt;&gt;</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="comment">// In header: &lt;<a class="link" href="../../proto/reference.html#header.boost.proto.transform.fold_tree_hpp" title="Header &lt;boost/proto/transform/fold_tree.hpp&gt;">boost/proto/transform/fold_tree.hpp</a>&gt;

</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Sequence<span class="special">,</span> <span class="keyword">typename</span> State0<span class="special">,</span> <span class="keyword">typename</span> Fun<span class="special">&gt;</span> 
<span class="keyword">struct</span> <a class="link" href="reverse_fold_tree.html" title="Struct template reverse_fold_tree">reverse_fold_tree</a> <span class="special">:</span>
  <span class="keyword"></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a><span class="special">&lt;</span> <span class="identifier">reverse_fold_tree</span><span class="special">&lt;</span><span class="identifier">Sequence</span><span class="special">,</span> <span class="identifier">State0</span><span class="special">,</span> <span class="identifier">Fun</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{</span>
  <span class="comment">// member classes/structs/unions</span>
  <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <a class="link" href="../../Expr.html" title="Concept Expr">Expr</a><span class="special">,</span> <span class="keyword">typename</span> State<span class="special">,</span> <span class="keyword">typename</span> Data<span class="special">&gt;</span> 
  <span class="keyword">struct</span> <a class="link" href="reverse_fold_tree/impl.html" title="Struct template impl">impl</a> <span class="special">:</span> <span class="keyword"></span> 
    <a class="link" href="reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold</a><span class="special">&lt;</span><span class="identifier">Sequence</span><span class="special">,</span> <span class="identifier">State0</span><span class="special">,</span> <span class="identifier">recurse_if_</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">,</span> <span class="identifier">Fun</span><span class="special">&gt;</span> <span class="special">&gt;</span>
      <span class="special">::</span><span class="keyword">template</span> <span class="identifier">impl</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">State</span><span class="special">,</span> <span class="identifier">Data</span><span class="special">&gt;</span>
  <span class="special">{</span>
  <span class="special">}</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id-1.3.34.5.48.5.4"></a><h2>Description</h2>
<p>
            <code class="computeroutput">proto::reverse_fold_tree&lt;&gt;</code> is useful for flattening trees
            into lists; for example, you might use <code class="computeroutput">proto::reverse_fold_tree&lt;&gt;</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&lt;&gt;</code> is easily understood in terms of
            a <code class="computeroutput">recurse_if_&lt;&gt;</code> helper, defined as follows:
            </p>
<pre class="programlisting"> <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Fun</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">recurse_if_</span> <span class="special">:</span>
  <a class="link" href="if_.html" title="Struct template if_">proto::if_</a><span class="special">&lt;</span>
    <span class="comment">// If the current node has type type "Tag" ...</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special">&lt;</span><a class="link" href="tag_of.html" title="Struct template tag_of">proto::tag_of</a><span class="special">&lt;</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">&gt;</span><span class="special">,</span> <span class="identifier">Tag</span><span class="special">&gt;</span><span class="special">(</span><span class="special">)</span><span class="special">,</span>
    <span class="comment">// ... recurse, otherwise ...</span>
    <a class="link" href="reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold</a><span class="special">&lt;</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">,</span> <a class="link" href="_state.html" title="Struct _state">proto::_state</a><span class="special">,</span> <span class="identifier">recurse_if_</span><span class="special">&lt;</span><span class="identifier">Tag</span><span class="special">,</span> <span class="identifier">Fun</span><span class="special">&gt;</span> <span class="special">&gt;</span><span class="special">,</span>
    <span class="comment">// ... apply the Fun transform.</span>
    <span class="identifier">Fun</span>
  <span class="special">&gt;</span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span></pre>
<p>
          </p>
<p>
            With <code class="computeroutput">recurse_if_&lt;&gt;</code> as defined above,
            <code class="computeroutput">proto::reverse_fold_tree&lt;Sequence, State0, Fun&gt;()(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><span class="special">&lt;</span>
  <span class="identifier">Sequence</span><span class="special">,</span>
  <span class="identifier">State0</span><span class="special">,</span>
  <span class="identifier">recurse_if_</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span><span class="special">,</span> <span class="identifier">Fun</span><span class="special">&gt;</span>
<span class="special">&gt;</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">expr</span><span class="special">,</span> <span class="identifier">state</span><span class="special">,</span> <span class="identifier">data</span><span class="special">)</span><span class="special">.</span></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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