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  26 <div class="refentry">
  27 <a name="boost.proto.fold_tree"></a><div class="titlepage"></div>
  28 <div class="refnamediv">
  29 <h2><span class="refentrytitle">Struct template fold_tree</span></h2>
  30 <p>boost::proto::fold_tree &#8212; A <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a> that recursively applies the
  31           <code class="computeroutput"><a class="link" href="fold.html" title="Struct template fold">proto::fold</a>&lt;&gt;</code> transform to sub-trees
  32           that all share a common tag type.</p>
  33 </div>
  34 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
  35 <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;
  36
  37 </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>
  38 <span class="keyword">struct</span> <a class="link" href="fold_tree.html" title="Struct template fold_tree">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">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>
  39   <span class="comment">// member classes/structs/unions</span>
  40   <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>
  41   <span class="keyword">struct</span> <a class="link" href="fold_tree/impl.html" title="Struct template impl">impl</a> <span class="special">:</span> <span class="keyword"></span>
  42     <a class="link" href="fold.html" title="Struct template fold">proto::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>
  43       <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>
  44   <span class="special">{</span>
  45   <span class="special">}</span><span class="special">;</span>
  46 <span class="special">}</span><span class="special">;</span></pre></div>
  47 <div class="refsect1">
  48 <a name="idp482316384"></a><h2>Description</h2>
  49 <p>
  50             <code class="computeroutput">proto::fold_tree&lt;&gt;</code> is useful for flattening trees into lists;
  51             for example, you might use <code class="computeroutput">proto::fold_tree&lt;&gt;</code> to flatten an
  52             expression tree like <code class="computeroutput">a | b | c</code> into a Fusion list like
  53             <code class="computeroutput">cons(c, cons(b, cons(a)))</code>.
  54           </p>
  55 <p>
  56             <code class="computeroutput">proto::fold_tree&lt;&gt;</code> is easily understood in terms of a
  57             <code class="computeroutput">recurse_if_&lt;&gt;</code> helper, defined as follows:
  58             </p>
  59 <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>
  60 <span class="keyword">struct</span> <span class="identifier">recurse_if_</span> <span class="special">:</span>
  61   <a class="link" href="if_.html" title="Struct template if_">proto::if_</a><span class="special">&lt;</span>
  62     <span class="comment">// If the current node has type type "Tag" ...</span>
  63     <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>
  64     <span class="comment">// ... recurse, otherwise ...</span>
  65     <a class="link" href="fold.html" title="Struct template fold">proto::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>
  66     <span class="comment">// ... apply the Fun transform.</span>
  67     <span class="identifier">Fun</span>
  68   <span class="special">&gt;</span>
  69 <span class="special">{</span><span class="special">}</span><span class="special">;</span></pre>
  70 <p>
  71           </p>
  72 <p>
  73             With <code class="computeroutput">recurse_if_&lt;&gt;</code> as defined above,
  74             <code class="computeroutput">proto::fold_tree&lt;Sequence, State0, Fun&gt;()(expr, state, data)</code>
  75             is equivalent to:
  76             </p>
  77 <pre class="programlisting"><a class="link" href="fold.html" title="Struct template fold">proto::fold</a><span class="special">&lt;</span>
  78   <span class="identifier">Sequence</span><span class="special">,</span>
  79   <span class="identifier">State0</span><span class="special">,</span>
  80   <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>
  81 <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>
  82 <p>
  83             It has the effect of folding a tree front-to-back, recursing into child nodes that share a
  84             tag type with the parent node.
  85           </p>
  86 </div>
  87 </div>
  88 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
  89 <td align="left"></td>
  90 <td align="right"><div class="copyright-footer">Copyright &#169; 2008 Eric Niebler<p>
  91         Distributed under the Boost Software License, Version 1.0. (See accompanying
  92         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>)
  93       </p>
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