1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Struct template reverse_fold_tree</title> 6<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../../proto/reference.html#header.boost.proto.transform.fold_tree_hpp" title="Header <boost/proto/transform/fold_tree.hpp>"> 10<link rel="prev" href="fold_tree/impl.html" title="Struct template impl"> 11<link rel="next" href="reverse_fold_tree/impl.html" title="Struct template impl"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" 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name="boost.proto.reverse_fold_tree"></a><div class="titlepage"></div> 28<div class="refnamediv"> 29<h2><span class="refentrytitle">Struct template reverse_fold_tree</span></h2> 30<p>boost::proto::reverse_fold_tree — A <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a> that recursively applies the 31 <code class="computeroutput"><a class="link" href="reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold<></a></code> transform to 32 sub-trees 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: <<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>> 36 37</span><span class="keyword">template</span><span class="special"><</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">></span> 38<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> 39 <span class="keyword"></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a><span class="special"><</span> <span class="identifier">reverse_fold_tree</span><span class="special"><</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">></span> <span class="special">></span> 40<span class="special">{</span> 41 <span class="comment">// member classes/structs/unions</span> 42 <span class="keyword">template</span><span class="special"><</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">></span> 43 <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> 44 <a class="link" href="reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold</a><span class="special"><</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"><</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">></span> <span class="special">></span> 45 <span class="special">::</span><span class="keyword">template</span> <span class="identifier">impl</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> 46 <span class="special">{</span> 47 <span class="special">}</span><span class="special">;</span> 48<span class="special">}</span><span class="special">;</span></pre></div> 49<div class="refsect1"> 50<a name="id-1.3.33.5.48.5.4"></a><h2>Description</h2> 51<p> 52 <code class="computeroutput">proto::reverse_fold_tree<></code> is useful for flattening trees 53 into lists; for example, you might use <code class="computeroutput">proto::reverse_fold_tree<></code> 54 to flatten an expression tree like <code class="computeroutput">a | b | c</code> into a Fusion list like 55 <code class="computeroutput">cons(a, cons(b, cons(c)))</code>. 56 </p> 57<p> 58 <code class="computeroutput">proto::reverse_fold_tree<></code> is easily understood in terms of 59 a <code class="computeroutput">recurse_if_<></code> helper, defined as follows: 60 </p> 61<pre class="programlisting"> <span class="keyword">template</span><span class="special"><</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">></span> 62<span class="keyword">struct</span> <span class="identifier">recurse_if_</span> <span class="special">:</span> 63 <a class="link" href="if_.html" title="Struct template if_">proto::if_</a><span class="special"><</span> 64 <span class="comment">// If the current node has type type "Tag" ...</span> 65 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span><a class="link" href="tag_of.html" title="Struct template tag_of">proto::tag_of</a><span class="special"><</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">></span><span class="special">,</span> <span class="identifier">Tag</span><span class="special">></span><span class="special">(</span><span class="special">)</span><span class="special">,</span> 66 <span class="comment">// ... recurse, otherwise ...</span> 67 <a class="link" href="reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold</a><span class="special"><</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"><</span><span class="identifier">Tag</span><span class="special">,</span> <span class="identifier">Fun</span><span class="special">></span> <span class="special">></span><span class="special">,</span> 68 <span class="comment">// ... apply the Fun transform.</span> 69 <span class="identifier">Fun</span> 70 <span class="special">></span> 71<span class="special">{</span><span class="special">}</span><span class="special">;</span></pre> 72<p> 73 </p> 74<p> 75 With <code class="computeroutput">recurse_if_<></code> as defined above, 76 <code class="computeroutput">proto::reverse_fold_tree<Sequence, State0, Fun>()(expr, state, data)</code> 77 is equivalent to: 78 </p> 79<pre class="programlisting"><a class="link" href="reverse_fold.html" title="Struct template reverse_fold">proto::reverse_fold</a><span class="special"><</span> 80 <span class="identifier">Sequence</span><span class="special">,</span> 81 <span class="identifier">State0</span><span class="special">,</span> 82 <span class="identifier">recurse_if_</span><span class="special"><</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">></span> 83<span class="special">></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> 84<p> 85 It has the effect of folding a tree back-to-front, recursing into child nodes that share a 86 tag type with the parent node. 87 </p> 88</div> 89</div> 90<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 91<td align="left"></td> 92<td align="right"><div class="copyright-footer">Copyright © 2008 Eric Niebler<p> 93 Distributed under the Boost Software License, Version 1.0. (See accompanying 94 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>) 95 </p> 96</div></td> 97</tr></table> 98<hr> 99<div class="spirit-nav"> 100<a accesskey="p" href="fold_tree/impl.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../proto/reference.html#header.boost.proto.transform.fold_tree_hpp"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="reverse_fold_tree/impl.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 101</div> 102</body> 103</html> 104
