<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Struct template fold_tree</title> <link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="../../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../../proto/reference.html#header.boost.proto.transform.fold_tree_hpp" title="Header <boost/proto/transform/fold_tree.hpp>"> <link rel="prev" href="reverse_fold.html" title="Struct template reverse_fold"> <link rel="next" href="fold_tree/impl.html" title="Struct template impl"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" 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class="titlepage"></div> <div class="refnamediv"> <h2><span class="refentrytitle">Struct template fold_tree</span></h2> <p>boost::proto::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="fold.html" title="Struct template fold">proto::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="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>> </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> <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"><</span> <span class="identifier">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> <span class="special">{</span> <span class="comment">// member classes/structs/unions</span> <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> <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> <a class="link" href="fold.html" title="Struct template fold">proto::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> <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> <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="idm45555171950576"></a><h2>Description</h2> <p> <code class="computeroutput">proto::fold_tree<></code> is useful for flattening trees into lists; for example, you might use <code class="computeroutput">proto::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(c, cons(b, cons(a)))</code>. </p> <p> <code class="computeroutput">proto::fold_tree<></code> is easily understood in terms of a <code class="computeroutput">recurse_if_<></code> helper, defined as follows: </p> <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> <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"><</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"><</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> <span class="comment">// ... recurse, otherwise ...</span> <a class="link" href="fold.html" title="Struct template fold">proto::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> <span class="comment">// ... apply the Fun transform.</span> <span class="identifier">Fun</span> <span class="special">></span> <span class="special">{</span><span class="special">}</span><span class="special">;</span></pre> <p> </p> <p> With <code class="computeroutput">recurse_if_<></code> as defined above, <code class="computeroutput">proto::fold_tree<Sequence, State0, Fun>()(expr, state, data)</code> is equivalent to: </p> <pre class="programlisting"><a class="link" href="fold.html" title="Struct template fold">proto::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><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 front-to-back, recursing into child nodes that share a tag type with the parent node. </p> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <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>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="reverse_fold.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="fold_tree/impl.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>