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 external_transforms</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.when_hpp" title="Header <boost/proto/transform/when.hpp>"> 10<link rel="prev" href="external_transform.html" title="Struct external_transform"> 11<link rel="next" href="external_transforms/when.html" title="Struct template when"> 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" 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class="refentry"> 27<a name="boost.proto.external_transforms"></a><div class="titlepage"></div> 28<div class="refnamediv"> 29<h2><span class="refentrytitle">Struct template external_transforms</span></h2> 30<p>boost::proto::external_transforms — A map from grammars to transforms, used as a way to externally associate transforms.</p> 31</div> 32<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2> 33<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.when_hpp" title="Header <boost/proto/transform/when.hpp>">boost/proto/transform/when.hpp</a>> 34 35</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span><span class="special">...</span> When<span class="special">></span> 36<span class="keyword">struct</span> <a class="link" href="external_transforms.html" title="Struct template external_transforms">external_transforms</a> <span class="special">{</span> 37 <span class="comment">// types</span> 38 <span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">to_mpl_pair</span><span class="special"><</span> <span class="identifier">When</span> <span class="special">></span><span class="special">::</span><span class="identifier">type</span><span class="special">...</span> <span class="special">></span> <a name="boost.proto.external_transforms.map_type"></a><span class="identifier">map_type</span><span class="special">;</span> <span class="comment">// For exposition only.</span> 39 40 <span class="comment">// member classes/structs/unions</span> 41 <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> Grammar<span class="special">></span> 42 <span class="keyword">struct</span> <a class="link" href="external_transforms/when.html" title="Struct template when">when</a> <span class="special">:</span> 43 <span class="keyword"></span> <a class="link" href="otherwise.html" title="Struct template otherwise">proto::otherwise</a>< typename mpl::at< map_type, Grammar >::type > 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="id-1.3.33.5.54.13.4"></a><h2>Description</h2> 49<p> 50 It is sometimes desirable to define a grammar that can be customized with different sets of transforms. 51 To do that, where you would normally specify a transform within a grammar, you can instead put 52 <code class="computeroutput"><a class="link" href="external_transform.html" title="Struct external_transform">proto::external_transform</a></code>; for example: 53 <code class="computeroutput">proto::when< some_grammar, proto::external_transform ></code>. Then, when 54 invoking the grammar, you can pass an approriately-defined instance of <code class="computeroutput">proto::external_transforms</code> 55 as the Data parameter. When an expression matches <code class="computeroutput">some_grammar</code>, Proto 56 will look up the approprite transform in the Data parameter using <code class="computeroutput">some_grammar</code> 57 as a key. 58 </p> 59<p> 60 </p> 61<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">int_terminal</span> 62 <span class="special">:</span> <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special"><</span><span class="keyword">int</span><span class="special">></span> 63<span class="special">{</span><span class="special">}</span><span class="special">;</span> 64 65<span class="keyword">struct</span> <span class="identifier">char_terminal</span> 66 <span class="special">:</span> <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special"><</span><span class="keyword">char</span><span class="special">></span> 67<span class="special">{</span><span class="special">}</span><span class="special">;</span> 68 69<span class="keyword">struct</span> <span class="identifier">my_grammar</span> 70 <span class="special">:</span> <a class="link" href="or_.html" title="Struct template or_">proto::or_</a><span class="special"><</span> 71 <span class="comment">// The next two grammar rules are customization points.</span> 72 <span class="comment">// The associated transforms are specified externally</span> 73 <span class="comment">// using external_transforms below.</span> 74 <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special"><</span> <span class="identifier">int_terminal</span><span class="special">,</span> <a class="link" href="external_transform.html" title="Struct external_transform">proto::external_transform</a> <span class="special">></span> 75 <span class="special">,</span> <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special"><</span> <span class="identifier">char_terminal</span><span class="special">,</span> <a class="link" href="external_transform.html" title="Struct external_transform">proto::external_transform</a> <span class="special">></span> 76 <span class="special">,</span> <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special"><</span> 77 <a class="link" href="plus.html" title="Struct template plus">proto::plus</a><span class="special"><</span> <span class="identifier">my_grammar</span><span class="special">,</span> <span class="identifier">my_grammar</span> <span class="special">></span> 78 <span class="special">,</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> <span class="keyword">int</span><span class="special">(</span><span class="special">)</span><span class="special">,</span> <span class="identifier">my_grammar</span> <span class="special">></span> 79 <span class="special">></span> 80 <span class="special">></span> 81<span class="special">{</span><span class="special">}</span><span class="special">;</span> 82 83<span class="comment">// Here is where the transforms are associated with the</span> 84<span class="comment">// grammar rules above.</span> 85<span class="keyword">struct</span> <span class="identifier">my_transforms</span> 86 <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special"><</span> 87 <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special"><</span><span class="identifier">int_terminal</span><span class="special">,</span> <span class="identifier">print</span><span class="special">(</span><a class="link" href="_value.html" title="Struct _value">proto::_value</a><span class="special">)</span><span class="special">></span> 88 <span class="special">,</span> <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special"><</span><span class="identifier">char_terminal</span><span class="special">,</span> <span class="identifier">print</span><span class="special">(</span><a class="link" href="_value.html" title="Struct _value">proto::_value</a><span class="special">)</span><span class="special">></span> 89 <span class="special">></span> 90<span class="special">{</span><span class="special">}</span><span class="special">;</span> 91 92<span class="comment">// ...</span> 93 94<a class="link" href="literal.html" title="Struct template literal">proto::literal</a><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">i</span><span class="special">(</span><span class="number">1</span><span class="special">)</span><span class="special">;</span> 95<a class="link" href="literal.html" title="Struct template literal">proto::literal</a><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">c</span><span class="special">(</span><span class="char">'a'</span><span class="special">)</span><span class="special">;</span> 96<span class="identifier">my_transforms</span> <span class="identifier">trx</span><span class="special">;</span> 97 98<span class="comment">// Evaluate "i+c" using my_grammar with the specified transforms:</span> 99<span class="identifier">my_grammar</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">i</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">trx</span><span class="special">)</span><span class="special">;</span> 100 101<span class="comment">// If you would also like to pass arbitrary data along with the</span> 102<span class="comment">// transforms, you can use a transform environment, as so:</span> 103<span class="identifier">my_grammar</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">i</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="special">(</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">data</span> <span class="special">=</span> <span class="number">42</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">transforms</span> <span class="special">=</span> <span class="identifier">trx</span><span class="special">)</span><span class="special">)</span><span class="special">;</span></pre> 104<p> 105 </p> 106</div> 107</div> 108<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 109<td align="left"></td> 110<td align="right"><div class="copyright-footer">Copyright © 2008 Eric Niebler<p> 111 Distributed under the Boost Software License, Version 1.0. (See accompanying 112 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>) 113 </p> 114</div></td> 115</tr></table> 116<hr> 117<div class="spirit-nav"> 118<a accesskey="p" href="external_transform.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../proto/reference.html#header.boost.proto.transform.when_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="external_transforms/when.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 119</div> 120</body> 121</html> 122
