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  25 <div class="refentry">
  26 <a name="boost.proto.external_transforms"></a><div class="titlepage"></div>
  27 <div class="refnamediv">
  28 <h2><span class="refentrytitle">Struct template external_transforms</span></h2>
  29 <p>boost::proto::external_transforms &#8212; A map from grammars to transforms, used as a way to externally associate transforms.</p>
  30 </div>
  31 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
  32 <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.when_hpp" title="Header &lt;boost/proto/transform/when.hpp&gt;">boost/proto/transform/when.hpp</a>&gt;
  33
  34 </span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span><span class="special">...</span> When<span class="special">&gt;</span>
  35 <span class="keyword">struct</span> <a class="link" href="external_transforms.html" title="Struct template external_transforms">external_transforms</a> <span class="special">{</span>
  36   <span class="comment">// types</span>
  37   <span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">to_mpl_pair</span><span class="special">&lt;</span> <span class="identifier">When</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span><span class="special">...</span> <span class="special">&gt;</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>
  38
  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> Grammar<span class="special">&gt;</span>
  41   <span class="keyword">struct</span> <a class="link" href="external_transforms/when.html" title="Struct template when">when</a> <span class="special">:</span>
  42     <span class="keyword"></span> <a class="link" href="otherwise.html" title="Struct template otherwise">proto::otherwise</a>&lt; typename mpl::at&lt; map_type, Grammar &gt;::type &gt;
  43   <span class="special">{</span>
  44   <span class="special">}</span><span class="special">;</span>
  45 <span class="special">}</span><span class="special">;</span></pre></div>
  46 <div class="refsect1">
  47 <a name="id2886865"></a><h2>Description</h2>
  48 <p>
  49             It is sometimes desirable to define a grammar that can be customized with different sets of transforms.
  50             To do that, where you would normally specify a transform within a grammar, you can instead put
  51             <code class="computeroutput"><a class="link" href="external_transform.html" title="Struct external_transform">proto::external_transform</a></code>; for example:
  52             <code class="computeroutput">proto::when&lt; some_grammar, proto::external_transform &gt;</code>. Then, when
  53             invoking the grammar, you can pass an approriately-defined instance of <code class="computeroutput">proto::external_transforms</code>
  54             as the Data parameter. When an expression matches <code class="computeroutput">some_grammar</code>, Proto
  55             will look up the approprite transform in the Data parameter using <code class="computeroutput">some_grammar</code>
  56             as a key.
  57           </p>
  58 <p>
  59             </p>
  60 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">int_terminal</span>
  61   <span class="special">:</span> <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>
  62 <span class="special">{</span><span class="special">}</span><span class="special">;</span>
  63
  64 <span class="keyword">struct</span> <span class="identifier">char_terminal</span>
  65   <span class="special">:</span> <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span>
  66 <span class="special">{</span><span class="special">}</span><span class="special">;</span>
  67
  68 <span class="keyword">struct</span> <span class="identifier">my_grammar</span>
  69   <span class="special">:</span> <a class="link" href="or_.html" title="Struct template or_">proto::or_</a><span class="special">&lt;</span>
  70         <span class="comment">// The next two grammar rules are customization points.</span>
  71         <span class="comment">// The associated transforms are specified externally</span>
  72         <span class="comment">// using external_transforms below.</span>
  73         <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</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">&gt;</span>
  74       <span class="special">,</span> <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</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">&gt;</span>
  75       <span class="special">,</span> <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span>
  76             <a class="link" href="plus.html" title="Struct template plus">proto::plus</a><span class="special">&lt;</span> <span class="identifier">my_grammar</span><span class="special">,</span> <span class="identifier">my_grammar</span> <span class="special">&gt;</span>
  77           <span class="special">,</span> <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> <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">&gt;</span>
  78         <span class="special">&gt;</span>
  79     <span class="special">&gt;</span>
  80 <span class="special">{</span><span class="special">}</span><span class="special">;</span>
  81
  82 <span class="comment">// Here is where the transforms are associated with the</span>
  83 <span class="comment">// grammar rules above.</span>
  84 <span class="keyword">struct</span> <span class="identifier">my_transforms</span>
  85   <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special">&lt;</span>
  86         <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</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">&gt;</span>
  87       <span class="special">,</span> <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</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">&gt;</span>
  88     <span class="special">&gt;</span>
  89 <span class="special">{</span><span class="special">}</span><span class="special">;</span>
  90
  91 <span class="comment">// ...</span>
  92
  93 <a class="link" href="literal.html" title="Struct template literal">proto::literal</a><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">i</span><span class="special">(</span><span class="number">1</span><span class="special">)</span><span class="special">;</span>
  94 <a class="link" href="literal.html" title="Struct template literal">proto::literal</a><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span> <span class="identifier">c</span><span class="special">(</span><span class="char">'a'</span><span class="special">)</span><span class="special">;</span>
  95 <span class="identifier">my_transforms</span> <span class="identifier">trx</span><span class="special">;</span>
  96
  97 <span class="comment">// Evaluate "i+c" using my_grammar with the specified transforms:</span>
  98 <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></pre>
  99 <p>
 100           </p>
 101 </div>
 102 </div>
 103 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 104 <td align="left"></td>
 105 <td align="right"><div class="copyright-footer">Copyright &#169; 2008 Eric Niebler<p>
 106         Distributed under the Boost Software License, Version 1.0. (See accompanying
 107         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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