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<a name="boost.proto.external_transforms"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Struct template external_transforms</span></h2>
<p>boost::proto::external_transforms — A map from grammars to transforms, used as a way to externally associate transforms.</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.when_hpp" title="Header <boost/proto/transform/when.hpp>">boost/proto/transform/when.hpp</a>>
</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span><span class="special">...</span> When<span class="special">></span>
<span class="keyword">struct</span> <a class="link" href="external_transforms.html" title="Struct template external_transforms">external_transforms</a> <span class="special">{</span>
<span class="comment">// types</span>
<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>
<span class="comment">// member classes/structs/unions</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> Grammar<span class="special">></span>
<span class="keyword">struct</span> <a class="link" href="external_transforms/when.html" title="Struct template when">when</a> <span class="special">:</span>
<span class="keyword"></span> <a class="link" href="otherwise.html" title="Struct template otherwise">proto::otherwise</a>< typename mpl::at< map_type, Grammar >::type >
<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="idm45555168896528"></a><h2>Description</h2>
<p>
It is sometimes desirable to define a grammar that can be customized with different sets of transforms.
To do that, where you would normally specify a transform within a grammar, you can instead put
<code class="computeroutput"><a class="link" href="external_transform.html" title="Struct external_transform">proto::external_transform</a></code>; for example:
<code class="computeroutput">proto::when< some_grammar, proto::external_transform ></code>. Then, when
invoking the grammar, you can pass an approriately-defined instance of <code class="computeroutput">proto::external_transforms</code>
as the Data parameter. When an expression matches <code class="computeroutput">some_grammar</code>, Proto
will look up the approprite transform in the Data parameter using <code class="computeroutput">some_grammar</code>
as a key.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">int_terminal</span>
<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>
<span class="special">{</span><span class="special">}</span><span class="special">;</span>
<span class="keyword">struct</span> <span class="identifier">char_terminal</span>
<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>
<span class="special">{</span><span class="special">}</span><span class="special">;</span>
<span class="keyword">struct</span> <span class="identifier">my_grammar</span>
<span class="special">:</span> <a class="link" href="or_.html" title="Struct template or_">proto::or_</a><span class="special"><</span>
<span class="comment">// The next two grammar rules are customization points.</span>
<span class="comment">// The associated transforms are specified externally</span>
<span class="comment">// using external_transforms below.</span>
<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>
<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>
<span class="special">,</span> <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special"><</span>
<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>
<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>
<span class="special">></span>
<span class="special">></span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span>
<span class="comment">// Here is where the transforms are associated with the</span>
<span class="comment">// grammar rules above.</span>
<span class="keyword">struct</span> <span class="identifier">my_transforms</span>
<span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">external_transforms</span><span class="special"><</span>
<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>
<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>
<span class="special">></span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span>
<span class="comment">// ...</span>
<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>
<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>
<span class="identifier">my_transforms</span> <span class="identifier">trx</span><span class="special">;</span>
<span class="comment">// Evaluate "i+c" using my_grammar with the specified transforms:</span>
<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>
<span class="comment">// If you would also like to pass arbitrary data along with the</span>
<span class="comment">// transforms, you can use a transform environment, as so:</span>
<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>
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<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>)
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