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| <h2><span class="refentrytitle">Struct template when<Grammar, proto::external_transform></span></h2> |
| <p>boost::proto::when<Grammar, > — A grammar element that associates an externally-specified transform with the grammar. |
| The transform is looked up in the Data parameter using the Grammar as a key.</p> |
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| <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> Grammar<span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="when_Grammar__idp203407488.html" title="Struct template when<Grammar, proto::external_transform>">when</a><span class="special"><</span><span class="identifier">Grammar</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> |
| <span class="keyword"></span> |
| <a class="link" href="transform.html" title="Struct template transform">proto::transform</a>< when<Grammar, <a class="link" href="external_transform.html" title="Struct external_transform">proto::external_transform</a>> > |
| <span class="special">{</span> |
| <span class="comment">// types</span> |
| <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Grammar</span><span class="special">::</span><span class="identifier">proto_grammar</span> <a name="boost.proto.when_Grammar,_idp203407488.proto_grammar"></a><span class="identifier">proto_grammar</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="when_Grammar__idp203407488/impl.html" title="Struct template impl">impl</a> <span class="special">:</span> <span class="keyword"></span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span> |
| <span class="keyword">typename</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">eval_if_c</span><span class="special"><</span> |
| <a class="link" href="result_of/has_env_var.html" title="Struct template has_env_var">proto::result_of::has_env_var</a><span class="special"><</span><span class="identifier">Data</span><span class="special">,</span> <a class="link" href="transforms_type.html" title="Struct transforms_type">proto::transforms_type</a><span class="special">></span><span class="special">::</span><span class="identifier">value</span><span class="special">,</span> |
| <a class="link" href="result_of/env_var.html" title="Struct template env_var">proto::result_of::env_var</a><span class="special"><</span><span class="identifier">Data</span><span class="special">,</span> <a class="link" href="transforms_type.html" title="Struct transforms_type">proto::transforms_type</a><span class="special">></span><span class="special">,</span> |
| <a class="link" href="result_of/env_var.html" title="Struct template env_var">proto::result_of::env_var</a><span class="special"><</span><span class="identifier">Data</span><span class="special">,</span> <a class="link" href="data_type.html" title="Struct data_type">proto::data_type</a><span class="special">></span> |
| <span class="special">></span><span class="special">::</span><span class="identifier">type</span> |
| <span class="special">></span><span class="special">::</span><span class="identifier">type</span> |
| <span class="special">::</span><span class="keyword">template</span> <span class="identifier">when</span><span class="special"><</span> <span class="identifier">Grammar</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="idp385852304"></a><h2>Description</h2> |
| <p> |
| Use <code class="computeroutput">proto::when<></code> to override a grammar's default |
| transform with a custom transform. It is for use when composing larger transforms by associating |
| smaller transforms with individual rules in your grammar. |
| </p> |
| <p> |
| The <code class="computeroutput">when<G, <a class="link" href="external_transform.html" title="Struct external_transform">proto::external_transform</a>></code> |
| indicates that the associated transform is not yet known. It should be looked up when the transform |
| is about to be applied. It is found by looking it up in the passed-in Data parameter, which |
| behaves like a compile-time map from grammar types to transform types. The map is indexed using |
| <code class="computeroutput">Grammar</code> as a key. The associated value type is used as the transform |
| to apply. In this way, the same grammar can be used to define multiple evaluating strategies that |
| can be added post-hoc. |
| </p> |
| <p> |
| See <code class="computeroutput"><a class="link" href="external_transforms.html" title="Struct template external_transforms">proto::external_transforms</a></code> for an example. |
| </p> |
| </div> |
| </div> |
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| <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>) |
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