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26 <div class="refentry">
27 <a name="boost.proto.when_Grammar,_idp931972016"></a><div class="titlepage"></div>
28 <div class="refnamediv">
29 <h2><span class="refentrytitle">Struct template when<Grammar, R(A...)></span></h2>
30 <p>boost::proto::when<Grammar, R(A...)> — A grammar element and a <a class="link" href="../../Transform.html" title="Concept Transform">Transform</a> that associates a
31 transform with the grammar. </p>
33 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
34 <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>>
36 </span><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> Grammar<span class="special">,</span> <span class="keyword">typename</span> R<span class="special">,</span> <span class="keyword">typename</span><span class="special">...</span> A<span class="special">></span>
37 <span class="keyword">struct</span> <a class="link" href="when_Grammar__idp931972016.html" title="Struct template when<Grammar, R(A...)>">when</a><span class="special"><</span><span class="identifier">Grammar</span><span class="special">,</span> <span class="identifier">R</span><span class="special">(</span><span class="identifier">A</span><span class="special">...</span><span class="special">)</span><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, R(A...)> > <span class="special">{</span>
38 <span class="comment">// types</span>
39 <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,_idp931972016.proto_grammar"></a><span class="identifier">proto_grammar</span><span class="special">;</span>
41 <span class="comment">// member classes/structs/unions</span>
42 <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>
43 <span class="keyword">struct</span> <a class="link" href="when_Grammar__idp931972016/impl.html" title="Struct template impl">impl</a> <span class="special">:</span> <span class="keyword"></span> <a class="link" href="transform_impl.html" title="Struct template transform_impl">proto::transform_impl</a>< Expr, State, Data > <span class="special">{</span>
44 <span class="comment">// types</span>
45 <span class="keyword">typedef</span> <a class="link" href="call.html" title="Struct template call">proto::call</a><span class="special"><</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">A</span><span class="special">...</span><span class="special">)</span><span class="special">></span> <a class="link" href="when_Grammar__idp931972016/impl.html#boost.proto.when_Grammar,_idp931972016.impl.call_"><span class="identifier">call_</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
46 <span class="keyword">typedef</span> <a class="link" href="make.html" title="Struct template make">proto::make</a><span class="special"><</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">A</span><span class="special">...</span><span class="special">)</span><span class="special">></span> <a class="link" href="when_Grammar__idp931972016/impl.html#boost.proto.when_Grammar,_idp931972016.impl.make_"><span class="identifier">make_</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
47 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">if_</span><span class="special"><</span><a class="link" href="is_callable.html" title="Struct template is_callable">proto::is_callable</a><span class="special"><</span><span class="identifier">R</span><span class="special">></span><span class="special">,</span><span class="identifier">call_</span><span class="special">,</span><span class="identifier">make_</span><span class="special">></span><span class="special">::</span><span class="identifier">type</span> <a class="link" href="when_Grammar__idp931972016/impl.html#boost.proto.when_Grammar,_idp931972016.impl.which"><span class="identifier">which</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
48 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">which</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="identifier">type</span> <a class="link" href="when_Grammar__idp931972016/impl.html#boost.proto.when_Grammar,_idp931972016.impl.result_type"><span class="identifier">result_type</span></a><span class="special">;</span>
50 <span class="comment">// <a class="link" href="when_Grammar__idp931972016/impl.html#idp931995488-bb">public member functions</a></span>
51 <span class="identifier">result_type</span> <a class="link" href="when_Grammar__idp931972016/impl.html#idp931996048-bb"><span class="keyword">operator</span><span class="special">(</span><span class="special">)</span></a><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">expr_param</span><span class="special">,</span>
52 <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">state_param</span><span class="special">,</span>
53 <span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">data_param</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
54 <span class="special">}</span><span class="special">;</span>
55 <span class="special">}</span><span class="special">;</span></pre></div>
56 <div class="refsect1">
57 <a name="idp572504592"></a><h2>Description</h2>
59 Use <code class="computeroutput">proto::when<></code> to override a grammar's default
60 transform with a custom transform. It is for use when composing larger transforms by associating
61 smaller transforms with individual rules in your grammar.
64 The <code class="computeroutput">when<G, R(A...)></code> form accepts either a
65 <a class="link" href="../../CallableTransform.html" title="Concept CallableTransform">CallableTransform</a> or an <a class="link" href="../../ObjectTransform.html" title="Concept ObjectTransform">ObjectTransform</a> as its
66 second parameter. <code class="computeroutput">proto::when<></code> uses
67 <code class="computeroutput"><a class="link" href="is_callable.html" title="Struct template is_callable">proto::is_callable</a><R>::value</code> to
68 distinguish between the two, and uses
69 <code class="computeroutput"><a class="link" href="call.html" title="Struct template call">proto::call<></a></code> to evaluate
70 <a class="link" href="../../CallableTransform.html" title="Concept CallableTransform">CallableTransform</a>s and
71 <code class="computeroutput"><a class="link" href="make.html" title="Struct template make">proto::make<></a></code> to evaluate
72 <a class="link" href="../../ObjectTransform.html" title="Concept ObjectTransform">ObjectTransform</a>s.
76 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
77 <td align="left"></td>
78 <td align="right"><div class="copyright-footer">Copyright © 2008 Eric Niebler<p>
79 Distributed under the Boost Software License, Version 1.0. (See accompanying
80 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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