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<h2><span class="refentrytitle">Struct template when<Grammar, R(A...)></span></h2>
<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
transform with the grammar. </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> 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>
<span class="keyword">struct</span> <a class="link" href="when_Gra_idm45506983690064.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>
<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_Gra_idm45506983690064.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_Gra_idm45506983690064/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>
<span class="comment">// types</span>
<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_Gra_idm45506983690064/impl.html#boost.proto.when_Gra_idm45506983690064.impl.call_"><span class="identifier">call_</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
<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_Gra_idm45506983690064/impl.html#boost.proto.when_Gra_idm45506983690064.impl.make_"><span class="identifier">make_</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
<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_Gra_idm45506983690064/impl.html#boost.proto.when_Gra_idm45506983690064.impl.which"><span class="identifier">which</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
<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_Gra_idm45506983690064/impl.html#boost.proto.when_Gra_idm45506983690064.impl.result_type"><span class="identifier">result_type</span></a><span class="special">;</span>
<span class="comment">// <a class="link" href="when_Gra_idm45506983690064/impl.html#idm45506983666592-bb">public member functions</a></span>
<span class="identifier">result_type</span> <a class="link" href="when_Gra_idm45506983690064/impl.html#idm45506983666032-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>
<span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">state_param</span><span class="special">,</span>
<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>
<span class="special">}</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="idm45555169444096"></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, R(A...)></code> form accepts either a
<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
second parameter. <code class="computeroutput">proto::when<></code> uses
<code class="computeroutput"><a class="link" href="is_callable.html" title="Struct template is_callable">proto::is_callable</a><R>::value</code> to
distinguish between the two, and uses
<code class="computeroutput"><a class="link" href="call.html" title="Struct template call">proto::call<></a></code> to evaluate
<a class="link" href="../../CallableTransform.html" title="Concept CallableTransform">CallableTransform</a>s and
<code class="computeroutput"><a class="link" href="make.html" title="Struct template make">proto::make<></a></code> to evaluate
<a class="link" href="../../ObjectTransform.html" title="Concept ObjectTransform">ObjectTransform</a>s.
</p>
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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>)
</p>
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