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<title>Struct template when<Grammar, R(A...)></title>
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<div class="refentry" title="Struct template when<Grammar, R(A...)>">
<a name="boost.proto.when_Grammar,_R(A_)_id1272228"></a><div class="titlepage"></div>
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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="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</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="emphasis"><em>// 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>>
</em></span><span class="bold"><strong>template</strong></span><<span class="bold"><strong>typename</strong></span> Grammar, <span class="bold"><strong>typename</strong></span> R, <span class="bold"><strong>typename</strong></span>... A>
<span class="bold"><strong>struct</strong></span> <a class="link" href="when_Grammar__R_A___id1272228.html" title="Struct template when<Grammar, R(A...)>">when</a><Grammar, R(A...)> : <span class="bold"><strong></strong></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a>< when<Grammar, R(A...)> > {
<span class="emphasis"><em>// types</em></span>
<span class="bold"><strong>typedef</strong></span> <span class="bold"><strong>typename</strong></span> Grammar::proto_base_expr <a name="boost.proto.when_Grammar,_R(A_)_id1272228.proto_base_expr"></a>proto_base_expr;
<span class="emphasis"><em>// member classes/structs/unions</em></span>
<span class="bold"><strong>template</strong></span><<span class="bold"><strong>typename</strong></span> Expr, <span class="bold"><strong>typename</strong></span> State, <span class="bold"><strong>typename</strong></span> Data>
<span class="bold"><strong>struct</strong></span> <a class="link" href="when_Grammar__R_A___id1272228/impl.html" title="Struct template impl">impl</a> : <span class="bold"><strong></strong></span> <a class="link" href="transform_impl.html" title="Struct template transform_impl">proto::transform_impl</a>< Expr, State, Data > {
<span class="emphasis"><em>// types</em></span>
<span class="bold"><strong>typedef</strong></span> <a class="link" href="call.html" title="Struct template call">proto::call</a><R(A...)> <a class="link" href="when_Grammar__R_A___id1272228/impl.html#boost.proto.when_Grammar,_R(A_)_id1272228.impl.call_">call_</a>; <span class="emphasis"><em>// For exposition only</em></span>
<span class="bold"><strong>typedef</strong></span> <a class="link" href="make.html" title="Struct template make">proto::make</a><R(A...)> <a class="link" href="when_Grammar__R_A___id1272228/impl.html#boost.proto.when_Grammar,_R(A_)_id1272228.impl.make_">make_</a>; <span class="emphasis"><em>// For exposition only</em></span>
<span class="bold"><strong>typedef</strong></span> <span class="bold"><strong>typename</strong></span> mpl::if_<<a class="link" href="is_callable.html" title="Struct template is_callable">proto::is_callable</a><R>,call_,make_>::type <a class="link" href="when_Grammar__R_A___id1272228/impl.html#boost.proto.when_Grammar,_R(A_)_id1272228.impl.which">which</a>; <span class="emphasis"><em>// For exposition only</em></span>
<span class="bold"><strong>typedef</strong></span> <span class="bold"><strong>typename</strong></span> boost::result_of<which(Expr, State, Data)>::type <a class="link" href="when_Grammar__R_A___id1272228/impl.html#boost.proto.when_Grammar,_R(A_)_id1272228.impl.result_type">result_type</a>;
<span class="emphasis"><em>// <a class="link" href="when_Grammar__R_A___id1272228/impl.html#id1272429-bb">public member functions</a></em></span>
<span class="type">result_type</span> <a class="link" href="when_Grammar__R_A___id1272228/impl.html#id1272434-bb"><span class="bold"><strong>operator</strong></span>()</a>(<span class="bold"><strong>typename</strong></span> impl::expr_param,
<span class="bold"><strong>typename</strong></span> impl::state_param,
<span class="bold"><strong>typename</strong></span> impl::data_param) <span class="bold"><strong>const</strong></span>;
};
};</pre></div>
<div class="refsect1" title="Description">
<a name="id1678061"></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>)
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