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<div class="refentry" title="Struct template if_">
<a name="boost.proto.if_"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2><span class="refentrytitle">Struct template if_</span></h2>
<p>boost::proto::if_ — Used to select one grammar or another based on the result of a compile-time Boolean.
When used as a transform, <code class="computeroutput">proto::if_<></code> selects between two
transforms based on a compile-time Boolean.</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.matches_hpp" title="Header <boost/proto/matches.hpp>">boost/proto/matches.hpp</a>>
</em></span><span class="bold"><strong>template</strong></span><<span class="bold"><strong>typename</strong></span> If, <span class="bold"><strong>typename</strong></span> Then = <span class="type"><a class="link" href="_.html" title="Struct _">proto::_</a></span>,
<span class="bold"><strong>typename</strong></span> Else = <span class="type"><a class="link" href="not_.html" title="Struct template not_">proto::not_</a><<a class="link" href="_.html" title="Struct _">proto::_</a>></span> >
<span class="bold"><strong>struct</strong></span> <a class="link" href="if_.html" title="Struct template if_">if_</a> : <span class="bold"><strong></strong></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a><if_<If, Then, Else> > {
<span class="emphasis"><em>// types</em></span>
<span class="bold"><strong>typedef</strong></span> if_ <a name="boost.proto.if_.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="if_/impl.html" title="Struct template impl">impl</a> : <span class="bold"><strong></strong></span> <span class="type"><a class="link" href="transform_impl.html" title="Struct template transform_impl">proto::transform_impl</a>< Expr, State, Data ></span> {
<span class="emphasis"><em>// types</em></span>
<span class="bold"><strong>typedef</strong></span> <span class="bold"><strong>typename</strong></span> mpl::if_<
<span class="bold"><strong>typename</strong></span> boost::result_of<<a class="link" href="when.html" title="Struct template when">proto::when</a><<a class="link" href="_.html" title="Struct _">proto::_</a>, If>(Expr, State, Data)>::type,
<span class="bold"><strong>typename</strong></span> boost::result_of<<a class="link" href="when.html" title="Struct template when">proto::when</a><<a class="link" href="_.html" title="Struct _">proto::_</a>, Then>(Expr, State, Data)>::type,
<span class="bold"><strong>typename</strong></span> boost::result_of<<a class="link" href="when.html" title="Struct template when">proto::when</a><<a class="link" href="_.html" title="Struct _">proto::_</a>, Else>(Expr, State, Data)>::type
>::type <a class="link" href="if_/impl.html#boost.proto.if_.impl.result_type">result_type</a>;
<span class="emphasis"><em>// <a class="link" href="if_/impl.html#id1243287-bb">public member functions</a></em></span>
<span class="type">result_type</span> <a class="link" href="if_/impl.html#id1243292-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="id1598751"></a><h2>Description</h2>
<p>
When <code class="computeroutput">proto::if_<If, Then, Else></code> is used as a grammar,
<code class="computeroutput">If</code> must be a Proto transform and
<code class="computeroutput">Then</code> and <code class="computeroutput">Else</code> must be grammars.
An expression type <code class="computeroutput">E</code> matches
<code class="computeroutput">proto::if_<If, Then, Else></code> if
<code class="computeroutput">boost::result_of<<a class="link" href="when.html" title="Struct template when">proto::when</a><<a class="link" href="_.html" title="Struct _">proto::_</a>, If>(E)>::type::value</code>
is <code class="computeroutput">true</code> and
<code class="computeroutput">E</code> matches <code class="computeroutput">Then</code>; or, if
<code class="computeroutput">boost::result_of<<a class="link" href="when.html" title="Struct template when">proto::when</a><<a class="link" href="_.html" title="Struct _">proto::_</a>, If>(E)>::type::value</code>
is <code class="computeroutput">false</code> and <code class="computeroutput">E</code> matches <code class="computeroutput">Else</code>.
</p>
<p>
The template parameter <code class="computeroutput">Then</code> defaults to <code class="computeroutput"><a class="link" href="_.html" title="Struct _">proto::_</a></code>
and <code class="computeroutput">Else</code> defaults to
<code class="computeroutput"><a class="link" href="not_.html" title="Struct template not_">proto::not_</a><<a class="link" href="_.html" title="Struct _">proto::_</a>></code>,
so an expression type <code class="computeroutput">E</code> will match
<code class="computeroutput">proto::if_<If></code> if and only if
<code class="computeroutput">boost::result_of<<a class="link" href="when.html" title="Struct template when">proto::when</a><<a class="link" href="_.html" title="Struct _">proto::_</a>, If>(E)>::type::value</code>
is <code class="computeroutput">true</code>.
</p>
<p>
</p>
<pre class="programlisting">// A grammar that only matches integral terminals,
// using is_integral<> from Boost.Type_traits.
struct IsIntegral :
<a class="link" href="and_.html" title="Struct template and_">proto::and_</a><
<a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><<a class="link" href="_.html" title="Struct _">proto::_</a>>,
<a class="link" href="if_.html" title="Struct template if_">proto::if_</a>< boost::is_integral<<a class="link" href="_value.html" title="Struct _value">proto::_value</a>>()>
>
{};</pre>
<p>
</p>
<p>
When <code class="computeroutput">proto::if_<If, Then, Else></code> is used as a transform,
<code class="computeroutput">If</code>, <code class="computeroutput">Then</code> and
<code class="computeroutput">Else</code> must be Proto transforms. When applying the transform to
an expression <code class="computeroutput">E</code>, state <code class="computeroutput">S</code> and
data <code class="computeroutput">V</code>, if
<code class="computeroutput">boost::result_of<<a class="link" href="when.html" title="Struct template when">proto::when</a><<a class="link" href="_.html" title="Struct _">proto::_</a>, If>(E,S,V)>::type::value</code>
is <code class="computeroutput">true</code> then the <code class="computeroutput">Then</code> transform
is applied; otherwise the <code class="computeroutput">Else</code> transform is applied.
</p>
<pre class="programlisting">// Match a terminal. If the terminal is integral, return
// mpl::true_; otherwise, return mpl::false_.
struct IsIntegral2 :
<a class="link" href="when.html" title="Struct template when">proto::when</a><
<a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><_>,
proto::if_<
boost::is_integral<<a class="link" href="_value.html" title="Struct _value">proto::_value</a>>(),
mpl::true_(),
mpl::false_()
>
>
{};</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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