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<a name="boost.proto.if_"></a><div class="titlepage"></div>
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<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="comment">// 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>>
</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> If<span class="special">,</span> <span class="keyword">typename</span> Then <span class="special">=</span> <a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">,</span>
<span class="keyword">typename</span> Else <span class="special">=</span> <a class="link" href="not_.html" title="Struct template not_">proto::not_</a><span class="special"><</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">></span> <span class="special">></span>
<span class="keyword">struct</span> <a class="link" href="if_.html" title="Struct template if_">if_</a> <span class="special">:</span> <span class="keyword"></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a><span class="special"><</span><span class="identifier">if_</span><span class="special"><</span><span class="identifier">If</span><span class="special">,</span> <span class="identifier">Then</span><span class="special">,</span> <span class="identifier">Else</span><span class="special">></span> <span class="special">></span> <span class="special">{</span>
<span class="comment">// types</span>
<span class="keyword">typedef</span> <span class="identifier">if_</span> <a name="boost.proto.if_.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="if_/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> <span class="keyword">typename</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">if_</span><span class="special"><</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><a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special"><</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">,</span> <span class="identifier">If</span><span class="special">></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><span class="special">,</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><a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special"><</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">,</span> <span class="identifier">Then</span><span class="special">></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><span class="special">,</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><a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special"><</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">,</span> <span class="identifier">Else</span><span class="special">></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>
<span class="special">></span><span class="special">::</span><span class="identifier">type</span> <a class="link" href="if_/impl.html#boost.proto.if_.impl.result_type"><span class="identifier">result_type</span></a><span class="special">;</span>
<span class="comment">// <a class="link" href="if_/impl.html#idm45506988415264-bb">public member functions</a></span>
<span class="identifier">result_type</span> <a class="link" href="if_/impl.html#idm45506988414704-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="idm45555185573840"></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"><span class="comment">// A grammar that only matches integral terminals,</span>
<span class="comment">// using is_integral<> from Boost.Type_traits.</span>
<span class="keyword">struct</span> <span class="identifier">IsIntegral</span> <span class="special">:</span>
<a class="link" href="and_.html" title="Struct template and_">proto::and_</a><span class="special"><</span>
<a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special"><</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">></span><span class="special">,</span>
<a class="link" href="if_.html" title="Struct template if_">proto::if_</a><span class="special"><</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_integral</span><span class="special"><</span><a class="link" href="_value.html" title="Struct _value">proto::_value</a><span class="special">></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><span class="special">;</span></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"><span class="comment">// Match a terminal. If the terminal is integral, return</span>
<span class="comment">// mpl::true_; otherwise, return mpl::false_.</span>
<span class="keyword">struct</span> <span class="identifier">IsIntegral2</span> <span class="special">:</span>
<a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special"><</span>
<a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special"><</span><span class="identifier">_</span><span class="special">></span><span class="special">,</span>
<span class="identifier">proto</span><span class="special">::</span><span class="identifier">if_</span><span class="special"><</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_integral</span><span class="special"><</span><a class="link" href="_value.html" title="Struct _value">proto::_value</a><span class="special">></span><span class="special">(</span><span class="special">)</span><span class="special">,</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span><span class="special">(</span><span class="special">)</span><span class="special">,</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</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><span class="special">;</span></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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