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<title>Struct _</title>
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<div class="refentry" title="Struct _">
<a name="boost.proto._"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Struct _</span></h2>
<p>boost::proto::_ — A wildcard grammar element that matches any expression, and a transform that returns
the current expression unchanged.</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>struct</strong></span> <a class="link" href="_.html" title="Struct _">_</a> : <span class="bold"><strong></strong></span> <span class="type"><a class="link" href="transform.html" title="Struct template transform">proto::transform</a><_></span> {
<span class="emphasis"><em>// types</em></span>
<span class="bold"><strong>typedef</strong></span> _ <a name="boost.proto._.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="_/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> Expr <a class="link" href="_/impl.html#boost.proto._.impl.result_type">result_type</a>;
<span class="emphasis"><em>// <a class="link" href="_/impl.html#id1242632-bb">public member functions</a></em></span>
<span class="type">Expr</span> <a class="link" href="_/impl.html#id1242636-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="id1597439"></a><h2>Description</h2>
<p>
The wildcard type, <code class="computeroutput">proto::_</code>, is a grammar element such
that <code class="computeroutput"><a class="link" href="matches.html" title="Struct template matches">proto::matches</a><E, proto::_>::value</code>
is <code class="computeroutput">true</code> for any expression type <code class="computeroutput">E</code>.
</p>
<p>
The wildcard can also be used as a stand-in for a template argument when matching terminals.
For instance, the following is a grammar that will match any
<code class="computeroutput">std::complex<></code> terminal:</p>
<pre class="programlisting">BOOST_MPL_ASSERT((
<a class="link" href="matches.html" title="Struct template matches">proto::matches</a><
<a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><std::complex<double> >::type,
<span class="bold"><strong><a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><std::complex< proto::_ > ></strong></span>
>
));</pre>
<p>
</p>
<p>
When used as a transform, <code class="computeroutput">proto::_</code> returns the current expression
unchanged. For instance, in the following, <code class="computeroutput">proto::_</code> is used with
the <code class="computeroutput"><a class="link" href="fold.html" title="Struct template fold">proto::fold<></a></code>
transform to fold the children of a node:</p>
<pre class="programlisting">struct CountChildren :
<a class="link" href="or_.html" title="Struct template or_">proto::or_</a><
// Terminals have no children
<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::_>, mpl::int_<0>()>,
// Use proto::fold<> to count the children of non-terminals
<a class="link" href="otherwise.html" title="Struct template otherwise">proto::otherwise</a><
<a class="link" href="fold.html" title="Struct template fold">proto::fold</a><
proto::_, // <-- fold the current expression
mpl::int_<0>(),
mpl::plus<<a class="link" href="_state.html" title="Struct _state">proto::_state</a>, mpl::int_<1> >()
>
>
>
{};</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>)
</p>
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