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| <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="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">struct</span> <a class="link" href="_.html" title="Struct _">_</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="comment">// types</span> |
| <span class="keyword">typedef</span> <span class="identifier">_</span> <a name="boost.proto._.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="_/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="identifier">Expr</span> <a class="link" href="_/impl.html#boost.proto._.impl.result_type"><span class="identifier">result_type</span></a><span class="special">;</span> |
| |
| <span class="comment">// <a class="link" href="_/impl.html#id1266497-bb">public member functions</a></span> |
| <span class="identifier">Expr</span> <a class="link" href="_/impl.html#id1266501-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="id2325218"></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"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">(</span><span class="special">(</span> |
| <a class="link" href="matches.html" title="Struct template matches">proto::matches</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">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="special">></span><span class="special">::</span><span class="identifier">type</span><span class="special">,</span> |
| <span class="bold"><strong><a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">></span> <span class="special">></span></strong></span> |
| <span class="special">></span> |
| <span class="special">)</span><span class="special">)</span><span class="special">;</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"><span class="keyword">struct</span> <span class="identifier">CountChildren</span> <span class="special">:</span> |
| <a class="link" href="or_.html" title="Struct template or_">proto::or_</a><span class="special"><</span> |
| <span class="comment">// Terminals have no children</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">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">></span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">0</span><span class="special">></span><span class="special">(</span><span class="special">)</span><span class="special">></span><span class="special">,</span> |
| <span class="comment">// Use proto::fold<> to count the children of non-terminals</span> |
| <a class="link" href="otherwise.html" title="Struct template otherwise">proto::otherwise</a><span class="special"><</span> |
| <a class="link" href="fold.html" title="Struct template fold">proto::fold</a><span class="special"><</span> |
| <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="comment">// <-- fold the current expression</span> |
| <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">0</span><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">plus</span><span class="special"><</span><a class="link" href="_state.html" title="Struct _state">proto::_state</a><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special"><</span><span class="number">1</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> |
| <span class="special">{</span><span class="special">}</span><span class="special">;</span></pre> |
| <p> |
| </p> |
| </div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <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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