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  27 <a name="boost.proto._"></a><div class="titlepage"></div>
  28 <div class="refnamediv">
  29 <h2><span class="refentrytitle">Struct _</span></h2>
  30 <p>boost::proto::_ &#8212; A wildcard grammar element that matches any expression, and a transform that returns
  31           the current expression unchanged.</p>
  32 </div>
  33 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
  34 <div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../../proto/reference.html#header.boost.proto.matches_hpp" title="Header &lt;boost/proto/matches.hpp&gt;">boost/proto/matches.hpp</a>&gt;
  35
  36 </span>
  37 <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>&lt;_&gt; <span class="special">{</span>
  38   <span class="comment">// types</span>
  39   <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>
  40
  41   <span class="comment">// member classes/structs/unions</span>
  42   <span class="keyword">template</span><span class="special">&lt;</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">&gt;</span>
  43   <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>&lt;Expr, State, Data&gt; <span class="special">{</span>
  44     <span class="comment">// types</span>
  45     <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>
  46
  47     <span class="comment">// <a class="link" href="_/impl.html#idp701785872-bb">public member functions</a></span>
  48     <span class="identifier">Expr</span> <a class="link" href="_/impl.html#idp701786432-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>
  49                     <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>
  50   <span class="special">}</span><span class="special">;</span>
  51 <span class="special">}</span><span class="special">;</span></pre></div>
  52 <div class="refsect1">
  53 <a name="idp468346384"></a><h2>Description</h2>
  54 <p>
  55             The wildcard type, <code class="computeroutput">proto::_</code>, is a grammar element such
  56             that <code class="computeroutput"><a class="link" href="matches.html" title="Struct template matches">proto::matches</a>&lt;E, proto::_&gt;::value</code>
  57             is <code class="computeroutput">true</code> for any expression type <code class="computeroutput">E</code>.
  58           </p>
  59 <p>
  60             The wildcard can also be used as a stand-in for a template argument when matching terminals.
  61             For instance, the following is a grammar that will match any
  62             <code class="computeroutput">std::complex&lt;&gt;</code> terminal:</p>
  63 <pre class="programlisting"><span class="identifier">BOOST_MPL_ASSERT</span><span class="special">(</span><span class="special">(</span>
  64   <a class="link" href="matches.html" title="Struct template matches">proto::matches</a><span class="special">&lt;</span>
  65     <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span><span class="special">,</span>
  66     <span class="bold"><strong><a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&lt;</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span> <span class="special">&gt;</span> <span class="special">&gt;</span></strong></span>
  67   <span class="special">&gt;</span>
  68 <span class="special">)</span><span class="special">)</span><span class="special">;</span></pre>
  69 <p>
  70           </p>
  71 <p>
  72             When used as a transform, <code class="computeroutput">proto::_</code> returns the current expression
  73             unchanged. For instance, in the following, <code class="computeroutput">proto::_</code> is used with
  74             the <code class="computeroutput"><a class="link" href="fold.html" title="Struct template fold">proto::fold&lt;&gt;</a></code>
  75             transform to fold the children of a node:</p>
  76 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">CountChildren</span> <span class="special">:</span>
  77   <a class="link" href="or_.html" title="Struct template or_">proto::or_</a><span class="special">&lt;</span>
  78     <span class="comment">// Terminals have no children</span>
  79     <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span><a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">&gt;</span><span class="special">,</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span><span class="special">(</span><span class="special">)</span><span class="special">&gt;</span><span class="special">,</span>
  80     <span class="comment">// Use proto::fold&lt;&gt; to count the children of non-terminals</span>
  81     <a class="link" href="otherwise.html" title="Struct template otherwise">proto::otherwise</a><span class="special">&lt;</span>
  82       <a class="link" href="fold.html" title="Struct template fold">proto::fold</a><span class="special">&lt;</span>
  83         <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">,</span> <span class="comment">// &lt;-- fold the current expression</span>
  84         <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">int_</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;</span><span class="special">(</span><span class="special">)</span><span class="special">,</span>
  85         <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</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">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;</span><span class="special">(</span><span class="special">)</span>
  86       <span class="special">&gt;</span>
  87     <span class="special">&gt;</span>
  88   <span class="special">&gt;</span>
  89 <span class="special">{</span><span class="special">}</span><span class="special">;</span></pre>
  90 <p>
  91           </p>
  92 </div>
  93 </div>
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  95 <td align="left"></td>
  96 <td align="right"><div class="copyright-footer">Copyright &#169; 2008 Eric Niebler<p>
  97         Distributed under the Boost Software License, Version 1.0. (See accompanying
  98         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>)
  99       </p>
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