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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="phoenix.inside.custom_terminals"></a><a class="link" href="custom_terminals.html" title="Custom Terminals">Custom Terminals</a>
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  29 <p>
  30         Custom Terminals are used in Phoenix to handle special values transparently.
  31         For example, as Phoenix captures everything by value, we needed to use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span></code> to bring reference semantics
  32         into Phoenix.
  33       </p>
  34 <p>
  35         Custom terminals could be any wrapper class:
  36       </p>
  37 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
  38 <span class="keyword">struct</span> <span class="identifier">is_custom_terminal</span><span class="special">;</span>
  39 </pre>
  40 <p>
  41         needs to be specialized in order for Phoenix to recognize this wrapper type.
  42         <code class="computeroutput"><span class="identifier">default_action</span></code> calls <code class="computeroutput"><span class="identifier">custom_terminal</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>.
  43       </p>
  44 <p>
  45         Example:
  46       </p>
  47 <pre class="programlisting"><span class="comment">// Call out boost::reference_wrapper for special handling</span>
  48 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
  49 <span class="keyword">struct</span> <span class="identifier">is_custom_terminal</span><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;</span>
  50   <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
  51 <span class="special">{};</span>
  52
  53 <span class="comment">// Special handling for boost::reference_wrapper</span>
  54 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
  55 <span class="keyword">struct</span> <span class="identifier">custom_terminal</span><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;</span>
  56 <span class="special">{</span>
  57     <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="special">&amp;</span><span class="identifier">result_type</span><span class="special">;</span>
  58
  59     <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
  60     <span class="identifier">T</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">()(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">r</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
  61     <span class="special">{</span>
  62         <span class="keyword">return</span> <span class="identifier">r</span><span class="special">;</span>
  63     <span class="special">}</span>
  64 <span class="special">};</span>
  65 </pre>
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  69 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas
  70       Heller, John Fletcher<p>
  71         Distributed under the Boost Software License, Version 1.0. (See accompanying
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