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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="phoenix.examples.adding_an_expression"></a><a class="link" href="adding_an_expression.html" title="Adding an expression">Adding an expression</a>
  28 </h3></div></div></div>
  29 <p>
  30         This is not a toy example. This is actually part of the library. Remember
  31         the <a class="link" href="../modules/statement/while__statement.html" title="while_ Statement"><code class="computeroutput"><span class="keyword">while</span></code></a> lazy statement? Putting together
  32         everything we've learned so far, we eill present it here in its entirety
  33         (verbatim):
  34       </p>
  35 <pre class="programlisting"><span class="identifier">BOOST_PHOENIX_DEFINE_EXPRESSION</span><span class="special">(</span>
  36     <span class="special">(</span><span class="identifier">boost</span><span class="special">)(</span><span class="identifier">phoenix</span><span class="special">)(</span><span class="identifier">while_</span><span class="special">)</span>
  37   <span class="special">,</span> <span class="special">(</span><span class="identifier">meta_grammar</span><span class="special">)</span>           <span class="comment">// Cond</span>
  38     <span class="special">(</span><span class="identifier">meta_grammar</span><span class="special">)</span>           <span class="comment">// Do</span>
  39 <span class="special">)</span>
  40
  41 <span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">phoenix</span>
  42 <span class="special">{</span>
  43     <span class="keyword">struct</span> <span class="identifier">while_eval</span>
  44     <span class="special">{</span>
  45         <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>
  46
  47         <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Cond</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Do</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
  48         <span class="identifier">result_type</span>
  49         <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Cond</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">cond</span><span class="special">,</span> <span class="identifier">Do</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">do_</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;</span> <span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span>
  50         <span class="special">{</span>
  51             <span class="keyword">while</span><span class="special">(</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">cond</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">))</span>
  52             <span class="special">{</span>
  53                 <span class="identifier">eval</span><span class="special">(</span><span class="identifier">do_</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
  54             <span class="special">}</span>
  55         <span class="special">}</span>
  56     <span class="special">};</span>
  57
  58     <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Dummy</span><span class="special">&gt;</span>
  59     <span class="keyword">struct</span> <span class="identifier">default_actions</span><span class="special">::</span><span class="identifier">when</span><span class="special">&lt;</span><span class="identifier">rule</span><span class="special">::</span><span class="identifier">while_</span><span class="special">,</span> <span class="identifier">Dummy</span><span class="special">&gt;</span>
  60         <span class="special">:</span> <span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">while_eval</span><span class="special">,</span> <span class="identifier">Dummy</span><span class="special">&gt;</span>
  61     <span class="special">{};</span>
  62
  63     <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Cond</span><span class="special">&gt;</span>
  64     <span class="keyword">struct</span> <span class="identifier">while_gen</span>
  65     <span class="special">{</span>
  66         <span class="identifier">while_gen</span><span class="special">(</span><span class="identifier">Cond</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">cond</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">cond</span><span class="special">(</span><span class="identifier">cond</span><span class="special">)</span> <span class="special">{}</span>
  67
  68         <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Do</span><span class="special">&gt;</span>
  69         <span class="keyword">typename</span> <span class="identifier">expression</span><span class="special">::</span><span class="identifier">while_</span><span class="special">&lt;</span><span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span>
  70         <span class="keyword">operator</span><span class="special">[](</span><span class="identifier">Do</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">do_</span><span class="special">)</span> <span class="keyword">const</span>
  71         <span class="special">{</span>
  72             <span class="keyword">return</span> <span class="identifier">expression</span><span class="special">::</span><span class="identifier">while_</span><span class="special">&lt;</span><span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">&gt;::</span><span class="identifier">make</span><span class="special">(</span><span class="identifier">cond</span><span class="special">,</span> <span class="identifier">do_</span><span class="special">);</span>
  73         <span class="special">}</span>
  74
  75         <span class="identifier">Cond</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">cond</span><span class="special">;</span>
  76     <span class="special">};</span>
  77
  78     <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Cond</span><span class="special">&gt;</span>
  79     <span class="identifier">while_gen</span><span class="special">&lt;</span><span class="identifier">Cond</span><span class="special">&gt;</span> <span class="keyword">const</span>
  80     <span class="identifier">while_</span><span class="special">(</span><span class="identifier">Cond</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">cond</span><span class="special">)</span>
  81     <span class="special">{</span>
  82         <span class="keyword">return</span> <span class="identifier">while_gen</span><span class="special">&lt;</span><span class="identifier">Cond</span><span class="special">&gt;(</span><span class="identifier">cond</span><span class="special">);</span>
  83     <span class="special">}</span>
  84 <span class="special">}}</span>
  85 </pre>
  86 <p>
  87         <code class="computeroutput"><span class="identifier">while_eval</span></code> is an example
  88         of how to evaluate an expression. It gets called in the <code class="computeroutput"><span class="identifier">rule</span><span class="special">::</span><span class="keyword">while</span></code> action.
  89         <code class="computeroutput"><span class="identifier">while_gen</span></code> and <code class="computeroutput"><span class="identifier">while_</span></code> are the expression template front
  90         ends. Let's break this apart to undestand what's happening. Let's start at
  91         the bottom. It's easier that way.
  92       </p>
  93 <p>
  94         When you write:
  95       </p>
  96 <pre class="programlisting"><span class="identifier">while_</span><span class="special">(</span><span class="identifier">cond</span><span class="special">)</span>
  97 </pre>
  98 <p>
  99         we generate an instance of <code class="computeroutput"><span class="identifier">while_gen</span><span class="special">&lt;</span><span class="identifier">Cond</span><span class="special">&gt;</span></code>, where <code class="computeroutput"><span class="identifier">Cond</span></code>
 100         is the type of <code class="computeroutput"><span class="identifier">cond</span></code>. <code class="computeroutput"><span class="identifier">cond</span></code> can be an arbitrarily complex actor
 101         expression. The <code class="computeroutput"><span class="identifier">while_gen</span></code>
 102         template class has an <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code> accepting another expression. If we write:
 103       </p>
 104 <pre class="programlisting"><span class="identifier">while_</span><span class="special">(</span><span class="identifier">cond</span><span class="special">)</span>
 105 <span class="special">[</span>
 106     <span class="identifier">do_</span>
 107 <span class="special">]</span>
 108 </pre>
 109 <p>
 110         it will generate a proper composite with the type:
 111       </p>
 112 <pre class="programlisting"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">while_</span><span class="special">&lt;</span><span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">&gt;::</span><span class="identifier">type</span>
 113 </pre>
 114 <p>
 115         where <code class="computeroutput"><span class="identifier">Cond</span></code> is the type of
 116         <code class="computeroutput"><span class="identifier">cond</span></code> and <code class="computeroutput"><span class="identifier">Do</span></code>
 117         is the type of <code class="computeroutput"><span class="identifier">do_</span></code>. Notice
 118         how we are using Phoenix's <a class="link" href="../inside/expression.html" title="Phoenix Expressions">Expression</a>
 119         mechanism here
 120       </p>
 121 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Do</span><span class="special">&gt;</span>
 122 <span class="keyword">typename</span> <span class="identifier">expression</span><span class="special">::</span><span class="identifier">while_</span><span class="special">&lt;</span><span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span>
 123 <span class="keyword">operator</span><span class="special">[](</span><span class="identifier">Do</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">do_</span><span class="special">)</span> <span class="keyword">const</span>
 124 <span class="special">{</span>
 125     <span class="keyword">return</span> <span class="identifier">expression</span><span class="special">::</span><span class="identifier">while_</span><span class="special">&lt;</span><span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">&gt;::</span><span class="identifier">make</span><span class="special">(</span><span class="identifier">cond</span><span class="special">,</span> <span class="identifier">do_</span><span class="special">);</span>
 126 <span class="special">}</span>
 127 </pre>
 128 <p>
 129         Finally, the <code class="computeroutput"><span class="identifier">while_eval</span></code> does
 130         its thing:
 131       </p>
 132 <pre class="programlisting"><span class="keyword">while</span><span class="special">(</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">cond</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">))</span>
 133 <span class="special">{</span>
 134     <span class="identifier">eval</span><span class="special">(</span><span class="identifier">do_</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">);</span>
 135 <span class="special">}</span>
 136 </pre>
 137 <p>
 138         <code class="computeroutput"><span class="identifier">cond</span></code> and <code class="computeroutput"><span class="identifier">do_</span></code>,
 139         at this point, are instances of <a class="link" href="../inside/actor.html" title="Actors in Detail">Actor</a>.
 140         <code class="computeroutput"><span class="identifier">cond</span></code> and <code class="computeroutput"><span class="identifier">do_</span></code>
 141         are the <a class="link" href="../inside/actor.html" title="Actors in Detail">Actors</a> passed as parameters
 142         by <code class="computeroutput"><span class="identifier">call</span></code>, ctx is the <a class="link" href="../inside/actor.html" title="Actors in Detail">Context</a>
 143       </p>
 144 </div>
 145 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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 147 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas
 148       Heller, John Fletcher<p>
 149         Distributed under the Boost Software License, Version 1.0. (See accompanying
 150         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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