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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="phoenix.examples.adding_an_expression"></a><a class="link" href="adding_an_expression.html" title="Adding an expression">Adding an expression</a> |
| </h3></div></div></div> |
| <p> |
| This is not a toy example. This is actually part of the library. Remember |
| 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 |
| everything we've learned so far, we eill present it here in its entirety |
| (verbatim): |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_PHOENIX_DEFINE_EXPRESSION</span><span class="special">(</span> |
| <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> |
| <span class="special">,</span> <span class="special">(</span><span class="identifier">meta_grammar</span><span class="special">)</span> <span class="comment">// Cond</span> |
| <span class="special">(</span><span class="identifier">meta_grammar</span><span class="special">)</span> <span class="comment">// Do</span> |
| <span class="special">)</span> |
| |
| <span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">phoenix</span> |
| <span class="special">{</span> |
| <span class="keyword">struct</span> <span class="identifier">while_eval</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</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">></span> |
| <span class="identifier">result_type</span> |
| <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Cond</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">cond</span><span class="special">,</span> <span class="identifier">Do</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">do_</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&</span> <span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <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> |
| <span class="special">{</span> |
| <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> |
| <span class="special">}</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Dummy</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">default_actions</span><span class="special">::</span><span class="identifier">when</span><span class="special"><</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">></span> |
| <span class="special">:</span> <span class="identifier">call</span><span class="special"><</span><span class="identifier">while_eval</span><span class="special">,</span> <span class="identifier">Dummy</span><span class="special">></span> |
| <span class="special">{};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Cond</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">while_gen</span> |
| <span class="special">{</span> |
| <span class="identifier">while_gen</span><span class="special">(</span><span class="identifier">Cond</span> <span class="keyword">const</span><span class="special">&</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> |
| |
| <span class="keyword">template</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">expression</span><span class="special">::</span><span class="identifier">while_</span><span class="special"><</span><span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> |
| <span class="keyword">operator</span><span class="special">[](</span><span class="identifier">Do</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">do_</span><span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">expression</span><span class="special">::</span><span class="identifier">while_</span><span class="special"><</span><span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">>::</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> |
| <span class="special">}</span> |
| |
| <span class="identifier">Cond</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">cond</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Cond</span><span class="special">></span> |
| <span class="identifier">while_gen</span><span class="special"><</span><span class="identifier">Cond</span><span class="special">></span> <span class="keyword">const</span> |
| <span class="identifier">while_</span><span class="special">(</span><span class="identifier">Cond</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">cond</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">while_gen</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> |
| <span class="special">}}</span> |
| </pre> |
| <p> |
| <code class="computeroutput"><span class="identifier">while_eval</span></code> is an example |
| 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. |
| <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 |
| ends. Let's break this apart to undestand what's happening. Let's start at |
| the bottom. It's easier that way. |
| </p> |
| <p> |
| When you write: |
| </p> |
| <pre class="programlisting"><span class="identifier">while_</span><span class="special">(</span><span class="identifier">cond</span><span class="special">)</span> |
| </pre> |
| <p> |
| we generate an instance of <code class="computeroutput"><span class="identifier">while_gen</span><span class="special"><</span><span class="identifier">Cond</span><span class="special">></span></code>, where <code class="computeroutput"><span class="identifier">Cond</span></code> |
| 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 |
| expression. The <code class="computeroutput"><span class="identifier">while_gen</span></code> |
| template class has an <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code> accepting another expression. If we write: |
| </p> |
| <pre class="programlisting"><span class="identifier">while_</span><span class="special">(</span><span class="identifier">cond</span><span class="special">)</span> |
| <span class="special">[</span> |
| <span class="identifier">do_</span> |
| <span class="special">]</span> |
| </pre> |
| <p> |
| it will generate a proper composite with the type: |
| </p> |
| <pre class="programlisting"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">while_</span><span class="special"><</span><span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">>::</span><span class="identifier">type</span> |
| </pre> |
| <p> |
| where <code class="computeroutput"><span class="identifier">Cond</span></code> is the type of |
| <code class="computeroutput"><span class="identifier">cond</span></code> and <code class="computeroutput"><span class="identifier">Do</span></code> |
| is the type of <code class="computeroutput"><span class="identifier">do_</span></code>. Notice |
| how we are using Phoenix's <a class="link" href="../inside/expression.html" title="Phoenix Expressions">Expression</a> |
| mechanism here |
| </p> |
| <pre class="programlisting"><span class="keyword">template</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">expression</span><span class="special">::</span><span class="identifier">while_</span><span class="special"><</span><span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> |
| <span class="keyword">operator</span><span class="special">[](</span><span class="identifier">Do</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">do_</span><span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">expression</span><span class="special">::</span><span class="identifier">while_</span><span class="special"><</span><span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">>::</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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| Finally, the <code class="computeroutput"><span class="identifier">while_eval</span></code> does |
| its thing: |
| </p> |
| <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> |
| <span class="special">{</span> |
| <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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| <code class="computeroutput"><span class="identifier">cond</span></code> and <code class="computeroutput"><span class="identifier">do_</span></code>, |
| at this point, are instances of <a class="link" href="../inside/actor.html" title="Actors in Detail">Actor</a>. |
| <code class="computeroutput"><span class="identifier">cond</span></code> and <code class="computeroutput"><span class="identifier">do_</span></code> |
| are the <a class="link" href="../inside/actor.html" title="Actors in Detail">Actors</a> passed as parameters |
| 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> |
| </p> |
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| Heller, John Fletcher<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>) |
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