<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Adding an expression</title> <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="../../index.html" title="Chapter 1. Phoenix 3.2.0"> <link rel="up" href="../examples.html" title="Advanced Examples"> <link rel="prev" href="extending_actors.html" title="Extending Actors"> <link rel="next" href="transforming_the_expression_tree.html" title="Transforming the Expression Tree"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> <td align="center"><a href="../../../../../../index.html">Home</a></td> <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="extending_actors.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../examples.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="transforming_the_expression_tree.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <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> </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 © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas 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>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="extending_actors.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../examples.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="transforming_the_expression_tree.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>