projects / ceph.git / blame
| Commit | Line | Data |
|---|---|---|
| 7c673cae FG |
1 | <html> |
| 2 | <head> | |
| 3 | <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> | |
| 4 | <title>Adding an expression</title> | |
| 5 | <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> | |
| 6 | <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> | |
| 7 | <link rel="home" href="../../index.html" title="Chapter 1. Phoenix 3.2.0"> | |
| 8 | <link rel="up" href="../examples.html" title="Advanced Examples"> | |
| 9 | <link rel="prev" href="extending_actors.html" title="Extending Actors"> | |
| 10 | <link rel="next" href="transforming_the_expression_tree.html" title="Transforming the Expression Tree"> | |
| 11 | </head> | |
| 12 | <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> | |
| 13 | <table cellpadding="2" width="100%"><tr> | |
| 14 | <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> | |
| 15 | <td align="center"><a href="../../../../../../index.html">Home</a></td> | |
| 16 | <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> | |
| 17 | <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> | |
| 18 | <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> | |
| 19 | <td align="center"><a href="../../../../../../more/index.htm">More</a></td> | |
| 20 | </tr></table> | |
| 21 | <hr> | |
| 22 | <div class="spirit-nav"> | |
| 23 | <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> | |
| 24 | </div> | |
| 25 | <div class="section"> | |
| 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"><</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> | |
| 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">&</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> | |
| 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"><</span><span class="keyword">typename</span> <span class="identifier">Dummy</span><span class="special">></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"><</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> | |
| 60 | <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> | |
| 61 | <span class="special">{};</span> | |
| 62 | ||
| 63 | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Cond</span><span class="special">></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">&</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"><</span><span class="keyword">typename</span> <span class="identifier">Do</span><span class="special">></span> | |
| 69 | <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> | |
| 70 | <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> | |
| 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"><</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> | |
| 73 | <span class="special">}</span> | |
| 74 | ||
| 75 | <span class="identifier">Cond</span> <span class="keyword">const</span><span class="special">&</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"><</span><span class="keyword">typename</span> <span class="identifier">Cond</span><span class="special">></span> | |
| 79 | <span class="identifier">while_gen</span><span class="special"><</span><span class="identifier">Cond</span><span class="special">></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">&</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"><</span><span class="identifier">Cond</span><span class="special">>(</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"><</span><span class="identifier">Cond</span><span class="special">></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"><</span><span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">>::</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"><</span><span class="keyword">typename</span> <span class="identifier">Do</span><span class="special">></span> | |
| 122 | <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> | |
| 123 | <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> | |
| 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"><</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> | |
| 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> | |
| 146 | <td align="left"></td> | |
| 147 | <td align="right"><div class="copyright-footer">Copyright © 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>) | |
| 151 | </p> | |
| 152 | </div></td> | |
| 153 | </tr></table> | |
| 154 | <hr> | |
| 155 | <div class="spirit-nav"> | |
| 156 | <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> | |
| 157 | </div> | |
| 158 | </body> | |
| 159 | </html> |