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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> | |
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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>) | |
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