/Src/Dependencies/Boost/libs/phoenix/doc/html/phoenix/examples/adding_an_expression.html

http://hadesmem.googlecode.com/ · HTML · 158 lines · 151 code · 7 blank · 0 comment · 0 complexity · 5beae5f3efa38be73d0dd2fd9ad3902e MD5 · raw file 

    

  1. <html>
    2. 

  2. <head>
    3. 

  3. <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
    4. 

  4. <title>Adding an expression</title>
    5. 

  5. <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
    6. 

  6. <meta name="generator" content="DocBook XSL Stylesheets V1.76.1">
    7. 

  7. <link rel="home" href="../../index.html" title="Chapter&#160;1.&#160;Phoenix 3.0">
    8. 

  8. <link rel="up" href="../examples.html" title="Advanced Examples">
    9. 

  9. <link rel="prev" href="extending_actors.html" title="Extending Actors">
    10. 

  10. <link rel="next" href="transforming_the_expression_tree.html" title="Transforming the Expression Tree">
    11. 

  11. </head>
    12. 

  12. <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
    13. 

  13. <table cellpadding="2" width="100%"><tr>
    14. 

  14. <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
    15. 

  15. <td align="center"><a href="../../../../../../index.html">Home</a></td>
    16. 

  16. <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
    17. 

  17. <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
    18. 

  18. <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
    19. 

  19. <td align="center"><a href="../../../../../../more/index.htm">More</a></td>
    20. 

  20. </tr></table>
    21. 

  21. <hr>
    22. 

  22. <div class="spirit-nav">
    23. 

  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. 

  24. </div>
    25. 

  25. <div class="section">
    26. 

  26. <div class="titlepage"><div><div><h3 class="title">
    27. 

  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. 

  28. </h3></div></div></div>
    29. 

  29. <p>
    30. 

  30.         This is not a toy example. This is actually part of the library. Remember
    31. 

  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. 

  32.         everything we've learned so far, we eill present it here in its entirety
    33. 

  33.         (verbatim):
    34. 

  34.       </p>
    35. 

  35. <pre class="programlisting"><span class="identifier">BOOST_PHOENIX_DEFINE_EXPRESSION</span><span class="special">(</span>
    36. 

  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. 

  37.   <span class="special">,</span> <span class="special">(</span><span class="identifier">meta_grammar</span><span class="special">)</span>           <span class="comment">// Cond
    38. 

  38. </span>    <span class="special">(</span><span class="identifier">meta_grammar</span><span class="special">)</span>           <span class="comment">// Do
    39. 

  39. </span><span class="special">)</span>
    40. 

  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. 

  42. <span class="special">{</span>
    43. 

  43.     <span class="keyword">struct</span> <span class="identifier">while_eval</span>
    44. 

  44.     <span class="special">{</span>
    45. 

  45.         <span class="keyword">typedef</span> <span class="keyword">void</span> <span class="identifier">result_type</span><span class="special">;</span>
    46. 

  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. 

  48.         <span class="identifier">result_type</span>
    49. 

  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. 

  50.         <span class="special">{</span>
    51. 

  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. 

  52.             <span class="special">{</span>
    53. 

  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. 

  54.             <span class="special">}</span>
    55. 

  55.         <span class="special">}</span>
    56. 

  56.     <span class="special">};</span>
    57. 

  57.     
    58. 

  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. 

  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. 

  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. 

  61.     <span class="special">{};</span>
    62. 

  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. 

  64.     <span class="keyword">struct</span> <span class="identifier">while_gen</span>
    65. 

  65.     <span class="special">{</span>
    66. 

  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. 

  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. 

  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. 

  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. 

  71.         <span class="special">{</span>
    72. 

  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. 

  73.         <span class="special">}</span>
    74. 

  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. 

  76.     <span class="special">};</span>
    77. 

  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. 

  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. 

  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. 

  81.     <span class="special">{</span>
    82. 

  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. 

  83.     <span class="special">}</span>
    84. 

  84. <span class="special">}}</span>
    85. 

  85. </pre>
    86. 

  86. <p>
    87. 

  87.         <code class="computeroutput"><span class="identifier">while_eval</span></code> is an example
    88. 

  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. 

  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. 

  90.         ends. Let's break this apart to undestand what's happening. Let's start at
    91. 

  91.         the bottom. It's easier that way.
    92. 

  92.       </p>
    93. 

  93. <p>
    94. 

  94.         When you write:
    95. 

  95.       </p>
    96. 

  96. <pre class="programlisting"><span class="identifier">while_</span><span class="special">(</span><span class="identifier">cond</span><span class="special">)</span>
    97. 

  97. </pre>
    98. 

  98. <p>
    99. 

  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. 

  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. 

  101.         expression. The <code class="computeroutput"><span class="identifier">while_gen</span></code>
    102. 

  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. 

  103.       </p>
    104. 

  104. <pre class="programlisting"><span class="identifier">while_</span><span class="special">(</span><span class="identifier">cond</span><span class="special">)</span>
    105. 

  105. <span class="special">[</span>
    106. 

  106.     <span class="identifier">do_</span>
    107. 

  107. <span class="special">]</span>
    108. 

  108. </pre>
    109. 

  109. <p>
    110. 

  110.         it will generate a proper composite with the type:
    111. 

  111.       </p>
    112. 

  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. 

  113. </pre>
    114. 

  114. <p>
    115. 

  115.         where <code class="computeroutput"><span class="identifier">Cond</span></code> is the type of
    116. 

  116.         <code class="computeroutput"><span class="identifier">cond</span></code> and <code class="computeroutput"><span class="identifier">Do</span></code>
    117. 

  117.         is the type of <code class="computeroutput"><span class="identifier">do_</span></code>. Notice
    118. 

  118.         how we are using Phoenix's <a class="link" href="../inside/expression.html" title="Phoenix Expressions">Expression</a>
    119. 

  119.         mechanism here
    120. 

  120.       </p>
    121. 

  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. 

  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. 

  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. 

  124. <span class="special">{</span>
    125. 

  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. 

  126. <span class="special">}</span>
    127. 

  127. </pre>
    128. 

  128. <p>
    129. 

  129.         Finally, the <code class="computeroutput"><span class="identifier">while_eval</span></code> does
    130. 

  130.         its thing:
    131. 

  131.       </p>
    132. 

  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. 

  133. <span class="special">{</span>
    134. 

  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. 

  135. <span class="special">}</span>
    136. 

  136. </pre>
    137. 

  137. <p>
    138. 

  138.         <code class="computeroutput"><span class="identifier">cond</span></code> and <code class="computeroutput"><span class="identifier">do_</span></code>,
    139. 

  139.         at this point, are instances of <a class="link" href="../inside/actor.html" title="Actors in Detail">Actor</a>.
    140. 

  140.         <code class="computeroutput"><span class="identifier">cond</span></code> and <code class="computeroutput"><span class="identifier">do_</span></code>
    141. 

  141.         are the <a class="link" href="../inside/actor.html" title="Actors in Detail">Actors</a> passed as parameters
    142. 

  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. 

  143.       </p>
    144. 

  144. </div>
    145. 

  145. <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
    146. 

  146. <td align="left"></td>
    147. 

  147. <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<p>
    148. 

  148.         Distributed under the Boost Software License, Version 1.0. (See accompanying
    149. 

  149.         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>)
    150. 

  150.       </p>
    151. 

  151. </div></td>
    152. 

  152. </tr></table>
    153. 

  153. <hr>
    154. 

  154. <div class="spirit-nav">
    155. 

  155. <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>
    156. 

  156. </div>
    157. 

  157. </body>
    158. 

  158. </html>
    159.