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  25. <div class="section">
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  26. <div class="titlepage"><div><div><h4 class="title">
    27. 

  27. <a name="phoenix.inside.expression.boost_phoenix_define_expression"></a><a class="link" href="boost_phoenix_define_expression.html" title="BOOST_PHOENIX_DEFINE_EXPRESSION">BOOST_PHOENIX_DEFINE_EXPRESSION</a>
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  28. </h4></div></div></div>
    29. 

  29. <a name="phoenix.inside.expression.boost_phoenix_define_expression.description"></a><h6>
    30. 

  30. <a name="id623241"></a>
    31. 

  31.           <a class="link" href="boost_phoenix_define_expression.html#phoenix.inside.expression.boost_phoenix_define_expression.description">Description</a>
    32. 

  32.         </h6>
    33. 

  33. <p>
    34. 

  34.           <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_DEFINE_EXPRESSION</span></code>
    35. 

  35.           is a macro that can be used to generate all the necessary boilerplate to
    36. 

  36.           create Phoenix Expressions
    37. 

  37.         </p>
    38. 

  38. <a name="phoenix.inside.expression.boost_phoenix_define_expression.synopsis"></a><h6>
    39. 

  39. <a name="id623276"></a>
    40. 

  40.           <a class="link" href="boost_phoenix_define_expression.html#phoenix.inside.expression.boost_phoenix_define_expression.synopsis">Synopsis</a>
    41. 

  41.         </h6>
    42. 

  42. <pre class="programlisting"><span class="identifier">BOOST_PHOENIX_DEFINE_EXPRESSION</span><span class="special">(</span>
    43. 

  43.     <span class="special">(</span><span class="identifier">namespace_seq</span><span class="special">)(</span><span class="identifier">name</span><span class="special">)</span>
    44. 

  44.   <span class="special">,</span> <span class="special">(</span><span class="identifier">child_grammar0</span><span class="special">)</span>
    45. 

  45.     <span class="special">(</span><span class="identifier">child_grammar1</span><span class="special">)</span>
    46. 

  46.     <span class="special">...</span>
    47. 

  47. <span class="special">)</span>
    48. 

  48. </pre>
    49. 

  49. <a name="phoenix.inside.expression.boost_phoenix_define_expression.semantics"></a><h6>
    50. 

  50. <a name="id623381"></a>
    51. 

  51.           <a class="link" href="boost_phoenix_define_expression.html#phoenix.inside.expression.boost_phoenix_define_expression.semantics">Semantics</a>
    52. 

  52.         </h6>
    53. 

  53. <p>
    54. 

  54.           The above macro generates the necessary code for an expression <code class="computeroutput"><span class="identifier">name</span></code> in namespace <code class="computeroutput"><span class="identifier">namespace_seq</span></code>.
    55. 

  55.           The sequence of <code class="computeroutput"><span class="special">(</span><span class="identifier">child_grammarN</span><span class="special">)</span></code> declares how many children the expression
    56. 

  56.           will have and what <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">grammar</span></code>
    57. 

  57.           they match.
    58. 

  58.         </p>
    59. 

  59. <p>
    60. 

  60.           The macro should be used at global scope. <code class="computeroutput"><span class="identifier">namespace_seq</span></code>
    61. 

  61.           shall be the sequence of namespaces under which the following symbols will
    62. 

  62.           be defined:
    63. 

  63.         </p>
    64. 

  64. <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">tag</span>
    65. 

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

  66.     <span class="keyword">struct</span> <span class="identifier">name</span><span class="special">;</span>
    67. 

  67. <span class="special">}</span>
    68. 

  68. 

  69. <span class="keyword">namespace</span> <span class="identifier">expression</span>
    70. 

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

  71.     <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A1</span> <span class="special">...</span> <span class="keyword">typename</span> <span class="identifier">AN</span><span class="special">&gt;</span>
    72. 

  72.     <span class="keyword">struct</span> <span class="identifier">name</span>
    73. 

  73.         <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">expr</span><span class="special">&lt;</span>
    74. 

  74.             <span class="identifier">tag</span><span class="special">::</span><span class="identifier">name</span>
    75. 

  75.           <span class="special">,</span> <span class="identifier">A0</span>
    76. 

  76.           <span class="special">,</span> <span class="identifier">A1</span>
    77. 

  77.             <span class="special">...</span>
    78. 

  78.           <span class="special">,</span> <span class="identifier">AN</span>
    79. 

  79.         <span class="special">&gt;</span>
    80. 

  80. <span class="special">}</span>
    81. 

  81. 

  82. <span class="keyword">namespace</span> <span class="identifier">rule</span>
    83. 

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

  84.     <span class="keyword">struct</span> <span class="identifier">name</span>
    85. 

  85.         <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">expr</span><span class="special">&lt;</span>
    86. 

  86.             <span class="identifier">child_grammar0</span>
    87. 

  87.           <span class="special">,</span> <span class="identifier">child_grammar1</span>
    88. 

  88.             <span class="special">...</span>
    89. 

  89.           <span class="special">,</span> <span class="identifier">child_grammarN</span>
    90. 

  90.         <span class="special">&gt;</span>
    91. 

  91.     <span class="special">{};</span>
    92. 

  92. <span class="special">}</span>
    93. 

  93. </pre>
    94. 

  94. <p>
    95. 

  95.           This macros also adds a specialization for <code class="computeroutput"><span class="identifier">meta_grammar</span><span class="special">::</span><span class="identifier">case_</span><span class="special">&lt;</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">name</span><span class="special">&gt;</span></code>.
    96. 

  96.         </p>
    97. 

  97. <a name="phoenix.inside.expression.boost_phoenix_define_expression.header"></a><h6>
    98. 

  98. <a name="id623869"></a>
    99. 

  99.           <a class="link" href="boost_phoenix_define_expression.html#phoenix.inside.expression.boost_phoenix_define_expression.header">Header</a>
    100. 

  100.         </h6>
    101. 

  101. <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">phoenix</span><span class="special">/</span><span class="identifier">core</span><span class="special">/</span><span class="identifier">expression</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
    102. 

  102. </pre>
    103. 

  103. <a name="phoenix.inside.expression.boost_phoenix_define_expression.example"></a><h6>
    104. 

  104. <a name="id623951"></a>
    105. 

  105.           <a class="link" href="boost_phoenix_define_expression.html#phoenix.inside.expression.boost_phoenix_define_expression.example">Example</a>
    106. 

  106.         </h6>
    107. 

  107. <p>
    108. 

  108.           The example from the previous section can be rewritten as:
    109. 

  109.         </p>
    110. 

  110. <pre class="programlisting"><span class="identifier">BOOST_PHOENIX_DEFINE_EXPRESSION</span><span class="special">(</span>
    111. 

  111.     <span class="special">(</span><span class="identifier">plus</span><span class="special">)</span>
    112. 

  112.   <span class="special">,</span> <span class="special">(</span><span class="identifier">meta_grammar</span><span class="special">)</span>               <span class="comment">// Lhs
    113. 

  113. </span>    <span class="special">(</span><span class="identifier">meta_grammar</span><span class="special">)</span>               <span class="comment">// Rhs
    114. 

  114. </span><span class="special">)</span>
    115. 

  115. 

  116. <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Lhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">&gt;</span>
    117. 

  117. <span class="keyword">typename</span> <span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Lhs</span><span class="special">,</span> <span class="identifier">Rhs</span><span class="special">&gt;::</span><span class="identifier">type</span>
    118. 

  118. <span class="identifier">plus</span><span class="special">(</span><span class="identifier">Lhs</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">Rhs</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">rhs</span><span class="special">)</span>
    119. 

  119. <span class="special">{</span>
    120. 

  120.     <span class="keyword">return</span> <span class="identifier">expression</span><span class="special">::</span><span class="identifier">plus</span><span class="special">&lt;</span><span class="identifier">Lhs</span><span class="special">,</span> <span class="identifier">Rhs</span><span class="special">&gt;::</span><span class="identifier">make</span><span class="special">(</span><span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">rhs</span><span class="special">);</span>
    121. 

  121. <span class="special">}</span>
    122. 

  122. </pre>
    123. 

  123. </div>
    124. 

  124. <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
    125. 

  125. <td align="left"></td>
    126. 

  126. <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<p>
    127. 

  127.         Distributed under the Boost Software License, Version 1.0. (See accompanying
    128. 

  128.         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>)
    129. 

  129.       </p>
    130. 

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    131. 

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