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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.modules.core.arguments"></a><a class="link" href="arguments.html" title="Arguments">Arguments</a>
    28. 

  28. </h4></div></div></div>
    29. 

  29. <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">argument</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
    30. 

  30. </pre>
    31. 

  31. <p>
    32. 

  32.           We use an instance of:
    33. 

  33.         </p>
    34. 

  34. <pre class="programlisting"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span>
    35. 

  35. </pre>
    36. 

  36. <p>
    37. 

  37.           to represent the Nth function argument. The argument placeholder acts as
    38. 

  38.           an imaginary data-bin where a function argument will be placed.
    39. 

  39.         </p>
    40. 

  40. <a name="phoenix.modules.core.arguments.predefined_arguments"></a><h6>
    41. 

  41. <a name="id808492"></a>
    42. 

  42.           <a class="link" href="arguments.html#phoenix.modules.core.arguments.predefined_arguments">Predefined
    43. 

  43.           Arguments</a>
    44. 

  44.         </h6>
    45. 

  45. <p>
    46. 

  46.           There are a few predefined instances of <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
    47. 

  47.           named <code class="computeroutput"><span class="identifier">arg1</span></code>..<code class="computeroutput"><span class="identifier">argN</span></code>, and its <a href="http://www.boost.org/libs/lambda/doc/index.html" target="_top">BLL</a>
    48. 

  48.           counterpart <code class="computeroutput"><span class="identifier">_1</span></code>..<code class="computeroutput"><span class="identifier">_N</span></code>. (where N is a predefined maximum).
    49. 

  49.         </p>
    50. 

  50. <p>
    51. 

  51.           Here are some sample preset definitions of <code class="computeroutput"><span class="identifier">arg1</span></code>..<code class="computeroutput"><span class="identifier">argN</span></code>
    52. 

  52.         </p>
    53. 

  53. <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">placeholders</span>
    54. 

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

  55.     <span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">arg1</span> <span class="special">=</span> <span class="special">{};</span>
    56. 

  56.     <span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special">&lt;</span><span class="number">2</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">arg2</span> <span class="special">=</span> <span class="special">{};</span>
    57. 

  57.     <span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special">&lt;</span><span class="number">3</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">arg3</span> <span class="special">=</span> <span class="special">{};</span>
    58. 

  58. <span class="special">}</span>
    59. 

  59. </pre>
    60. 

  60. <p>
    61. 

  61.           and its <a href="http://www.boost.org/libs/lambda/doc/index.html" target="_top">BLL</a>
    62. 

  62.           <code class="computeroutput"><span class="identifier">_1</span></code>..<code class="computeroutput"><span class="identifier">_N</span></code>
    63. 

  63.           style counterparts:
    64. 

  64.         </p>
    65. 

  65. <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">placeholders</span>
    66. 

  66. <span class="special">{</span>
    67. 

  67.     <span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_1</span> <span class="special">=</span> <span class="special">{};</span>
    68. 

  68.     <span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special">&lt;</span><span class="number">2</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_2</span> <span class="special">=</span> <span class="special">{};</span>
    69. 

  69.     <span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special">&lt;</span><span class="number">3</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_3</span> <span class="special">=</span> <span class="special">{};</span>
    70. 

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

  71. </pre>
    72. 

  72. <div class="note"><table border="0" summary="Note">
    73. 

  73. <tr>
    74. 

  74. <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../../doc/src/images/note.png"></td>
    75. 

  75. <th align="left">Note</th>
    76. 

  76. </tr>
    77. 

  77. <tr><td align="left" valign="top"><p>
    78. 

  78.             You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_ARG_LIMIT</span></code>,
    79. 

  79.             the predefined maximum placeholder index. By default, <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_ARG_LIMIT</span></code>
    80. 

  80.             is set to <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>
    81. 

  81.             (See <a class="link" href="../../actor.html" title="Actor">Actor</a>).
    82. 

  82.           </p></td></tr>
    83. 

  83. </table></div>
    84. 

  84. <a name="phoenix.modules.core.arguments.user_defined_arguments"></a><h6>
    85. 

  85. <a name="id809017"></a>
    86. 

  86.           <a class="link" href="arguments.html#phoenix.modules.core.arguments.user_defined_arguments">User
    87. 

  87.           Defined Arguments</a>
    88. 

  88.         </h6>
    89. 

  89. <p>
    90. 

  90.           When appropriate, you can define your own <code class="computeroutput"><span class="identifier">argument</span></code>
    91. 

  91.           names. For example:
    92. 

  92.         </p>
    93. 

  93. <pre class="programlisting"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">x</span><span class="special">;</span> <span class="comment">// note one based index
    94. 

  94. </span></pre>
    95. 

  95. <p>
    96. 

  96.           <code class="computeroutput"><span class="identifier">x</span></code> may now be used as a
    97. 

  97.           parameter to a lazy function:
    98. 

  98.         </p>
    99. 

  99. <pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="number">6</span><span class="special">)</span>
    100. 

  100. </pre>
    101. 

  101. <p>
    102. 

  102.           which is equivalent to:
    103. 

  103.         </p>
    104. 

  104. <pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="number">6</span><span class="special">)</span>
    105. 

  105. </pre>
    106. 

  106. <a name="phoenix.modules.core.arguments.evaluating_an_argument"></a><h6>
    107. 

  107. <a name="id809168"></a>
    108. 

  108.           <a class="link" href="arguments.html#phoenix.modules.core.arguments.evaluating_an_argument">Evaluating
    109. 

  109.           an Argument</a>
    110. 

  110.         </h6>
    111. 

  111. <p>
    112. 

  112.           An argument, when evaluated, selects the Nth argument from the those passed
    113. 

  113.           in by the client.
    114. 

  114.         </p>
    115. 

  115. <p>
    116. 

  116.           For example:
    117. 

  117.         </p>
    118. 

  118. <pre class="programlisting"><span class="keyword">char</span>        <span class="identifier">c</span> <span class="special">=</span> <span class="char">'A'</span><span class="special">;</span>
    119. 

  119. <span class="keyword">int</span>         <span class="identifier">i</span> <span class="special">=</span> <span class="number">123</span><span class="special">;</span>
    120. 

  120. <span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">s</span> <span class="special">=</span> <span class="string">"Hello World"</span><span class="special">;</span>
    121. 

  121. 

  122. <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">arg1</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>        <span class="comment">//  Get the 1st argument: c
    123. 

  123. </span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">arg1</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">s</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>     <span class="comment">//  Get the 1st argument: i
    124. 

  124. </span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">arg2</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">s</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>     <span class="comment">//  Get the 2nd argument: s
    125. 

  125. </span></pre>
    126. 

  126. <p>
    127. 

  127.           will print out:
    128. 

  128.         </p>
    129. 

  129. <pre class="programlisting"><span class="identifier">A</span>
    130. 

  130. <span class="number">123</span>
    131. 

  131. <span class="identifier">Hello</span> <span class="identifier">World</span>
    132. 

  132. </pre>
    133. 

  133. <a name="phoenix.modules.core.arguments.extra_arguments"></a><h6>
    134. 

  134. <a name="id809466"></a>
    135. 

  135.           <a class="link" href="arguments.html#phoenix.modules.core.arguments.extra_arguments">Extra Arguments</a>
    136. 

  136.         </h6>
    137. 

  137. <p>
    138. 

  138.           In C and C++, a function can have extra arguments that are not at all used
    139. 

  139.           by the function body itself. These extra arguments are simply ignored.
    140. 

  140.         </p>
    141. 

  141. <p>
    142. 

  142.           Phoenix also allows extra arguments to be passed. For example, recall our
    143. 

  143.           original <code class="computeroutput"><span class="identifier">add</span></code> function:
    144. 

  144.         </p>
    145. 

  145. <pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">arg2</span><span class="special">)</span>
    146. 

  146. </pre>
    147. 

  147. <p>
    148. 

  148.           We know now that partially applying this function results to a function
    149. 

  149.           that expects 2 arguments. However, the library is a bit more lenient and
    150. 

  150.           allows the caller to supply more arguments than is actually required. Thus,
    151. 

  151.           <code class="computeroutput"><span class="identifier">add</span></code> actually allows 2
    152. 

  152.           <span class="emphasis"><em>or more</em></span> arguments. For instance, with:
    153. 

  153.         </p>
    154. 

  154. <pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">arg2</span><span class="special">)(</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">y</span><span class="special">,</span> <span class="identifier">z</span><span class="special">)</span>
    155. 

  155. </pre>
    156. 

  156. <p>
    157. 

  157.           the third argument <code class="computeroutput"><span class="identifier">z</span></code> is
    158. 

  158.           ignored. Taking this further, in-between arguments are also ignored. Example:
    159. 

  159.         </p>
    160. 

  160. <pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">arg5</span><span class="special">)(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">c</span><span class="special">,</span> <span class="identifier">d</span><span class="special">,</span> <span class="identifier">e</span><span class="special">)</span>
    161. 

  161. </pre>
    162. 

  162. <p>
    163. 

  163.           Here, arguments b, c, and d are ignored. The function <code class="computeroutput"><span class="identifier">add</span></code>
    164. 

  164.           takes in the first argument (<code class="computeroutput"><span class="identifier">arg1</span></code>)
    165. 

  165.           and the fifth argument (<code class="computeroutput"><span class="identifier">arg5</span></code>).
    166. 

  166.         </p>
    167. 

  167. <div class="note"><table border="0" summary="Note">
    168. 

  168. <tr>
    169. 

  169. <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../../doc/src/images/note.png"></td>
    170. 

  170. <th align="left">Note</th>
    171. 

  171. </tr>
    172. 

  172. <tr><td align="left" valign="top"><p>
    173. 

  173.             There are a few reasons why enforcing strict arity is not desirable.
    174. 

  174.             A case in point is the callback function. Typical callback functions
    175. 

  175.             provide more information than is actually needed. Lambda functions are
    176. 

  176.             often used as callbacks.
    177. 

  177.           </p></td></tr>
    178. 

  178. </table></div>
    179. 

  179. </div>
    180. 

  180. <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
    181. 

  181. <td align="left"></td>
    182. 

  182. <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<p>
    183. 

  183.         Distributed under the Boost Software License, Version 1.0. (See accompanying
    184. 

  184.         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>)
    185. 

  185.       </p>
    186. 

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    187. 

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    188. 

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