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lambda_expressions.html

lambda_expressions.html 13 KB
Riwayat Mentahan

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  6. <title>Boost.Flyweight Documentation - Tutorial - Annex - MPL lambda expressions</title>
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  15. <h1><img src="../../../../boost.png" alt="Boost logo" align=
    16. 

  16. "middle" width="277" height="86">Boost.Flyweight Tutorial Annex: MPL lambda expressions</h1>
    17. 

  17. 

  18. <div class="prev_link"><a href="technical.html"><img src="../prev.gif" alt="technical issues" border="0"><br>
    19. 

  19. Technical issues
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    21. 

  21. <div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight tutorial" border="0"><br>
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  22. Boost.Flyweight tutorial
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  24. <div class="next_link"><a href="../reference/index.html"><img src="../next.gif" alt="Boost.Flyweight reference" border="0"><br>
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  25. Boost.Flyweight reference
    26. 

  26. </a></div><br clear="all" style="clear: all;">
    27. 

  27. 

  28. <hr>
    29. 

  29. 

  30. <p>This short introduction to lambda expressions is meant for readers unfamiliar
    31. 

  31. with the <a href="../../../mpl/doc/index.html">Boost MPL Library</a> who
    32. 

  32. want to rapidly acquire a working knowledge of the basic concepts for the purposes
    33. 

  33. of using them in Boost.Flyweight. Please refer to the Boost.MPL documentation
    34. 

  34. for further information beyond these introductory notes.
    35. 

  35. </p>
    36. 

  36. 

  37. <p>
    38. 

  38. The specifiers defined by Boost.Flyweight rely heavily on the
    39. 

  39. <a href="../../../mpl/doc/refmanual/lambda-expression.html"><code>Lambda
    40. 

  40. Expression</code></a> concept defined by the 
    41. 

  41. <a href="../../../mpl/doc/index.html">Boost MPL Library</a>. A lambda
    42. 

  42. expression can be thought of as a compile-time "type function", an entity (a
    43. 

  43. concrete type, actually) that can be invoked with a list of types and returns
    44. 

  44. some associated type in its turn. Consider for instance an arbitrary class
    45. 

  45. template:
    46. 

  46. </p>
    47. 

  47. 

  48. <blockquote><pre>
    49. 

  49. <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>T</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Q</span><span class=special>&gt;</span>
    50. 

  50. <span class=keyword>class</span> <span class=identifier>foo</span>
    51. 

  51. <span class=special>{</span>
    52. 

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

  53. <span class=special>};</span>
    54. 

  54. </pre></blockquote>
    55. 

  55. 

  56. <p>
    57. 

  57. and suppose we want to have a lambda expression that, when invoked
    58. 

  58. with some generic types <code>Arg1</code> and <code>Arg2</code>,
    59. 

  59. returns <code>foo&lt;Arg1,Arg2&gt;</code>. Such a lambda expression
    60. 

  60. can be implemented in two ways
    61. 

  61. <ol>
    62. 

  62.   <li>As a
    63. 

  63.     <a href="../../../mpl/doc/refmanual/metafunction-class.html"><code>MPL
    64. 

  64.     Metafunction Class</code></a>, a type with a special nested class template
    65. 

  65.     named <code>apply</code>:
    66. 

  66. <blockquote><pre>
    67. 

  67. <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
    68. 

  68. <span class=special>{</span>
    69. 

  69.   <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>&gt;</span>
    70. 

  70.   <span class=keyword>struct</span> <span class=identifier>apply</span>
    71. 

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

  72.     <span class=comment>// this is the &quot;return type&quot; of foo_specifier</span>
    73. 

  73.     <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>Arg1</span><span class=special>,</span><span class=identifier>Arg2</span><span class=special>&gt;</span> <span class=identifier>type</span><span class=special>;</span>
    74. 

  74.   <span class=special>};</span>
    75. 

  75. <span class=special>};</span>
    76. 

  76. </pre></blockquote>
    77. 

  77.   </li>
    78. 

  78.   <li>
    79. 

  79.     As a
    80. 

  80.     <a href="../../../mpl/doc/refmanual/placeholder-expression.html"><code>MPL
    81. 

  81.     Placeholder Expression</code></a>, a class template instantiated with one or
    82. 

  82.     more <i>placeholders</i>:
    83. 

  83. <blockquote><pre>
    84. 

  84. <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>&gt;</span> <span class=identifier>foo_specifier</span><span class=special>;</span>
    85. 

  85. </pre></blockquote>
    86. 

  86.     Note that, in this case, <code>foo_specifier</code> is a concrete type, much
    87. 

  87.     as <code>int</code> or <code>std::set&lt;std::string&gt;</code> are; yet,
    88. 

  88.     MPL internal mechanisms are able to detect that this type has been gotten
    89. 

  89.     from instantiating a class template with placeholders <code>boost::mpl::_1</code>
    90. 

  90.     and <code>boost::mpl::_2</code> and take these placeholders as slots to
    91. 

  91.     be substituted for actual types (the first and second type supplied,
    92. 

  92.     respectively) when <code>foo_specifier</code> is
    93. 

  93.     invoked. So, an instantiation of <code>foo</code> can be used
    94. 

  94.     to refer back to the <code>foo</code> class template itself! The net
    95. 

  95.     effect is the same as with metafunctions, but placeholder expressions spare
    96. 

  96.     us the need to write boilerplate metafunction classes
    97. 

  97.     --and the kind of metaprogramming magic they depend on has an undeniable
    98. 

  98.     beauty to it.
    99. 

  99.   </li>
    100. 

  100. </ol>
    101. 

  101. So far the examples shown just forward the arguments <code>Arg1</code> and
    102. 

  102. <code>Arg2</code> directly to a class template without further elaboration,
    103. 

  103. but there is nothing preventing us from doing some argument manipulation,
    104. 

  104. like, for instance, switching their places:
    105. 

  105. </p>
    106. 

  106. 

  107. <blockquote><pre>
    108. 

  108. <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
    109. 

  109. <span class=special>{</span>
    110. 

  110.   <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>&gt;</span>
    111. 

  111.   <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>Arg2</span><span class=special>,</span><span class=identifier>Arg1</span><span class=special>&gt;</span> <span class=identifier>type</span><span class=special>;};</span>
    112. 

  112. <span class=special>};</span>
    113. 

  113. 

  114. <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>,</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>&gt;</span> <span class=identifier>foo_specifier</span><span class=special>;</span>
    115. 

  115. </pre></blockquote>
    116. 

  116. 

  117. <p>
    118. 

  118. passing placeholder subexpressions as arguments to the overall expression:
    119. 

  119. </p>
    120. 

  120. 

  121. <blockquote><pre>
    122. 

  122. <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
    123. 

  123. <span class=special>{</span>
    124. 

  124.   <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>&gt;</span>
    125. 

  125.   <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>shared_ptr</span><span class=special>&lt;</span><span class=identifier>Arg1</span><span class=special>&gt;,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special>&lt;</span><span class=identifier>Arg2</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=identifier>type</span><span class=special>;};</span>
    126. 

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

  127. 

  128. <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span>
    129. 

  129.   <span class=identifier>boost</span><span class=special>::</span><span class=identifier>shared_ptr</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>&gt;,</span>
    130. 

  130.   <span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>&gt;</span>
    131. 

  131. <span class=special>&gt;</span> <span class=identifier>foo_specifier</span><span class=special>;</span>
    132. 

  132. </pre></blockquote>
    133. 

  133. 

  134. <p>
    135. 

  135. or accepting less or more arguments than the class template itself
    136. 

  136. (the number of parameters of a lambda expression is called its <i>arity</i>):
    137. 

  137. </p>
    138. 

  138. 

  139. <blockquote><pre>
    140. 

  140. <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
    141. 

  141. <span class=special>{</span>
    142. 

  142.   <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>&gt;</span>
    143. 

  143.   <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>Arg1</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special>&lt;</span><span class=identifier>Arg1</span><span class=special>&gt;</span> <span class=identifier>type</span><span class=special>;};</span>
    144. 

  144. <span class=special>};</span>
    145. 

  145. 

  146. <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=identifier>foo_specifier</span><span class=special>;</span>
    147. 

  147. 

  148. <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
    149. 

  149. <span class=special>{</span>
    150. 

  150.   <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg3</span><span class=special>&gt;</span>
    151. 

  151.   <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>Arg1</span><span class=special>,</span><span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>Arg2</span><span class=special>,</span><span class=identifier>Arg3</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=identifier>type</span><span class=special>;};</span>
    152. 

  152. <span class=special>};</span>
    153. 

  153. 

  154. <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>,</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_3</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=identifier>foo_specifier</span><span class=special>;</span>
    155. 

  155. </pre></blockquote>
    156. 

  156. 

  157. <hr>
    158. 

  158. 

  159. <div class="prev_link"><a href="technical.html"><img src="../prev.gif" alt="technical issues" border="0"><br>
    160. 

  160. Technical issues
    161. 

  161. </a></div>
    162. 

  162. <div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight tutorial" border="0"><br>
    163. 

  163. Boost.Flyweight tutorial
    164. 

  164. </a></div>
    165. 

  165. <div class="next_link"><a href="../reference/index.html"><img src="../next.gif" alt="Boost.Flyweight reference" border="0"><br>
    166. 

  166. Boost.Flyweight reference
    167. 

  167. </a></div><br clear="all" style="clear: all;">
    168. 

  168. 

  169. <br>
    170. 

  170. 

  171. <p>Revised August 13th 2008</p>
    172. 

  172. 

  173. <p>&copy; Copyright 2006-2008 Joaqu&iacute;n M L&oacute;pez Mu&ntilde;oz.
    174. 

  174. Distributed under the Boost Software 
    175. 

  175. License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt">
    176. 

  176. LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
    177. 

  177. http://www.boost.org/LICENSE_1_0.txt</a>)
    178. 

  178. </p>
    179. 

  179. 

  180. </body>
    181. 

  181. </html>
    182.