ceph/src/boost/libs/functional/function_traits.html

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  33
  34   <h1>Function Object Traits</h1>
  35
  36   <p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
  37   provides two traits class templates for functions and function objects:</p>
  38
  39   <table border="1" summary="">
  40     <tr>
  41       <th>Type</th>
  42
  43       <th>Contents</th>
  44
  45       <th>Description</th>
  46     </tr>
  47
  48     <tr>
  49       <td valign="top" rowspan="4">
  50       <tt>template&nbsp;&lt;typename&nbsp;T&gt;<br>
  51       struct&nbsp;unary_traits</tt></td>
  52
  53       <td valign="top"><tt>function_type</tt></td>
  54
  55       <td valign="top">The type of the function or function object itself
  56       (i.e., <tt>T</tt>).</td>
  57     </tr>
  58
  59     <tr>
  60       <td valign="top"><tt>param_type</tt></td>
  61
  62       <td valign="top">The type that should be used to pass the function or
  63       function object as a parameter.</td>
  64     </tr>
  65
  66     <tr>
  67       <td valign="top"><tt>result_type</tt></td>
  68
  69       <td valign="top">The type returned by the function or function
  70       object.</td>
  71     </tr>
  72
  73     <tr>
  74       <td valign="top"><tt>argument_type</tt></td>
  75
  76       <td valign="top">The type of the argument to the function or function
  77       object.</td>
  78     </tr>
  79
  80     <tr>
  81       <td valign="top" rowspan="5">
  82       <tt>template&nbsp;&lt;typename&nbsp;T&gt;<br>
  83       struct&nbsp;binary_traits</tt></td>
  84
  85       <td valign="top"><tt>function_type</tt></td>
  86
  87       <td valign="top">The type of the function or function object itself
  88       (i.e., <tt>T</tt>).</td>
  89     </tr>
  90
  91     <tr>
  92       <td valign="top"><tt>param_type</tt></td>
  93
  94       <td valign="top">The type that should be used to pass the function or
  95       function object as a parameter.</td>
  96     </tr>
  97
  98     <tr>
  99       <td valign="top"><tt>result_type</tt></td>
 100
 101       <td valign="top">The type returned by the function or function
 102       object.</td>
 103     </tr>
 104
 105     <tr>
 106       <td valign="top"><tt>first_argument_type</tt></td>
 107
 108       <td valign="top">The type of the first argument to the function or
 109       function object.</td>
 110     </tr>
 111
 112     <tr>
 113       <td valign="top"><tt>second_argument_type</tt></td>
 114
 115       <td valign="top">The type of the second argument to the function or
 116       function object.</td>
 117     </tr>
 118   </table>
 119
 120   <h3>Usage</h3>
 121
 122   <p><tt>unary_traits</tt> should be instantiated with either a function
 123   taking a single parameter, or an adaptable unary function object (i.e., a
 124   class derived from <tt>std::unary_function</tt> or one which provides the
 125   same typedefs). (See &sect;20.3.1 in the C++ Standard.)</p>
 126
 127   <p><tt>binary_traits</tt> should be instantiated with either a function
 128   taking two parameters, or an adaptable binary function object (i.e., a
 129   class derived from <tt>std::binary_function</tt> or one which provides the
 130   same typedefs). (See &sect;20.3.1 in the C++ Standard.)</p>
 131
 132   <p>The most common usage of these templates is in function object adapters,
 133   thus allowing them to adapt plain functions as well as function objects.
 134   You can do this by wherever you would normally write, for example,</p>
 135
 136   <blockquote>
 137     <pre>
 138 typename Operation::argument_type
 139 </pre>
 140   </blockquote>
 141
 142   <p>simply writing</p>
 143
 144   <blockquote>
 145     <pre>
 146 typename boost::unary_traits&lt;Operation&gt;::argument_type
 147 </pre>
 148   </blockquote>
 149
 150   <p>instead.</p>
 151
 152   <h3>Additional Types Defined</h3>
 153
 154   <p>In addition to the standard result and argument typedefs, these traits
 155   templates define two additional types.</p>
 156
 157   <h4><tt>function_type</tt></h4>
 158
 159   <p>This is the type of the function or function object, and can be used in
 160   declarations such as</p>
 161
 162   <blockquote>
 163     <pre>
 164 template &lt;class Predicate&gt;
 165 class unary_negate : // ...
 166 {
 167   // ...
 168   private:
 169     <strong>typename unary_traits&lt;Predicate&gt;::function_type</strong> pred;
 170 };
 171 </pre>
 172   </blockquote>
 173
 174   <p>If this typedef were not provided, it would not be possible to declare
 175   <tt>pred</tt> in a way that would allow <tt>unary_negate</tt> to be
 176   instantiated with a function type (see the C++ Standard &sect;14.3.1
 177   &para;3).</p>
 178
 179   <h4><tt>param_type</tt></h4>
 180
 181   <p>This is a type suitable for passing the function or function object as a
 182   parameter to another function. For example,</p>
 183
 184   <blockquote>
 185     <pre>
 186 template &lt;class Predicate&gt;
 187 class unary_negate : // ...
 188 {
 189   public:
 190     explicit unary_negate(<strong>typename unary_traits&lt;Predicate&gt;::param_type</strong> x)
 191         :
 192         pred(x)
 193     {}
 194     // ...
 195 };
 196 </pre>
 197   </blockquote>
 198
 199   <p>Function objects are passed by reference to const; function pointers are
 200   passed by value.</p>
 201
 202   <h3>Limitations</h3>
 203
 204   <p>This library uses these traits within all function object adapters,
 205   theoretically rendering <tt>ptr_fun</tt> obsolete. However, third party
 206   adapters probably won't take advantage of this mechanism, and so
 207   <tt>ptr_fun</tt> may still be required. Accordingly, this library also
 208   provides <a href="ptr_fun.html">improved versions of the standard function
 209   pointer adapters</a>.</p>
 210
 211   <p>These traits templates will also not work with compilers that fail to
 212   support partial specialisation of templates. With these compilers, the
 213   traits templates can only be instantiated with adaptable function objects,
 214   thus requiring <tt>ptr_fun</tt> to be used, even with the function object
 215   adapters in this library.</p>
 216   <hr>
 217
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 221
 222   <p>Revised
 223   <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02
 224   December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p>
 225
 226   <p><i>Copyright &copy; 2000 Cadenza New Zealand Ltd.</i></p>
 227
 228   <p><i>Distributed under the Boost Software License, Version 1.0. (See
 229   accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
 230   copy at <a href=
 231   "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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