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10 <title>Boost Function Object Adapter Library</title>
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35
36 <h1>Improved Function Object Adapters</h1>
37
38 <p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
39 provides enhancements to the function object adapters specified in the C++
40 Standard Library (sections 20.3.5, through to 20.3.8). The enhancements are
41 principally possible due to two changes:</p>
42
43 <ol>
44 <li>We use the Boost <tt><a href=
45 "../utility/call_traits.htm">call_traits</a></tt> templates to avoid the
46 problem of <a href="binders.html#refref">references to references</a>,
47 and to improve the efficiency of <a href="mem_fun.html#args">parameter
48 passing</a>.</li>
49
50 <li>We use two <a href="function_traits.html">function object traits</a>
51 class templates to avoid the need for <tt><a href=
52 "ptr_fun.html">ptr_fun</a></tt> with the adapters in this library.</li>
53 </ol>
54
55 <h3>Contents</h3>
56
57 <p>The header contains the following function and class templates:</p>
58
59 <table border="1" cellpadding="5" summary="">
60 <tr>
61 <th align="left"><a href="function_traits.html">Function object
62 traits</a></th>
63
64 <td valign="top"><tt>unary_traits<br>
65 binary_traits</tt></td>
66
67 <td valign="top">Used to determine the types of function objects' and
68 functions' arguments. Eliminate the necessity for
69 <tt>ptr_fun</tt>.</td>
70 </tr>
71
72 <tr>
73 <th align="left"><a href="negators.html">Negators</a></th>
74
75 <td valign="top"><tt>unary_negate<br>
76 binary_negate<br>
77 not1<br>
78 not2</tt></td>
79
80 <td valign="top">Based on section 20.3.5 of the standard.</td>
81 </tr>
82
83 <tr>
84 <th align="left"><a href="binders.html">Binders</a></th>
85
86 <td valign="top"><tt>binder1st<br>
87 binder2nd<br>
88 bind1st<br>
89 bind2nd</tt></td>
90
91 <td valign="top">Based on section 20.3.6 of the standard.</td>
92 </tr>
93
94 <tr>
95 <th align="left"><a href="ptr_fun.html">Adapters for pointers to
96 functions</a></th>
97
98 <td valign="top"><tt>pointer_to_unary_function<br>
99 pointer_to_binary_function<br>
100 ptr_fun</tt></td>
101
102 <td valign="top">Based on section 20.3.7 of the standard. Not required
103 for use with this library since the binders and negators can adapt
104 functions, but may be needed with third party adapters.</td>
105 </tr>
106
107 <tr>
108 <th align="left"><a href="mem_fun.html">Adapters for pointers to member
109 functions</a></th>
110
111 <td valign="top"><tt>mem_fun_t<br>
112 mem_fun1_t<br>
113 const_mem_fun_t<br>
114 const_mem_fun1_t<br>
115 mem_fun_ref_t<br>
116 mem_fun1_ref_t<br>
117 const_mem_fun_ref_t<br>
118 const_mem_fun1_ref_t<br>
119 mem_fun<br>
120 mem_fun_ref</tt></td>
121
122 <td valign="top">Based on section 20.3.8 of the standard.</td>
123 </tr>
124 </table>
125
126 <h3>Usage</h3>
127
128 <p>Using these adapters should be pretty much the same as using the
129 standard function object adapters; the only differences are that you need
130 to write <tt>boost::</tt> instead of <tt>std::</tt>, and that you will get
131 fewer headaches.</p>
132
133 <p>For example, suppose you had a <tt>Person</tt> class that contained a
134 <tt>set_name</tt> function:</p>
135
136 <blockquote>
137 <pre>
138class Person
139{
140 public:
141 void set_name(const std::string &amp;name);
142 // ...
143};
144</pre>
145 </blockquote>
146
147 <p>You could rename a bunch of people in a collection, <tt>c</tt>, by
148 writing</p>
149
150 <blockquote>
151 <pre>
152std::for_each(c.begin(), c.end(),
153 boost::bind2nd(boost::mem_fun_ref(&amp;Person::set_name), "Fred"));
154</pre>
155 </blockquote>
156
157 <p>If the standard adapters had been used instead then this code would
158 normally fail to compile, because <tt>set_name</tt> takes a reference
159 argument. Refer to the comments in the <a href="binders.html#refref">binder
160 documentation</a> to explain why this is so.</p>
161
162 <h3>Compiler Compatibility</h3>
163
164 <p>The header and <a href="test/function_test.cpp">test program</a> have been
165 compiled with the following compilers:</p>
166
167 <table border="1" cellpadding="5" summary="">
168 <tr>
169 <th>Compiler</th>
170
171 <th>Comments</th>
172 </tr>
173
174 <tr>
175 <td valign="top">Borland C++Builder 4 Update 2</td>
176
177 <td valign="top">No known issues.</td>
178 </tr>
179
180 <tr>
181 <td valign="top">Borland C++ 5.5</td>
182
183 <td valign="top">No known issues.</td>
184 </tr>
185
186 <tr>
187 <td valign="top">g++ 2.95.2</td>
188
189 <td valign="top">No known issues.</td>
190 </tr>
191
192 <tr>
193 <td valign="top">Microsoft Visual C++ Service Pack 3</td>
194
195 <td valign="top">
196 Compiler lacks partial specialisation, so this library offers little
197 more than is provided by the standard adapters:
198
199 <ul>
200 <li>The <tt>call_traits</tt> mechanism is unable to prevent
201 references to references, and so the adapters in this library will
202 be usable in fewer situations.</li>
203
204 <li>The <tt>function_traits</tt> mechanism is unable to determine
205 the argument and result types of functions, therefore
206 <tt>ptr_fun</tt> continues to be required to adapt functions.</li>
207 </ul>
208 </td>
209 </tr>
210 </table>
211
212 <h3>Future Directions</h3>
213
214 <p>This library's primary focus is to solve the problem of references to
215 references while maintaining as much compatibility as possible with the
216 standard library. This allows you to use the techniques you read about in
217 books and magazines with many of today's compilers.</p>
218
219 <p>In the longer term, even better solutions are likely:</p>
220
221 <ol>
222 <li>Several Boost members are working on expression template libraries.
223 These will allow a more natural syntax for combining and adapting
224 functions. As this is a new technology, it may be some time before it has
225 matured and is widely supported by major compilers but shows great
226 promise. In the meantime, the functional.hpp library fills the gap.</li>
227
228 <li>The Standard Committee has recognised the problem of references to
229 references occurring during template instantiation and has moved to fix
230 the standard (see the <a href=
231 "http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#106">C++
232 standard core language active issues list</a>).</li>
233 </ol>
234
235 <h3>Author</h3>
236
237 <p><a href="http://www.boost.org/people/mark_rodgers.htm">Mark Rodgers</a></p>
238
239 <h3>Acknowledgements</h3>
240
241 <p>Thanks to <a href="http://www.boost.org/people/john_maddock.htm">John Maddock</a> for
242 suggesting the mechanism that allowed the function objects traits to work
243 correctly. <a href="http://www.boost.org/people/jens_maurer.htm">Jens Maurer</a> provided
244 invaluable feedback during the <a href=
245 "http://www.boost.org/more/formal_review_process.htm">formal review process</a>.</p>
246 <hr>
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252 <p>Revised
253 <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02
254 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p>
255
256 <p><i>Copyright &copy; 2000 Cadenza New Zealand Ltd.</i></p>
257
258 <p><i>Distributed under the Boost Software License, Version 1.0. (See
259 accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
260 copy at <a href=
261 "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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