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8 | <title>Boost Function Object Adapter Library</title> | |
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33 | ||
34 | <h1>Member Function Adapters</h1> | |
35 | ||
36 | <p>The header <a href="../../boost/functional.hpp">functional.hpp</a> | |
37 | includes improved versions of the full range of member function adapters | |
38 | from the the C++ Standard Library (§20.3.8):</p> | |
39 | ||
40 | <ul> | |
41 | <li><tt>mem_fun_t</tt></li> | |
42 | ||
43 | <li><tt>mem_fun1_t</tt></li> | |
44 | ||
45 | <li><tt>const_mem_fun_t</tt></li> | |
46 | ||
47 | <li><tt>const_mem_fun1_t</tt></li> | |
48 | ||
49 | <li><tt>mem_fun_ref_t</tt></li> | |
50 | ||
51 | <li><tt>mem_fun1_ref_t</tt></li> | |
52 | ||
53 | <li><tt>const_mem_fun_ref_t</tt></li> | |
54 | ||
55 | <li><tt>const_mem_fun1_ref_t</tt></li> | |
56 | </ul> | |
57 | ||
58 | <p>as well as the corresponding overloaded helper functions</p> | |
59 | ||
60 | <ul> | |
61 | <li><tt>mem_fun</tt></li> | |
62 | ||
63 | <li><tt>mem_fun_ref</tt></li> | |
64 | </ul> | |
65 | ||
66 | <p>The following changes have been made to the adapters as specified in the | |
67 | Standard:</p> | |
68 | ||
69 | <ul> | |
70 | <li>The <tt>first_argument_type</tt> typedef has been corrected for the | |
71 | <tt>const_</tt> family of member function adapters (see <a href= | |
72 | "#firstarg">below</a>).</li> | |
73 | ||
74 | <li>The argument passed to <tt>mem_fun1_t</tt> and its variants is passed | |
75 | using the <tt>call_traits::param_type</tt> for the member function's | |
76 | argument type.</li> | |
77 | </ul> | |
78 | ||
79 | <h3 id="firstarg">first_argument_type</h3> | |
80 | ||
81 | <p>The standard specifies <tt>const_mem_fun1_t</tt>, for example, like | |
82 | this:</p> | |
83 | ||
84 | <blockquote> | |
85 | <pre> | |
86 | template <class S, class T, class A> class const_mem_fun1_t | |
87 | : public binary_function<<strong>T*</strong>, A, S> { | |
88 | public: | |
89 | explicit const_mem_fun1_t(S (T::*p)(A) const); | |
90 | S operator()(<strong>const T*</strong> p, A x) const; | |
91 | }; | |
92 | </pre> | |
93 | </blockquote> | |
94 | ||
95 | <p>Note that the first argument to <tt>binary_function</tt> is <tt>T*</tt> | |
96 | despite the fact that the first argument to <tt>operator()</tt> is actually | |
97 | of type <tt><em>const</em> T*</tt>.</p> | |
98 | ||
99 | <p>Does this matter? Well, consider what happens when we write</p> | |
100 | ||
101 | <blockquote> | |
102 | <pre> | |
103 | struct Foo { void bar(int) const; }; | |
104 | const Foo *cp = new Foo; | |
105 | std::bind1st(std::mem_fun(&Foo::bar), cp); | |
106 | </pre> | |
107 | </blockquote> | |
108 | ||
109 | <p>We have created a <tt>const_mem_fun1_t</tt> object which will | |
110 | effectively contain the following</p> | |
111 | ||
112 | <blockquote> | |
113 | <pre> | |
114 | typedef Foo* first_argument_type; | |
115 | </pre> | |
116 | </blockquote> | |
117 | ||
118 | <p>The <tt>bind1st</tt> will then create a <tt>binder1st</tt> object that | |
119 | will use this <tt>typedef</tt> as the type of a member which will be | |
120 | initialised with <tt>cp</tt>. In other words, we will need to initialise a | |
121 | <tt>Foo*</tt> member with a <tt>const Foo*</tt> pointer! Clearly this | |
122 | is not possible, so to implement this your Standard Library vendor will | |
123 | have had to cast away the constness of <tt>cp</tt>, probably within the | |
124 | body of <tt>bind1st</tt>.</p> | |
125 | ||
126 | <p>This hack will not suffice with the improved <a href= | |
127 | "binders.html">binders</a> in this library, so we have had to provide | |
128 | corrected versions of the member function adapters as well.</p> | |
129 | ||
130 | <h3 id="args">Argument Types</h3> | |
131 | ||
132 | <p>The standard defines <tt>mem_fun1_t</tt>, for example, like this | |
133 | (§20.3.8 ¶2):</p> | |
134 | ||
135 | <blockquote> | |
136 | <pre> | |
137 | template <class S, class T, class A> class mem_fun1_t | |
138 | : public binary_function<T*, A, S> { | |
139 | public: | |
140 | explicit mem_fun1_t(S (T::*p)(<strong>A</strong>)); | |
141 | S operator()(T* p, <strong>A</strong> x) const; | |
142 | }; | |
143 | </pre> | |
144 | </blockquote> | |
145 | ||
146 | <p>Note that the second argument to <tt>operator()</tt> is exactly the same | |
147 | type as the argument to the member function. If this is a value type, the | |
148 | argument will be passed by value and copied twice.</p> | |
149 | ||
150 | <p>However, if we were to try and eliminate this inefficiency by instead | |
151 | declaring the argument as <tt>const A&</tt>, then if A were a | |
152 | reference type, we would have a reference to a reference, which is | |
153 | currently illegal (but see <a href= | |
154 | "http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ core | |
155 | language issue number 106)</a></p> | |
156 | ||
157 | <p>So the way in which we want to declare the second argument for | |
158 | <tt>operator()</tt> depends on whether or not the member function's | |
159 | argument is a reference. If it is a reference, we want to declare it simply | |
160 | as <tt>A</tt>; if it is a value we want to declare it as | |
161 | <tt>const A&</tt>.</p> | |
162 | ||
163 | <p>The Boost <a href="../utility/call_traits.htm">call_traits</a> class | |
164 | template contains a <tt>param_type</tt> typedef, which uses partial | |
165 | specialisation to make precisely this decision. By declaring the | |
166 | <tt>operator()</tt> as</p> | |
167 | ||
168 | <blockquote> | |
169 | <pre> | |
170 | S operator()(T* p, typename call_traits<A>::param_type x) const | |
171 | </pre> | |
172 | </blockquote> | |
173 | ||
174 | <p>we achieve the desired result - we improve efficiency without generating | |
175 | references to references.</p> | |
176 | ||
177 | <h3>Limitations</h3> | |
178 | ||
179 | <p>The call traits template used to realise some improvements relies on | |
180 | partial specialisation, so these improvements are only available on | |
181 | compilers that support that feature. With other compilers, the argument | |
182 | passed to the member function (in the <tt>mem_fun1_t</tt> family) will | |
183 | always be passed by reference, thus generating the possibility of | |
184 | references to references.</p> | |
185 | <hr> | |
186 | ||
187 | <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src= | |
188 | "../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional" | |
189 | height="31" width="88"></a></p> | |
190 | ||
191 | <p>Revised | |
192 | <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p> | |
193 | ||
194 | <p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p> | |
195 | ||
196 | <p><i>Distributed under the Boost Software License, Version 1.0. (See | |
197 | accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or | |
198 | copy at <a href= | |
199 | "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p> | |
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