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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 (&sect;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>
86template &lt;class S, class T, class A&gt; class const_mem_fun1_t
87  : public binary_function&lt;<strong>T*</strong>, A, S&gt; {
88public:
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>&nbsp;T*</tt>.</p>
98
99  <p>Does this matter? Well, consider what happens when we write</p>
100
101  <blockquote>
102    <pre>
103struct Foo { void bar(int) const; };
104const Foo *cp = new Foo;
105std::bind1st(std::mem_fun(&amp;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>
114typedef 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&nbsp;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  (&sect;20.3.8&nbsp;&para;2):</p>
134
135  <blockquote>
136    <pre>
137template &lt;class S, class T, class A&gt; class mem_fun1_t
138  : public binary_function&lt;T*, A, S&gt; {
139public:
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&nbsp;A&amp;</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://anubis.dkuug.dk/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&nbsp;A&amp;</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>
170S operator()(T* p, typename call_traits&lt;A&gt;::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
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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 &copy; 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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