1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
|
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Boost Function Object Adapter Library</title>
</head>
<body bgcolor="#FFFFFF" text="#000000">
<table border="1" bgcolor="#007F7F" cellpadding="2">
<tr>
<td bgcolor="#FFFFFF"><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" WIDTH="277" HEIGHT="86"></td>
<td><a href="../../index.htm"><font face="Arial" color="#FFFFFF"><big>Home </big></font></a></td>
<td><a href="../libraries.htm"><font face="Arial" color="#FFFFFF"><big>Libraries </big></font></a></td>
<td><a href="../../people/people.htm"><font face="Arial" color="#FFFFFF"><big>People </big></font></a></td>
<td><a href="../../more/faq.htm"><font face="Arial" color="#FFFFFF"><big>FAQ </big></font></a></td>
<td><a href="../../more/index.htm"><font face="Arial" color="#FFFFFF"><big>More </big></font></a></td>
</tr>
</table>
<h1>Member Function Adapters</h1>
<p>The header <nobr><a
href="../../boost/functional.hpp">functional.hpp</a></nobr> includes
improved versions of the full range of member function adapters from
the the C++ Standard Library <nobr>(§ 20.3.8):</nobr></p>
<ul>
<li><tt>mem_fun_t</tt></li>
<li><tt>mem_fun1_t</tt></li>
<li><tt>const_mem_fun_t</tt></li>
<li><tt>const_mem_fun1_t</tt></li>
<li><tt>mem_fun_ref_t</tt></li>
<li><tt>mem_fun1_ref_t</tt></li>
<li><tt>const_mem_fun_ref_t</tt></li>
<li><tt>const_mem_fun1_ref_t</tt></li>
</ul>
<p>as well as the corresponding overloaded helper functions<p>
<ul>
<li><tt>mem_fun</tt></li>
<li><tt>mem_fun_ref</tt></li>
</ul>
<p>The following changes have been made to the adapters as specified
in the Standard:</p>
<ul>
<li>The <tt><nobr>first_argument_type</nobr></tt> typedef has been
corrected for the <nobr><tt>const_</tt></nobr> family of member
function adapters (see <a href="#firstarg">below</a>).</li>
<li>The argument passed to <tt><nobr>mem_fun1_t</nobr></tt> and its
variants is passed using the
<tt><nobr>call_traits::param_type</nobr></tt> for the member
function's argument type.
</ul>
<h3 id="firstarg">first_argument_type</h3>
<p>The standard specifies <tt><nobr>const_mem_fun1_t</nobr></tt>, for example, like this:
<blockquote><pre>
template <class S, class T, class A> class const_mem_fun1_t
: public binary_function<<strong>T*</strong>, A, S> {
public:
explicit const_mem_fun1_t(S (T::*p)(A) const);
S operator()(<strong>const T*</strong> p, A x) const;
};
</pre></blockquote>
<p>Note that the first argument to
<tt><nobr>binary_function</nobr></tt> is <tt><nobr>T*</nobr></tt>
despite the fact that the first argument to <tt><nobr>operator()</nobr></tt> is
actually of type <tt><nobr><em>const</em> T*</nobr></tt>.
<p>Does this matter? Well, consider what happens when we write
<blockquote><pre>
struct Foo { void bar(int) const; };
const Foo *cp = new Foo;
std::bind1st(std::mem_fun(&Foo::bar), cp);
</pre></blockquote>
<p>We have created a <tt><nobr>const_mem_fun1_t</nobr></tt> object
which will effectively contain the following
<blockquote><pre>
typedef Foo* first_argument_type;
</pre></blockquote>
<p>The <tt><nobr>bind1st</nobr></tt> will then create a
<tt><nobr>binder1st</nobr></tt> object that will use this
<tt><nobr>typedef</nobr></tt> as the type of a member which will be
initialised with <tt><nobr>cp</nobr></tt>. In other words, we will
need to initialise a <tt><nobr>Foo*</nobr></tt> member with a
<tt><nobr>const Foo*</nobr></tt> pointer! Clearly this is not
possible, so to implement this your Standard Library vendor will have
had to cast away the constness of <tt><nobr>cp</nobr></tt>, probably
within the body of <tt><nobr>bind1st</nobr></tt>.
<p>This hack will not suffice with the improved <a
href="binders.html">binders</a> in this library, so we have had to
provide corrected versions of the member function adapters as well.
<h3 id="args">Argument Types</h3>
<p>The standard defines <nobr><tt>mem_fun1_t</tt></nobr>, for example, like this
<nobr>(§20.3.8 ¶2):</nobr>
<blockquote><pre>
template <class S, class T, class A> class mem_fun1_t
: public binary_function<T*, A, S> {
public:
explicit mem_fun1_t(S (T::*p)(<strong>A</strong>));
S operator()(T* p, <strong>A</strong> x) const;
};
</pre></blockquote>
<p>Note that the second argument to <nobr><tt>operator()</tt></nobr> is
exactly the same type as the argument to the member function. If this
is a value type, the argument will be passed by value and copied twice.
<p>However, if we were to try and eliminate this inefficiency by
instead declaring the argument as <nobr><tt>const A&</tt></nobr>, then
if A were a reference type, we would have a reference to a reference,
which is currently illegal (but see <a
href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#106">C++
core language issue number 106)</a>
<p>So the way in which we want to declare the second argument for
<nobr><tt>operator()</tt></nobr> depends on whether or not the member
function's argument is a reference. If it is a reference, we want to
declare it simply as <nobr><tt>A</tt></nobr>; if it is a value we want
to declare it as <nobr><tt>const A&</tt></nobr>.
<p>The Boost <nobr><a
href="../utility/call_traits.htm">call_traits</a></nobr> class
template contains a <tt><nobr>param_type</nobr></tt> typedef, which
uses partial specialisation to make precisely this decision. By
declaring the <nobr><tt>operator()</tt></nobr> as
<blockquote><pre>
S operator()(T* p, typename call_traits<A>::param_type x) const
</pre></blockquote>
<p>we achieve the desired result - we improve efficiency without
generating references to references.</p>
<h3>Limitations</h3>
<p>The call traits template used to realise some improvements relies
on partial specialisation, so these improvements are only available on
compilers that support that feature. With other compilers, the
argument passed to the member function (in the
<nobr><tt>mem_fun1_t</tt></nobr> family) will always be passed by
reference, thus generating the possibility of references to references.
<hr>
<p>Copyright © 2000 Cadenza New Zealand Ltd. Permission to copy,
use, modify, sell and distribute this document is granted provided
this copyright notice appears in all copies. This document is provided
"as is" without express or implied warranty, and with no claim as to
its suitability for any purpose.</p>
<p>Revised 28 June 2000</p>
</body>
</html>
|
