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
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Language" content="en-us">
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<title>Boost Function Object Adapter Library</title>
</head>
<body bgcolor="#FFFFFF" text="#000000">
<table border="1" bgcolor="#007F7F" cellpadding="2" summary="">
<tr>
<td bgcolor="#FFFFFF"><img src="../../boost.png" alt=
"boost.png (6897 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="http://www.boost.org/people/people.htm"><font face="Arial" color=
"#FFFFFF"><big>People</big></font></a></td>
<td><a href="http://www.boost.org/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>Binders</h1>
<p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
provides enhanced versions of both the binder function object adapters from
the C++ Standard Library (§20.3.6):</p>
<ul>
<li><tt>binder1st</tt></li>
<li><tt>binder2nd</tt></li>
</ul>
<p>As well as the corresponding helper functions</p>
<ul>
<li><tt>bind1st</tt></li>
<li><tt>bind2nd</tt></li>
</ul>
<p>The key benefit of these adapters over those in the Standard Library is
they avoid the problem of <a href="#refref">references to
references.</a></p>
<h3>Usage</h3>
<p>Usage is identical to the standard binders. For example,</p>
<blockquote>
<pre>
class Foo {
public:
void bar(std::ostream &);
// ...
};
// ...
std::vector<Foo> c;
// ...
std::for_each(c.begin(), c.end(),
boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout));
</pre>
</blockquote>
<h3 id="refref">References to References</h3>
<p>Consider the usage example above</p>
<blockquote>
<pre>
class Foo {
public:
void bar(<strong>std::ostream &</strong>);
// ...
};
// ...
std::for_each(c.begin(), c.end(),
boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout));
</pre>
</blockquote>
<p>If this had been written using <tt>std::bind2nd</tt> and
<tt>std::mem_fun_ref</tt>, it would be unlikely to compile.</p>
<p>The problem arises because <tt>bar</tt> takes a reference argument. The
Standard defines <tt>std::mem_fun_ref</tt> such that it creates a function
object whose <tt>second_argument_type</tt> will be
<tt>std::ostream&</tt>.</p>
<p>The call to <tt>bind2nd</tt> creates a <tt>binder2nd</tt> which the
Standard defines as follows:</p>
<blockquote>
<pre>
template <class Operation>
class binder2nd
: public unary_function<typename Operation::first_argument_type,
typename Operation::result_type> {
...
public:
binder2nd(const Operation& x,
<strong>const typename Operation::second_argument_type& y</strong>);
...
</pre>
</blockquote>
<p>Since our operation's <tt>second_argument_type</tt> is
<tt>std::ostream&</tt>, the type of <tt>y</tt> in the constructor would
be <tt>std::ostream&&</tt>. Since you cannot have a reference to a
reference, at this point we should get a compilation error because
references to references are illegal in C++ (but see <a href=
"http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#106">C++
Standard core language active issues list</a>).</p>
<p>The binders in this library avoid this problem by using the Boost
<tt><a href="../utility/call_traits.htm">call_traits</a></tt>
templates.</p>
<p>Our constructor is declared</p>
<blockquote>
<pre>
binder2nd(const Operation& x,
<strong>typename call_traits<
typename binary_traits<Operation>::second_argument_type
>::param_type y</strong>)
</pre>
</blockquote>
<p>As a result, <tt>y</tt> has a type of <tt>std::ostream&</tt>, and
our example compiles.</p>
<hr>
<p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
"http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01 Transitional"
height="31" width="88"></a></p>
<p>Revised
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02
December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p>
<p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p>
<p><i>Distributed under the Boost Software License, Version 1.0. (See
accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
copy at <a href=
"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
</body>
</html>
|
