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
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
|
<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>Function Object Traits</h1>
<p>The header <nobr><a
href="../../boost/functional.hpp">functional.hpp</a></nobr> provides two
traits class templates for functions and function objects:</p>
<table border="1">
<tr>
<th>Type</th>
<th>Contents</th>
<th>Description</th>
</tr>
<tr>
<td valign="top" rowspan="4"><tt><nobr>template <typename T></nobr><br><nobr>struct unary_traits<nobr></tt>
</td>
<td valign="top"><tt><nobr>function_type</nobr></tt>
</td>
<td valign="top">The type of the function or function object itself (i.e., <tt>T</tt>).
</td>
</tr>
<tr>
<td valign="top"><tt><nobr>param_type</nobr></tt>
</td>
<td valign="top">The type that should be used to pass the function or function object as a parameter.
</td>
</tr>
<tr>
<td valign="top"><tt><nobr>result_type</nobr></tt>
</td>
<td valign="top">The type returned by the function or function object.
</td>
</tr>
<tr>
<td valign="top"><tt><nobr>argument_type</nobr></tt>
</td>
<td valign="top">The type of the argument to the function or function object.
</td>
</tr>
<tr>
<td valign="top" rowspan="5"><tt><nobr>template <typename T></nobr><br><nobr>struct binary_traits<nobr></tt>
</td>
<td valign="top"><tt><nobr>function_type</nobr></tt>
</td>
<td valign="top">The type of the function or function object itself (i.e., <tt>T</tt>).
</td>
</tr>
<tr>
<td valign="top"><tt><nobr>param_type</nobr></tt>
</td>
<td valign="top">The type that should be used to pass the function or function object as a parameter.
</td>
</tr>
<tr>
<td valign="top"><tt><nobr>result_type</nobr></tt>
</td>
<td valign="top">The type returned by the function or function object.
</td>
</tr>
<tr>
<td valign="top"><tt><nobr>first_argument_type</nobr></tt>
</td>
<td valign="top">The type of the first argument to the function or function object.
</td>
</tr>
<tr>
<td valign="top"><tt><nobr>second_argument_type</nobr></tt>
</td>
<td valign="top">The type of the second argument to the function or function object.
</td>
</tr>
</table>
<h3>Usage</h3>
<p><tt><nobr>unary_traits</nobr></tt> should be instantiated with
either a function taking a single parameter, or an adaptable unary
function object (i.e., a class derived from
<tt><nobr>std::unary_function</nobr></tt> or one which provides the
same typedefs). (See §20.3.1 in the C++ Standard.)
<p><tt><nobr>binary_traits</nobr></tt> should be instantiated with
either a function taking two parameters, or an adaptable binary
function object (i.e., a class derived from
<tt><nobr>std::binary_function</nobr></tt> or one which provides the
same typedefs). (See §20.3.1 in the C++ Standard.)
<p>The most common usage of these templates is in function object
adapters, thus allowing them to adapt plain functions as well as
function objects. You can do this by wherever you would normally
write, for example,
<blockquote><pre>
typename Operation::argument_type
</pre></blockquote>
<p>simply writing
<blockquote><pre>
typename boost::unary_traits<Operation>::argument_type
</pre></blockquote>
<p>instead.
<h3>Additional Types Defined</h3>
<p>In addition to the standard result and argument typedefs, these
traits templates define two additional types.
<h4><tt>function_type</tt></h4>
<p>This is the type of the function or function object, and can be
used in declarations such as</p>
<blockquote><pre>
template <class Predicate>
class unary_negate : // ...
{
// ...
private:
<strong>typename unary_traits<Predicate>::function_type</strong> pred;
};
</pre></blockquote>
<p>If this typedef were not provided, it would not be possible to
declare <tt>pred</tt> in a way that would allow
<tt><nobr>unary_negate</nobr></tt> to be instantiated with a function
type (see the C++ Standard §14.3.1 ¶3).
<h4><tt>param_type</tt></h4>
<p>This is a type suitable for passing the function or function object
as a parameter to another function. For example,
<blockquote><pre>
template <class Predicate>
class unary_negate : // ...
{
public:
explicit unary_negate(<strong>typename unary_traits<Predicate>::param_type</strong> x)
:
pred(x)
{}
// ...
};
</pre></blockquote>
<p>Function objects are passed by reference to const; function
pointers are passed by value.</p>
<h3>Limitations</h3>
<p>This library uses these traits within all function object adapters,
theoretically rendering <tt><nobr>ptr_fun</nobr></tt> obsolete.
However, third party adapters probably won't take advantage of this
mechanism, and so <tt><nobr>ptr_fun</nobr></tt> may still be required.
Accordingly, this library also provides <a
href="ptr_fun.html">improved versions of the standard function pointer
adapters</a>.</p>
<p>These traits templates will also not work with compilers that fail
to support partial specialisation of templates. With these compilers,
the traits templates can only be instantiated with adaptable function
objects, thus requiring <tt><nobr>ptr_fun</nobr></tt> to be used, even
with the function object adapters in this library.
<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>
|
