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
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<html>
<head>
<title>Binary Logarithm Template</title>
</head>
<body bgcolor="white" text="black">
<h1><img src="../../../c++boost.gif" alt="c++boost.gif (8819 bytes)"
align="middle" width="277" height="86">Binary Logarithm Template</h1>
<p>The class template in <cite><a href="../../../boost/integer/static_log2.hpp"><boost/integer/static_log2.hpp></a></cite> determines the position of the highest bit in a given value. This facility is useful for solving generic programming problems.</p>
<h2><a name="contents">Contents</a></h2>
<ul>
<li><a href="#contents">Contents</a></li>
<li><a href="#synopsis">Synopsis</a></li>
<li><a href="#usage">Usage</a></li>
<li><a href="#example">Example</a></li>
<li><a href="#demo">Demonstration Program</a></li>
<li><a href="#rationale">Rationale</a></li>
<li><a href="#credits">Credits</a></li>
</ul>
<h2><a name="synopsis">Synopsis</a></h2>
<blockquote><pre>
namespace boost
{
template < unsigned long Value >
struct static_log2
{
static const int value = <em>implementation_defined</em>;
};
template < >
struct static_log2< 0ul >
{
// The logarithm of zero is undefined.
};
} // namespace boost
</pre></blockquote>
<h2><a name="usage">Usage</a></h2>
<p>The <code>boost::static_log2</code> class template takes one template
parameter, a value of type <code>unsigned long</code>. The template
only defines one member, <code>value</code>, that returns the truncated
base-two logarithm of the template parameter.</p>
<p>Since the logarithm of zero, for any base, is undefined, there is a
specialization of <code>static_log2</code> for a template parameter
of zero. This specialization has no members, so an attempt to use
the base-two logarithm of zero results in a compile-time error.</p>
<h2><a name="example">Example</a></h2>
<blockquote><pre>
#include <boost/integer/static_log2.hpp>
template < unsigned long Value >
bool is_it_what()
{
typedef boost::static_log2<Value> lb_type;
int temp = lb_type::value;
//...
return (temp % 2) != 0;
}
//...
int main()
{
bool temp = is_it_what<2000>();
//...
#if 0
temp = is_it_what<0>(); // would give an error
#endif
//...
temp = is_it_what<24>();
//...
}
</pre></blockquote>
<h2><a name="demo">Demonstration Program</a></h2>
<p>The program <a href="../test/static_log2_test.cpp">static_log2_test.cpp</a>
is a simplistic demonstration of the results from instantiating various
examples of the binary logarithm class template.</p>
<h2><a name="rationale">Rationale</a></h2>
<p>The base-two (binary) logarithm, abbreviated <dfn>lb</dfn>, function
is occasionally used to give order-estimates of computer algorithms.
The truncated logarithm can be considered the highest power-of-two in a
value, which corresponds to the value's highest set bit (for binary
integers). Sometimes the highest-bit position could be used in generic
programming, which requires the position to be statically (<i>i.e.</i>
at compile-time) available.</p>
<h2><a name="credits">Credits</a></h2>
<p>The author of the Boost binary logarithm class template is <a
href="../../../people/daryle_walker.html">Daryle Walker</a>.</p>
<hr>
<p>Revised October 1, 2001</p>
<p>© Copyright Daryle Walker 2001. 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>
</body>
</html>
|
