]>
Commit | Line | Data |
---|---|---|
7c673cae FG |
1 | <html> |
2 | <head> | |
3 | <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> | |
4 | <title>Compile Time log2 Calculation</title> | |
5 | <link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> | |
6 | <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> | |
7 | <link rel="home" href="../index.html" title="Boost.Integer"> | |
8 | <link rel="up" href="../index.html" title="Boost.Integer"> | |
9 | <link rel="prev" href="mask.html" title="Integer Masks"> | |
10 | <link rel="next" href="minmax.html" title="Compile time min/max calculation"> | |
11 | </head> | |
12 | <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> | |
13 | <table cellpadding="2" width="100%"><tr> | |
14 | <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> | |
15 | <td align="center"><a href="../../../../../index.html">Home</a></td> | |
16 | <td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> | |
17 | <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> | |
18 | <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> | |
19 | <td align="center"><a href="../../../../../more/index.htm">More</a></td> | |
20 | </tr></table> | |
21 | <hr> | |
22 | <div class="spirit-nav"> | |
23 | <a accesskey="p" href="mask.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="minmax.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> | |
24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | |
27 | <a name="boost_integer.log2"></a><a class="link" href="log2.html" title="Compile Time log2 Calculation">Compile Time log2 Calculation</a> | |
28 | </h2></div></div></div> | |
29 | <div class="toc"><dl class="toc"> | |
30 | <dt><span class="section"><a href="log2.html#boost_integer.log2.synopsis">Synopsis</a></span></dt> | |
31 | <dt><span class="section"><a href="log2.html#boost_integer.log2.usage">Usage</a></span></dt> | |
32 | <dt><span class="section"><a href="log2.html#boost_integer.log2.demonstration_program">Demonstration | |
33 | Program</a></span></dt> | |
34 | <dt><span class="section"><a href="log2.html#boost_integer.log2.rationale">Rationale</a></span></dt> | |
35 | <dt><span class="section"><a href="log2.html#boost_integer.log2.credits">Credits</a></span></dt> | |
36 | </dl></div> | |
37 | <p> | |
38 | The class template in <a href="../../../../../boost/integer/static_log2.hpp" target="_top"><boost/integer/static_log2.hpp></a> | |
39 | determines the position of the highest bit in a given value. This facility | |
40 | is useful for solving generic programming problems. | |
41 | </p> | |
42 | <div class="section"> | |
43 | <div class="titlepage"><div><div><h3 class="title"> | |
44 | <a name="boost_integer.log2.synopsis"></a><a class="link" href="log2.html#boost_integer.log2.synopsis" title="Synopsis">Synopsis</a> | |
45 | </h3></div></div></div> | |
46 | <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> | |
47 | <span class="special">{</span> | |
48 | ||
49 | <span class="keyword">typedef</span> <span class="emphasis"><em>implementation-defined</em></span> <span class="identifier">static_log2_argument_type</span><span class="special">;</span> | |
50 | <span class="keyword">typedef</span> <span class="emphasis"><em>implementation-defined</em></span> <span class="identifier">static_log2_result_type</span><span class="special">;</span> | |
51 | ||
52 | <span class="keyword">template</span> <span class="special"><</span><span class="identifier">static_log2_argument_type</span> <span class="identifier">arg</span><span class="special">></span> | |
53 | <span class="keyword">struct</span> <span class="identifier">static_log2</span> | |
54 | <span class="special">{</span> | |
55 | <span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">static_log2_result_type</span> <span class="identifier">value</span> <span class="special">=</span> <span class="emphasis"><em>implementation-defined</em></span><span class="special">;</span> | |
56 | <span class="special">};</span> | |
57 | ||
58 | ||
59 | <span class="keyword">template</span> <span class="special"><</span> <span class="special">></span> | |
60 | <span class="keyword">struct</span> <span class="identifier">static_log2</span><span class="special"><</span> <span class="number">0</span> <span class="special">></span> | |
61 | <span class="special">{</span> | |
62 | <span class="comment">// The logarithm of zero is undefined.</span> | |
63 | <span class="special">};</span> | |
64 | ||
65 | ||
66 | <span class="special">}</span> <span class="comment">// namespace boost</span> | |
67 | </pre> | |
68 | </div> | |
69 | <div class="section"> | |
70 | <div class="titlepage"><div><div><h3 class="title"> | |
71 | <a name="boost_integer.log2.usage"></a><a class="link" href="log2.html#boost_integer.log2.usage" title="Usage">Usage</a> | |
72 | </h3></div></div></div> | |
73 | <p> | |
74 | The <code class="literal">boost::static_log2</code> class template takes one template | |
75 | parameter, a value of type <code class="literal">static_log2_argument_type</code>. | |
76 | The template only defines one member, <code class="literal">value</code>, which gives | |
77 | the truncated, base-two logarithm of the template argument. | |
78 | </p> | |
79 | <p> | |
80 | Since the logarithm of zero, for any base, is undefined, there is a specialization | |
81 | of <code class="literal">static_log2</code> for a template argument of zero. This specialization | |
82 | has no members, so an attempt to use the base-two logarithm of zero results | |
83 | in a compile-time error. | |
84 | </p> | |
85 | <p> | |
86 | Note: | |
87 | </p> | |
88 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
89 | <li class="listitem"> | |
90 | <code class="literal">static_log2_argument_type</code> is an <span class="emphasis"><em>unsigned | |
91 | integer type</em></span> (C++ standard, 3.9.1p3). | |
92 | </li> | |
93 | <li class="listitem"> | |
94 | <code class="literal">static_log2_result_type</code> is an <span class="emphasis"><em>integer type</em></span> | |
95 | (C++ standard, 3.9.1p7). | |
96 | </li> | |
97 | </ul></div> | |
98 | </div> | |
99 | <div class="section"> | |
100 | <div class="titlepage"><div><div><h3 class="title"> | |
101 | <a name="boost_integer.log2.demonstration_program"></a><a class="link" href="log2.html#boost_integer.log2.demonstration_program" title="Demonstration Program">Demonstration | |
102 | Program</a> | |
103 | </h3></div></div></div> | |
104 | <p> | |
105 | The program <a href="../../../test/static_log2_test.cpp" target="_top">static_log2_test.cpp</a> | |
106 | is a simplistic demonstration of the results from instantiating various examples | |
107 | of the binary logarithm class template. | |
108 | </p> | |
109 | </div> | |
110 | <div class="section"> | |
111 | <div class="titlepage"><div><div><h3 class="title"> | |
112 | <a name="boost_integer.log2.rationale"></a><a class="link" href="log2.html#boost_integer.log2.rationale" title="Rationale">Rationale</a> | |
113 | </h3></div></div></div> | |
114 | <p> | |
115 | The base-two (binary) logarithm, abbreviated lb, function is occasionally | |
116 | used to give order-estimates of computer algorithms. The truncated logarithm | |
117 | can be considered the highest power-of-two in a value, which corresponds | |
118 | to the value's highest set bit (for binary integers). Sometimes the highest-bit | |
119 | position could be used in generic programming, which requires the position | |
120 | to be available statically (<span class="emphasis"><em>i.e.</em></span> at compile-time). | |
121 | </p> | |
122 | </div> | |
123 | <div class="section"> | |
124 | <div class="titlepage"><div><div><h3 class="title"> | |
125 | <a name="boost_integer.log2.credits"></a><a class="link" href="log2.html#boost_integer.log2.credits" title="Credits">Credits</a> | |
126 | </h3></div></div></div> | |
127 | <p> | |
128 | The original version of the Boost binary logarithm class template was written | |
129 | by <a href="http://www.boost.org/people/daryle_walker.html" target="_top">Daryle Walker</a> | |
130 | and then enhanced by Giovanni Bajo with support for compilers without partial | |
131 | template specialization. The current version was suggested, together with | |
132 | a reference implementation, by Vesa Karvonen. Gennaro Prota wrote the actual | |
133 | source file. | |
134 | </p> | |
135 | </div> | |
136 | </div> | |
137 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
138 | <td align="left"></td> | |
139 | <td align="right"><div class="copyright-footer">Copyright © 2001-2009 Beman | |
140 | Dawes, Daryle Walker, Gennaro Prota, John Maddock<p> | |
141 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
142 | file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) | |
143 | </p> | |
144 | </div></td> | |
145 | </tr></table> | |
146 | <hr> | |
147 | <div class="spirit-nav"> | |
148 | <a accesskey="p" href="mask.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="minmax.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> | |
149 | </div> | |
150 | </body> | |
151 | </html> |