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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>
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>
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37<p>
38 The class template in <a href="../../../../../boost/integer/static_log2.hpp" target="_top">&lt;boost/integer/static_log2.hpp&gt;</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">&lt;</span><span class="identifier">static_log2_argument_type</span> <span class="identifier">arg</span><span class="special">&gt;</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">&lt;</span> <span class="special">&gt;</span>
60 <span class="keyword">struct</span> <span class="identifier">static_log2</span><span class="special">&lt;</span> <span class="number">0</span> <span class="special">&gt;</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>
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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>
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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 &#169; 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>)
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