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123 lines
3.6 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
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<head>
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<title>Binary Logarithm Template</title>
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</head>
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<body bgcolor="white" text="black">
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<h1><img src="../../../c++boost.gif" alt="c++boost.gif (8819 bytes)"
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align="middle" width="277" height="86">Binary Logarithm Template</h1>
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<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>
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<h2><a name="contents">Contents</a></h2>
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<ul>
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<li><a href="#contents">Contents</a></li>
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<li><a href="#synopsis">Synopsis</a></li>
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<li><a href="#usage">Usage</a></li>
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<li><a href="#example">Example</a></li>
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<li><a href="#demo">Demonstration Program</a></li>
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<li><a href="#rationale">Rationale</a></li>
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<li><a href="#credits">Credits</a></li>
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</ul>
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<h2><a name="synopsis">Synopsis</a></h2>
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<blockquote><pre>
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namespace boost
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{
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template < unsigned long Value >
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struct static_log2
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{
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static const int value = <em>implementation_defined</em>;
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};
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template < >
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struct static_log2< 0ul >
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{
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// The logarithm of zero is undefined.
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};
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} // namespace boost
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</pre></blockquote>
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<h2><a name="usage">Usage</a></h2>
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<p>The <code>boost::static_log2</code> class template takes one template
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parameter, a value of type <code>unsigned long</code>. The template
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only defines one member, <code>value</code>, that returns the truncated
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base-two logarithm of the template parameter.</p>
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<p>Since the logarithm of zero, for any base, is undefined, there is a
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specialization of <code>static_log2</code> for a template parameter
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of zero. This specialization has no members, so an attempt to use
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the base-two logarithm of zero results in a compile-time error.</p>
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<h2><a name="example">Example</a></h2>
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<blockquote><pre>
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#include <boost/integer/static_log2.hpp>
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template < unsigned long Value >
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bool is_it_what()
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{
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typedef boost::static_log2<Value> lb_type;
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int temp = lb_type::value;
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//...
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return (temp % 2) != 0;
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}
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//...
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int main()
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{
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bool temp = is_it_what<2000>();
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//...
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#if 0
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temp = is_it_what<0>(); // would give an error
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#endif
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//...
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temp = is_it_what<24>();
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//...
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}
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</pre></blockquote>
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<h2><a name="demo">Demonstration Program</a></h2>
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<p>The program <a href="../test/static_log2_test.cpp">static_log2_test.cpp</a>
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is a simplistic demonstration of the results from instantiating various
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examples of the binary logarithm class template.</p>
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<h2><a name="rationale">Rationale</a></h2>
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<p>The base-two (binary) logarithm, abbreviated <dfn>lb</dfn>, function
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is occasionally used to give order-estimates of computer algorithms.
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The truncated logarithm can be considered the highest power-of-two in a
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value, which corresponds to the value's highest set bit (for binary
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integers). Sometimes the highest-bit position could be used in generic
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programming, which requires the position to be statically (<i>i.e.</i>
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at compile-time) available.</p>
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<h2><a name="credits">Credits</a></h2>
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<p>The author of the Boost binary logarithm class template is <a
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href="../../../people/daryle_walker.html">Daryle Walker</a>.</p>
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<hr>
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<p>Revised October 1, 2001</p>
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<p>© Copyright Daryle Walker 2001. Permission to copy, use,
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modify, sell and distribute this document is granted provided this
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copyright notice appears in all copies. This document is provided
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"as is" without express or implied warranty, and with no claim
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as to its suitability for any purpose.</p>
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</body>
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</html>
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