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<!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">&lt;boost/integer/static_log2.hpp&gt;</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 &lt; unsigned long Value &gt;
struct static_log2
{
static const int value = <em>implementation_defined</em>;
};
template &lt; &gt;
struct static_log2&lt; 0ul &gt;
{
// 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 &lt;boost/integer/static_log2.hpp&gt;
template &lt; unsigned long Value &gt;
bool is_it_what()
{
typedef boost::static_log2&lt;Value&gt; lb_type;
int temp = lb_type::value;
//...
return (temp % 2) != 0;
}
//...
int main()
{
bool temp = is_it_what&lt;2000&gt;();
//...
#if 0
temp = is_it_what&lt;0&gt;(); // would give an error
#endif
//...
temp = is_it_what&lt;24&gt;();
//...
}
</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>&copy; 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
&quot;as is&quot; without express or implied warranty, and with no claim
as to its suitability for any purpose.</p>
</body>
</html>