<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Compile Time log2 Calculation</title> <link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../index.html" title="Boost.Integer"> <link rel="up" href="../index.html" title="Boost.Integer"> <link rel="prev" href="mask.html" title="Integer Masks"> <link rel="next" href="minmax.html" title="Compile time min/max calculation"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> <td align="center"><a href="../../../../../index.html">Home</a></td> <td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <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> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="boost_integer.log2"></a><a class="link" href="log2.html" title="Compile Time log2 Calculation"> Compile Time log2 Calculation</a> </h2></div></div></div> <div class="toc"><dl> <dt><span class="section"><a href="log2.html#boost_integer.log2.synopsis">Synopsis</a></span></dt> <dt><span class="section"><a href="log2.html#boost_integer.log2.usage">Usage</a></span></dt> <dt><span class="section"><a href="log2.html#boost_integer.log2.demonstration_program">Demonstration Program</a></span></dt> <dt><span class="section"><a href="log2.html#boost_integer.log2.rationale">Rationale</a></span></dt> <dt><span class="section"><a href="log2.html#boost_integer.log2.credits">Credits</a></span></dt> </dl></div> <p> The class template in <a href="../../../../../boost/integer/static_log2.hpp" target="_top"><boost/integer/static_log2.hpp></a> determines the position of the highest bit in a given value. This facility is useful for solving generic programming problems. </p> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_integer.log2.synopsis"></a><a class="link" href="log2.html#boost_integer.log2.synopsis" title="Synopsis">Synopsis</a> </h3></div></div></div> <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <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> <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> <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> <span class="keyword">struct</span> <span class="identifier">static_log2</span> <span class="special">{</span> <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> <span class="special">};</span> <span class="keyword">template</span> <span class="special"><</span> <span class="special">></span> <span class="keyword">struct</span> <span class="identifier">static_log2</span><span class="special"><</span> <span class="number">0</span> <span class="special">></span> <span class="special">{</span> <span class="comment">// The logarithm of zero is undefined. </span> <span class="special">};</span> <span class="special">}</span> <span class="comment">// namespace boost </span></pre> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_integer.log2.usage"></a><a class="link" href="log2.html#boost_integer.log2.usage" title="Usage">Usage</a> </h3></div></div></div> <p> The <code class="literal">boost::static_log2</code> class template takes one template parameter, a value of type <code class="literal">static_log2_argument_type</code>. The template only defines one member, <code class="literal">value</code>, which gives the truncated, base-two logarithm of the template argument. </p> <p> Since the logarithm of zero, for any base, is undefined, there is a specialization of <code class="literal">static_log2</code> for a template argument 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> <p> Note: </p> <div class="itemizedlist"><ul class="itemizedlist" type="disc"> <li class="listitem"> <code class="literal">static_log2_argument_type</code> is an <span class="emphasis"><em>unsigned integer type</em></span> (C++ standard, 3.9.1p3). </li> <li class="listitem"> <code class="literal">static_log2_result_type</code> is an <span class="emphasis"><em>integer type</em></span> (C++ standard, 3.9.1p7). </li> </ul></div> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_integer.log2.demonstration_program"></a><a class="link" href="log2.html#boost_integer.log2.demonstration_program" title="Demonstration Program">Demonstration Program</a> </h3></div></div></div> <p> The program <a href="../../../test/static_log2_test.cpp" target="_top">static_log2_test.cpp</a> is a simplistic demonstration of the results from instantiating various examples of the binary logarithm class template. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_integer.log2.rationale"></a><a class="link" href="log2.html#boost_integer.log2.rationale" title="Rationale">Rationale</a> </h3></div></div></div> <p> The base-two (binary) logarithm, abbreviated lb, 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 available statically (<span class="emphasis"><em>i.e.</em></span> at compile-time). </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_integer.log2.credits"></a><a class="link" href="log2.html#boost_integer.log2.credits" title="Credits">Credits</a> </h3></div></div></div> <p> The original version of the Boost binary logarithm class template was written by <a href="http://www.boost.org/people/daryle_walker.html" target="_top">Daryle Walker</a> and then enhanced by Giovanni Bajo with support for compilers without partial template specialization. The current version was suggested, together with a reference implementation, by Vesa Karvonen. Gennaro Prota wrote the actual source file. </p> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2001 -2009 Beman Dawes, Daryle Walker, Gennaro Prota, John Maddock<p> Distributed under the Boost Software License, Version 1.0. (See accompanying 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>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <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> </div> </body> </html>