1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 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 V1.79.1"> 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> 152
