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| <a name="boost_integer.log2"></a><a class="link" href="log2.html" title="Compile Time log2 Calculation"> Compile Time log2 Calculation</a> |
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| <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> |
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| <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> |
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