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<title>Compile Time log2 Calculation</title>
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<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" lang="en">
<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>
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<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 type="disc">
<li>
<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>
<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" lang="en">
<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" lang="en">
<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" lang="en">
<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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