ceph/src/boost/libs/integer/doc/html/boost_integer/log2.html

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  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>
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  37 <p>
  38       The class template in <a href="../../../../../boost/integer/static_log2.hpp" target="_top">&lt;boost/integer/static_log2.hpp&gt;</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">&lt;</span><span class="identifier">static_log2_argument_type</span> <span class="identifier">arg</span><span class="special">&gt;</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">&lt;</span> <span class="special">&gt;</span>
  60   <span class="keyword">struct</span> <span class="identifier">static_log2</span><span class="special">&lt;</span> <span class="number">0</span> <span class="special">&gt;</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 &#169; 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>
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