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initial checkin [SVN r11972]
2001-12-07 13:19:38 +00:00
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
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<title>Integer Bit Mask Templates</title>
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c++boost.gif -> boost.png replacement [SVN r25573]
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<h1><img src="../../../boost.png" alt="boost.png (6897 bytes)"
initial checkin [SVN r11972]
2001-12-07 13:19:38 +00:00
align="middle" width="277" height="86">Integer Bit Mask Templates</h1>
<p>The class templates in <cite><a href="../../../boost/integer/integer_mask.hpp">&lt;boost/integer/integer_mask.hpp&gt;</a></cite> provide bit masks for a certain bit position or a contiguous-bit pack of a certain size. The types of the masking constants come from the <a href="../integer.htm">integer type selection templates</a> header.</p>
<h2><a name="contents">Contents</a></h2>
<ul>
<li><a href="#contents">Contents</a></li>
<li><a href="#synopsis">Synopsis</a></li>
<li><a href="#single">Single Bit-Mask Class Template</a></li>
<li><a href="#group">Group Bit-Mask Class Template</a></li>
<li><a href="#example">Example</a></li>
<li><a href="#demo">Demonstration Program</a></li>
<li><a href="#rationale">Rationale</a></li>
<li><a href="#credits">Credits</a></li>
</ul>
<h2><a name="synopsis">Synopsis</a></h2>
<blockquote><pre>
#include &lt;cstddef&gt; <i>// for std::size_t</i>
namespace boost
{
template &lt; std::size_t Bit &gt;
struct high_bit_mask_t
{
typedef <em>implementation_supplied</em> least;
typedef <em>implementation_supplied</em> fast;
static const least high_bit = <em>implementation_defined</em>;
static const fast high_bit_fast = <em>implementation_defined</em>;
static const std::size_t bit_position = Bit;
};
template &lt; std::size_t Bits &gt;
struct low_bits_mask_t
{
typedef <em>implementation_supplied</em> least;
typedef <em>implementation_supplied</em> fast;
static const least sig_bits = <em>implementation_defined</em>;
static const fast sig_bits_fast = <em>implementation_defined</em>;
static const std::size_t bit_count = Bits;
};
// Specializations for low_bits_mask_t exist for certain bit counts.
} // namespace boost
</pre></blockquote>
<h2><a name="single">Single Bit-Mask Class Template</a></h2>
<p>The <code>boost::high_bit_mask_t</code> class template provides
constants for bit masks representing the bit at a certain position. The
masks are equivalent to the value 2<sup><code>Bit</code></sup>, where
<code>Bit</code> is the template parameter. The bit position must be a
nonnegative number from zero to <i>Max</i>, where <dfn>Max</dfn> is one
less than the number of bits supported by the largest unsigned built-in
integral type. The following table describes the members of an
instantiation of <code>high_bit_mask_t</code>.</p>
<table border="1" cellpadding="5">
<caption>Members of the <code>boost::high_bit_mask_t</code> Class
Template</caption>
<tr>
<th>Member</th>
<th>Meaning</th>
</tr>
<tr>
<td><code>least</code></td>
<td>The smallest unsigned built-in type that supports the given
bit position.</td>
</tr>
<tr>
<td><code>fast</code></td>
<td>The quick-to-manipulate analog of <code>least</code>.</td>
</tr>
<tr>
<td><code>high_bit</code></td>
<td>A <code>least</code> constant of the desired bit-masking
value.</td>
</tr>
<tr>
<td><code>high_bit_fast</code></td>
<td>A <code>fast</code> analog of <code>high_bit</code>.</td>
</tr>
<tr>
<td><code>bit_position</code></td>
<td>The value of the template parameter, in case its needed from
a renamed instantiation of the class template.</td>
</tr>
</table>
<h2><a name="group">Group Bit-Mask Class Template</a></h2>
<p>The <code>boost::low_bits_mask_t</code> class template provides
constants for bit masks representing the lowest bits of a certain
amount. The masks are equivalent to the value
(2<sup><code>Bits</code></sup> - 1), where <code>Bits</code> is the
template parameter. The bit amount must be a nonnegative number from
zero to <i>Max</i>, where <dfn>Max</dfn> is the number of bits supported
by the largest unsigned built-in integral type. The following table
describes the members of an instantiation of
<code>low_bits_mask_t</code>.</p>
<table border="1" cellpadding="5">
<caption>Members of the <code>boost::low_bits_mask_t</code> Class
Template</caption>
<tr>
<th>Member</th>
<th>Meaning</th>
</tr>
<tr>
<td><code>least</code></td>
<td>The smallest unsigned built-in type that supports the given
bit count.</td>
</tr>
<tr>
<td><code>fast</code></td>
<td>The quick-to-manipulate analog of <code>least</code>.</td>
</tr>
<tr>
<td><code>sig_bits</code></td>
<td>A <code>least</code> constant of the desired bit-masking
value.</td>
</tr>
<tr>
<td><code>sig_bits_fast</code></td>
<td>A <code>fast</code> analog of <code>sig_bits</code>.</td>
</tr>
<tr>
<td><code>bit_count</code></td>
<td>The value of the template parameter, in case its needed from
a renamed instantiation of the class template.</td>
</tr>
</table>
<p><strong>Implementation Note</strong><br>
When <code>Bits</code> is the exact size of a built-in unsigned type,
the implementation has to change to prevent undefined behavior.
Therefore, there are specializations of <code>low_bits_mask_t</code> at
those bit counts.</p>
<h2><a name="example">Example</a></h2>
<blockquote><pre>
#include &lt;boost/integer/integer_mask.hpp&gt;
//...
int main()
{
typedef boost::high_bit_mask_t&lt;29&gt; mask1_type;
typedef boost::low_bits_mask_t&lt;15&gt; mask2_type;
mask1_type::least my_var1;
mask2_type::fast my_var2;
//...
my_var1 |= mask1_type::high_bit;
my_var2 &amp;= mask2_type::sig_bits_fast;
//...
}
</pre></blockquote>
<h2><a name="demo">Demonstration Program</a></h2>
<p>The program <a href="../test/integer_mask_test.cpp">integer_mask_test.cpp</a>
is a simplistic demonstration of the results from instantiating various
examples of the bit mask class templates.</p>
<h2><a name="rationale">Rationale</a></h2>
<p>The class templates in this header are an extension of the <a
href="../integer.htm">integer type selection class templates</a>. The new
class templates provide the same sized types, but also convienent masks
to use when extracting the highest or all the significant bits when the
containing built-in type contains more bits. This prevents
contaimination of values by the higher, unused bits.</p>
<h2><a name="credits">Credits</a></h2>
<p>The author of the Boost bit mask class templates is <a
href="../../../people/daryle_walker.html">Daryle Walker</a>.</p>
<hr>
<p>Revised September 23, 2001</p>
<p>&copy; Copyright Daryle Walker 2001. Permission to copy, use,
modify, sell and distribute this document is granted provided this
copyright notice appears in all copies. This document is provided
&quot;as is&quot; without express or implied warranty, and with no claim
as to its suitability for any purpose.</p>
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