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211 lines
6.8 KiB
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<title>Integer Bit Mask Templates</title>
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<body bgcolor="white" text="black" link="blue" vlink="purple" alink="red">
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<h1><img src="../../../boost.png" alt="boost.png (6897 bytes)"
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align="middle" width="277" height="86">Integer Bit Mask Templates</h1>
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<p>The class templates in <cite><a href="../../../boost/integer/integer_mask.hpp"><boost/integer/integer_mask.hpp></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>
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<h2><a name="contents">Contents</a></h2>
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<ul>
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<li><a href="#contents">Contents</a></li>
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<li><a href="#synopsis">Synopsis</a></li>
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<li><a href="#single">Single Bit-Mask Class Template</a></li>
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<li><a href="#group">Group Bit-Mask Class Template</a></li>
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<li><a href="#example">Example</a></li>
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<li><a href="#demo">Demonstration Program</a></li>
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<li><a href="#rationale">Rationale</a></li>
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<li><a href="#credits">Credits</a></li>
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</ul>
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<h2><a name="synopsis">Synopsis</a></h2>
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<blockquote><pre>
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#include <cstddef> <i>// for std::size_t</i>
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namespace boost
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{
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template < std::size_t Bit >
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struct high_bit_mask_t
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{
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typedef <em>implementation_supplied</em> least;
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typedef <em>implementation_supplied</em> fast;
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static const least high_bit = <em>implementation_defined</em>;
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static const fast high_bit_fast = <em>implementation_defined</em>;
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static const std::size_t bit_position = Bit;
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};
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template < std::size_t Bits >
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struct low_bits_mask_t
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{
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typedef <em>implementation_supplied</em> least;
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typedef <em>implementation_supplied</em> fast;
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static const least sig_bits = <em>implementation_defined</em>;
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static const fast sig_bits_fast = <em>implementation_defined</em>;
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static const std::size_t bit_count = Bits;
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};
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// Specializations for low_bits_mask_t exist for certain bit counts.
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} // namespace boost
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</pre></blockquote>
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<h2><a name="single">Single Bit-Mask Class Template</a></h2>
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<p>The <code>boost::high_bit_mask_t</code> class template provides
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constants for bit masks representing the bit at a certain position. The
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masks are equivalent to the value 2<sup><code>Bit</code></sup>, where
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<code>Bit</code> is the template parameter. The bit position must be a
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nonnegative number from zero to <i>Max</i>, where <dfn>Max</dfn> is one
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less than the number of bits supported by the largest unsigned built-in
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integral type. The following table describes the members of an
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instantiation of <code>high_bit_mask_t</code>.</p>
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<table border="1" cellpadding="5">
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<caption>Members of the <code>boost::high_bit_mask_t</code> Class
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Template</caption>
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<tr>
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<th>Member</th>
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<th>Meaning</th>
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</tr>
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<tr>
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<td><code>least</code></td>
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<td>The smallest unsigned built-in type that supports the given
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bit position.</td>
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</tr>
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<tr>
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<td><code>fast</code></td>
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<td>The quick-to-manipulate analog of <code>least</code>.</td>
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</tr>
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<tr>
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<td><code>high_bit</code></td>
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<td>A <code>least</code> constant of the desired bit-masking
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value.</td>
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</tr>
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<tr>
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<td><code>high_bit_fast</code></td>
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<td>A <code>fast</code> analog of <code>high_bit</code>.</td>
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</tr>
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<tr>
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<td><code>bit_position</code></td>
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<td>The value of the template parameter, in case its needed from
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a renamed instantiation of the class template.</td>
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</tr>
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</table>
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<h2><a name="group">Group Bit-Mask Class Template</a></h2>
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<p>The <code>boost::low_bits_mask_t</code> class template provides
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constants for bit masks representing the lowest bits of a certain
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amount. The masks are equivalent to the value
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(2<sup><code>Bits</code></sup> - 1), where <code>Bits</code> is the
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template parameter. The bit amount must be a nonnegative number from
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zero to <i>Max</i>, where <dfn>Max</dfn> is the number of bits supported
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by the largest unsigned built-in integral type. The following table
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describes the members of an instantiation of
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<code>low_bits_mask_t</code>.</p>
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<table border="1" cellpadding="5">
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<caption>Members of the <code>boost::low_bits_mask_t</code> Class
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Template</caption>
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<tr>
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<th>Member</th>
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<th>Meaning</th>
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</tr>
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<tr>
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<td><code>least</code></td>
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<td>The smallest unsigned built-in type that supports the given
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bit count.</td>
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</tr>
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<tr>
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<td><code>fast</code></td>
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<td>The quick-to-manipulate analog of <code>least</code>.</td>
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</tr>
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<tr>
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<td><code>sig_bits</code></td>
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<td>A <code>least</code> constant of the desired bit-masking
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value.</td>
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</tr>
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<tr>
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<td><code>sig_bits_fast</code></td>
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<td>A <code>fast</code> analog of <code>sig_bits</code>.</td>
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</tr>
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<tr>
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<td><code>bit_count</code></td>
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<td>The value of the template parameter, in case its needed from
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a renamed instantiation of the class template.</td>
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</tr>
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</table>
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<p><strong>Implementation Note</strong><br>
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When <code>Bits</code> is the exact size of a built-in unsigned type,
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the implementation has to change to prevent undefined behavior.
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Therefore, there are specializations of <code>low_bits_mask_t</code> at
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those bit counts.</p>
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<h2><a name="example">Example</a></h2>
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<blockquote><pre>
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#include <boost/integer/integer_mask.hpp>
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//...
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int main()
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{
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typedef boost::high_bit_mask_t<29> mask1_type;
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typedef boost::low_bits_mask_t<15> mask2_type;
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mask1_type::least my_var1;
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mask2_type::fast my_var2;
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//...
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my_var1 |= mask1_type::high_bit;
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my_var2 &= mask2_type::sig_bits_fast;
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//...
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}
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</pre></blockquote>
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<h2><a name="demo">Demonstration Program</a></h2>
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<p>The program <a href="../test/integer_mask_test.cpp">integer_mask_test.cpp</a>
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is a simplistic demonstration of the results from instantiating various
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examples of the bit mask class templates.</p>
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<h2><a name="rationale">Rationale</a></h2>
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<p>The class templates in this header are an extension of the <a
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href="../integer.htm">integer type selection class templates</a>. The new
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class templates provide the same sized types, but also convienent masks
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to use when extracting the highest or all the significant bits when the
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containing built-in type contains more bits. This prevents
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contaimination of values by the higher, unused bits.</p>
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<h2><a name="credits">Credits</a></h2>
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<p>The author of the Boost bit mask class templates is <a
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href="http://www.boost.org/people/daryle_walker.html">Daryle Walker</a>.</p>
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<hr>
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<p>Revised September 23, 2001</p>
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<p>© Copyright Daryle Walker 2001. Use, modification, and distribution are
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subject to the Boost Software License, Version 1.0. (See accompanying file <a
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href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or a copy at <<a
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href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>>.)</p>
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