| 1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> |
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| 2 | <html> |
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| 3 | <head> |
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| 4 | <title>Integer Bit Mask Templates</title> |
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| 5 | </head> |
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| 6 | |
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| 7 | <body bgcolor="white" text="black" link="blue" vlink="purple" alink="red"> |
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| 8 | <h1><img src="../../../boost.png" alt="boost.png (6897 bytes)" |
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| 9 | align="middle" width="277" height="86">Integer Bit Mask Templates</h1> |
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| 10 | |
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| 11 | <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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| 12 | |
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| 13 | <h2><a name="contents">Contents</a></h2> |
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| 14 | |
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| 15 | <ul> |
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| 16 | <li><a href="#contents">Contents</a></li> |
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| 17 | <li><a href="#synopsis">Synopsis</a></li> |
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| 18 | <li><a href="#single">Single Bit-Mask Class Template</a></li> |
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| 19 | <li><a href="#group">Group Bit-Mask Class Template</a></li> |
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| 20 | <li><a href="#example">Example</a></li> |
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| 21 | <li><a href="#demo">Demonstration Program</a></li> |
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| 22 | <li><a href="#rationale">Rationale</a></li> |
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| 23 | <li><a href="#credits">Credits</a></li> |
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| 24 | </ul> |
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| 25 | |
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| 26 | <h2><a name="synopsis">Synopsis</a></h2> |
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| 27 | |
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| 28 | <blockquote><pre> |
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| 29 | #include <cstddef> <i>// for std::size_t</i> |
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| 30 | |
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| 31 | namespace boost |
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| 32 | { |
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| 33 | |
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| 34 | template < std::size_t Bit > |
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| 35 | struct high_bit_mask_t |
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| 36 | { |
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| 37 | typedef <em>implementation_supplied</em> least; |
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| 38 | typedef <em>implementation_supplied</em> fast; |
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| 39 | |
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| 40 | static const least high_bit = <em>implementation_defined</em>; |
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| 41 | static const fast high_bit_fast = <em>implementation_defined</em>; |
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| 42 | |
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| 43 | static const std::size_t bit_position = Bit; |
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| 44 | |
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| 45 | }; |
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| 46 | |
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| 47 | template < std::size_t Bits > |
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| 48 | struct low_bits_mask_t |
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| 49 | { |
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| 50 | typedef <em>implementation_supplied</em> least; |
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| 51 | typedef <em>implementation_supplied</em> fast; |
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| 52 | |
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| 53 | static const least sig_bits = <em>implementation_defined</em>; |
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| 54 | static const fast sig_bits_fast = <em>implementation_defined</em>; |
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| 55 | |
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| 56 | static const std::size_t bit_count = Bits; |
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| 57 | |
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| 58 | }; |
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| 59 | |
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| 60 | // Specializations for low_bits_mask_t exist for certain bit counts. |
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| 61 | |
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| 62 | } // namespace boost |
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| 63 | </pre></blockquote> |
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| 64 | |
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| 65 | <h2><a name="single">Single Bit-Mask Class Template</a></h2> |
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| 66 | |
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| 67 | <p>The <code>boost::high_bit_mask_t</code> class template provides |
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| 68 | constants for bit masks representing the bit at a certain position. The |
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| 69 | masks are equivalent to the value 2<sup><code>Bit</code></sup>, where |
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| 70 | <code>Bit</code> is the template parameter. The bit position must be a |
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| 71 | nonnegative number from zero to <i>Max</i>, where <dfn>Max</dfn> is one |
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| 72 | less than the number of bits supported by the largest unsigned built-in |
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| 73 | integral type. The following table describes the members of an |
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| 74 | instantiation of <code>high_bit_mask_t</code>.</p> |
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| 75 | |
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| 76 | <table border="1" cellpadding="5"> |
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| 77 | <caption>Members of the <code>boost::high_bit_mask_t</code> Class |
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| 78 | Template</caption> |
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| 79 | <tr> |
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| 80 | <th>Member</th> |
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| 81 | <th>Meaning</th> |
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| 82 | </tr> |
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| 83 | <tr> |
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| 84 | <td><code>least</code></td> |
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| 85 | <td>The smallest unsigned built-in type that supports the given |
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| 86 | bit position.</td> |
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| 87 | </tr> |
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| 88 | <tr> |
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| 89 | <td><code>fast</code></td> |
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| 90 | <td>The quick-to-manipulate analog of <code>least</code>.</td> |
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| 91 | </tr> |
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| 92 | <tr> |
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| 93 | <td><code>high_bit</code></td> |
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| 94 | <td>A <code>least</code> constant of the desired bit-masking |
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| 95 | value.</td> |
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| 96 | </tr> |
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| 97 | <tr> |
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| 98 | <td><code>high_bit_fast</code></td> |
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| 99 | <td>A <code>fast</code> analog of <code>high_bit</code>.</td> |
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| 100 | </tr> |
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| 101 | <tr> |
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| 102 | <td><code>bit_position</code></td> |
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| 103 | <td>The value of the template parameter, in case its needed from |
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| 104 | a renamed instantiation of the class template.</td> |
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| 105 | </tr> |
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| 106 | </table> |
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| 107 | |
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| 108 | <h2><a name="group">Group Bit-Mask Class Template</a></h2> |
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| 109 | |
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| 110 | <p>The <code>boost::low_bits_mask_t</code> class template provides |
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| 111 | constants for bit masks representing the lowest bits of a certain |
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| 112 | amount. The masks are equivalent to the value |
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| 113 | (2<sup><code>Bits</code></sup> - 1), where <code>Bits</code> is the |
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| 114 | template parameter. The bit amount must be a nonnegative number from |
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| 115 | zero to <i>Max</i>, where <dfn>Max</dfn> is the number of bits supported |
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| 116 | by the largest unsigned built-in integral type. The following table |
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| 117 | describes the members of an instantiation of |
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| 118 | <code>low_bits_mask_t</code>.</p> |
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| 119 | |
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| 120 | <table border="1" cellpadding="5"> |
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| 121 | <caption>Members of the <code>boost::low_bits_mask_t</code> Class |
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| 122 | Template</caption> |
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| 123 | <tr> |
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| 124 | <th>Member</th> |
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| 125 | <th>Meaning</th> |
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| 126 | </tr> |
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| 127 | <tr> |
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| 128 | <td><code>least</code></td> |
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| 129 | <td>The smallest unsigned built-in type that supports the given |
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| 130 | bit count.</td> |
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| 131 | </tr> |
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| 132 | <tr> |
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| 133 | <td><code>fast</code></td> |
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| 134 | <td>The quick-to-manipulate analog of <code>least</code>.</td> |
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| 135 | </tr> |
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| 136 | <tr> |
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| 137 | <td><code>sig_bits</code></td> |
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| 138 | <td>A <code>least</code> constant of the desired bit-masking |
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| 139 | value.</td> |
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| 140 | </tr> |
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| 141 | <tr> |
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| 142 | <td><code>sig_bits_fast</code></td> |
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| 143 | <td>A <code>fast</code> analog of <code>sig_bits</code>.</td> |
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| 144 | </tr> |
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| 145 | <tr> |
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| 146 | <td><code>bit_count</code></td> |
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| 147 | <td>The value of the template parameter, in case its needed from |
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| 148 | a renamed instantiation of the class template.</td> |
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| 149 | </tr> |
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| 150 | </table> |
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| 151 | |
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| 152 | <p><strong>Implementation Note</strong><br> |
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| 153 | When <code>Bits</code> is the exact size of a built-in unsigned type, |
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| 154 | the implementation has to change to prevent undefined behavior. |
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| 155 | Therefore, there are specializations of <code>low_bits_mask_t</code> at |
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| 156 | those bit counts.</p> |
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| 157 | |
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| 158 | <h2><a name="example">Example</a></h2> |
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| 159 | |
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| 160 | <blockquote><pre> |
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| 161 | #include <boost/integer/integer_mask.hpp> |
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| 162 | |
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| 163 | //... |
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| 164 | |
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| 165 | int main() |
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| 166 | { |
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| 167 | typedef boost::high_bit_mask_t<29> mask1_type; |
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| 168 | typedef boost::low_bits_mask_t<15> mask2_type; |
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| 169 | |
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| 170 | mask1_type::least my_var1; |
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| 171 | mask2_type::fast my_var2; |
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| 172 | //... |
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| 173 | |
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| 174 | my_var1 |= mask1_type::high_bit; |
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| 175 | my_var2 &= mask2_type::sig_bits_fast; |
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| 176 | |
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| 177 | //... |
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| 178 | } |
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| 179 | </pre></blockquote> |
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| 180 | |
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| 181 | <h2><a name="demo">Demonstration Program</a></h2> |
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| 182 | |
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| 183 | <p>The program <a href="../test/integer_mask_test.cpp">integer_mask_test.cpp</a> |
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| 184 | is a simplistic demonstration of the results from instantiating various |
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| 185 | examples of the bit mask class templates.</p> |
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| 186 | |
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| 187 | <h2><a name="rationale">Rationale</a></h2> |
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| 188 | |
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| 189 | <p>The class templates in this header are an extension of the <a |
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| 190 | href="../integer.htm">integer type selection class templates</a>. The new |
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| 191 | class templates provide the same sized types, but also convienent masks |
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| 192 | to use when extracting the highest or all the significant bits when the |
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| 193 | containing built-in type contains more bits. This prevents |
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| 194 | contaimination of values by the higher, unused bits.</p> |
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| 195 | |
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| 196 | <h2><a name="credits">Credits</a></h2> |
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| 197 | |
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| 198 | <p>The author of the Boost bit mask class templates is <a |
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| 199 | href="../../../people/daryle_walker.html">Daryle Walker</a>.</p> |
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| 200 | |
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| 201 | <hr> |
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| 202 | |
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| 203 | <p>Revised September 23, 2001</p> |
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| 204 | |
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| 205 | <p>© Copyright Daryle Walker 2001. Permission to copy, use, |
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| 206 | modify, sell and distribute this document is granted provided this |
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| 207 | copyright notice appears in all copies. This document is provided |
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| 208 | "as is" without express or implied warranty, and with no claim |
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| 209 | as to its suitability for any purpose.</p> |
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| 210 | </body> |
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| 211 | </html> |
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