| 1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> |
|---|
| 2 | <html> |
|---|
| 3 | <head> |
|---|
| 4 | <title>Integer Type Selection Templates</title> |
|---|
| 5 | </head> |
|---|
| 6 | |
|---|
| 7 | <body bgcolor="white" text="black"> |
|---|
| 8 | <h1> |
|---|
| 9 | <img src="../../boost.png" alt="boost.png (6897 bytes)" |
|---|
| 10 | align="middle" width="277" height="86">Integer Type Selection |
|---|
| 11 | Templates</h1> |
|---|
| 12 | |
|---|
| 13 | <p>The <cite><a |
|---|
| 14 | href="../../boost/integer.hpp"><boost/integer.hpp></a></cite> type |
|---|
| 15 | selection templates allow integer types to be selected based on desired |
|---|
| 16 | characteristics such as number of bits or maximum value. This facility |
|---|
| 17 | is particularly useful for solving generic programming problems.</p> |
|---|
| 18 | |
|---|
| 19 | <h2><a name="contents">Contents</a></h2> |
|---|
| 20 | |
|---|
| 21 | <ul> |
|---|
| 22 | <li><a href="#contents">Contents</a></li> |
|---|
| 23 | <li><a href="#synopsis">Synopsis</a></li> |
|---|
| 24 | <li><a href="#easy">Easiest-to-Manipulate Types</a></li> |
|---|
| 25 | <li><a href="#sized">Sized Types</a></li> |
|---|
| 26 | <li><a href="#example">Example</a></li> |
|---|
| 27 | <li><a href="#demo">Demonstration Program</a></li> |
|---|
| 28 | <li><a href="#rationale">Rationale</a></li> |
|---|
| 29 | <li><a href="#alternative">Alternative</a></li> |
|---|
| 30 | <li><a href="#credits">Credits</a></li> |
|---|
| 31 | </ul> |
|---|
| 32 | |
|---|
| 33 | <h2><a name="synopsis">Synopsis</a></h2> |
|---|
| 34 | |
|---|
| 35 | <blockquote><pre>namespace boost |
|---|
| 36 | { |
|---|
| 37 | // fast integers from least integers |
|---|
| 38 | template< typename LeastInt > |
|---|
| 39 | struct int_fast_t |
|---|
| 40 | { |
|---|
| 41 | typedef <em>implementation_supplied</em> fast; |
|---|
| 42 | }; |
|---|
| 43 | |
|---|
| 44 | // signed |
|---|
| 45 | template< int Bits > |
|---|
| 46 | struct int_t |
|---|
| 47 | { |
|---|
| 48 | typedef <em>implementation_supplied</em> least; |
|---|
| 49 | typedef int_fast_t<least>::fast fast; |
|---|
| 50 | }; |
|---|
| 51 | |
|---|
| 52 | // unsigned |
|---|
| 53 | template< int Bits > |
|---|
| 54 | struct uint_t |
|---|
| 55 | { |
|---|
| 56 | typedef <em>implementation_supplied</em> least; |
|---|
| 57 | typedef int_fast_t<least>::fast fast; |
|---|
| 58 | }; |
|---|
| 59 | |
|---|
| 60 | // signed |
|---|
| 61 | template< long MaxValue > |
|---|
| 62 | struct int_max_value_t |
|---|
| 63 | { |
|---|
| 64 | typedef <em>implementation_supplied</em> least; |
|---|
| 65 | typedef int_fast_t<least>::fast fast; |
|---|
| 66 | }; |
|---|
| 67 | |
|---|
| 68 | template< long MinValue > |
|---|
| 69 | struct int_min_value_t |
|---|
| 70 | { |
|---|
| 71 | typedef <em>implementation_supplied</em> least; |
|---|
| 72 | typedef int_fast_t<least>::fast fast; |
|---|
| 73 | }; |
|---|
| 74 | |
|---|
| 75 | // unsigned |
|---|
| 76 | template< unsigned long Value > |
|---|
| 77 | struct uint_value_t |
|---|
| 78 | { |
|---|
| 79 | typedef <em>implementation_supplied</em> least; |
|---|
| 80 | typedef int_fast_t<least>::fast fast; |
|---|
| 81 | }; |
|---|
| 82 | } // namespace boost |
|---|
| 83 | </pre></blockquote> |
|---|
| 84 | |
|---|
| 85 | <h2><a name="easy">Easiest-to-Manipulate Types</a></h2> |
|---|
| 86 | |
|---|
| 87 | <p>The <code>int_fast_t</code> class template maps its input type to the |
|---|
| 88 | next-largest type that the processor can manipulate the easiest, or to |
|---|
| 89 | itself if the input type is already an easy-to-manipulate type. For |
|---|
| 90 | instance, processing a bunch of <code>char</code> objects may go faster |
|---|
| 91 | if they were converted to <code>int</code> objects before processing. |
|---|
| 92 | The input type, passed as the only template parameter, must be a |
|---|
| 93 | built-in integral type, except <code>bool</code>. Unsigned integral |
|---|
| 94 | types can be used, as well as signed integral types, despite the name. |
|---|
| 95 | The output type is given as the class member <code>fast</code>.</p> |
|---|
| 96 | |
|---|
| 97 | <p><strong>Implementation Notes</strong><br> |
|---|
| 98 | By default, the output type is identical to the input type. Eventually, |
|---|
| 99 | this code's implementation should be conditionalized for each platform |
|---|
| 100 | to give accurate mappings between the built-in types and the |
|---|
| 101 | easiest-to-manipulate built-in types. Also, there is no guarantee that |
|---|
| 102 | the output type actually is easier to manipulate than the input |
|---|
| 103 | type.</p> |
|---|
| 104 | |
|---|
| 105 | <h2><a name="sized">Sized Types</a></h2> |
|---|
| 106 | |
|---|
| 107 | <p>The <code>int_t</code>, <code>uint_t</code>, |
|---|
| 108 | <code>int_max_value_t</code>, <code>int_min_value_t</code>, and |
|---|
| 109 | <code>uint_value_t</code> class templates find the most appropiate |
|---|
| 110 | built-in integral type for the given template parameter. This type is |
|---|
| 111 | given by the class member <code>least</code>. The easiest-to-manipulate |
|---|
| 112 | version of that type is given by the class member <code>fast</code>. |
|---|
| 113 | The following table describes each template's criteria.</p> |
|---|
| 114 | |
|---|
| 115 | <table border="1" cellpadding="5"> |
|---|
| 116 | <caption>Criteria for the Sized Type Class Templates</caption> |
|---|
| 117 | <tr> |
|---|
| 118 | <th>Class Template</th> |
|---|
| 119 | <th>Template Parameter Mapping</th> |
|---|
| 120 | </tr> |
|---|
| 121 | <tr> |
|---|
| 122 | <td><code>boost::int_t</code></td> |
|---|
| 123 | <td>The smallest built-in signed integral type with at least the |
|---|
| 124 | given number of bits, including the sign bit. The parameter |
|---|
| 125 | should be a positive number. A compile-time error results if |
|---|
| 126 | the parameter is larger than the number of bits in a |
|---|
| 127 | <code>long</code>.</td> |
|---|
| 128 | </tr> |
|---|
| 129 | <tr> |
|---|
| 130 | <td><code>boost::uint_t</code></td> |
|---|
| 131 | <td>The smallest built-in unsigned integral type with at least |
|---|
| 132 | the given number of bits. The parameter should be a positive |
|---|
| 133 | number. A compile-time error results if the parameter is |
|---|
| 134 | larger than the number of bits in an <code>unsigned |
|---|
| 135 | long</code>.</td> |
|---|
| 136 | </tr> |
|---|
| 137 | <tr> |
|---|
| 138 | <td><code>boost::int_max_value_t</code></td> |
|---|
| 139 | <td>The smallest built-in signed integral type that supports the |
|---|
| 140 | given value as a maximum. The parameter should be a |
|---|
| 141 | positive number.</td> |
|---|
| 142 | </tr> |
|---|
| 143 | <tr> |
|---|
| 144 | <td><code>boost::int_min_value_t</code></td> |
|---|
| 145 | <td>The smallest built-in signed integral type that supports the |
|---|
| 146 | given value as a minimum. The parameter should be a |
|---|
| 147 | negative number.</td> |
|---|
| 148 | </tr> |
|---|
| 149 | <tr> |
|---|
| 150 | <td><code>boost::uint_value_t</code></td> |
|---|
| 151 | <td>The smallest built-in unsigned integral type that supports |
|---|
| 152 | the given value as a maximum. The parameter should be a |
|---|
| 153 | positive number.</td> |
|---|
| 154 | </tr> |
|---|
| 155 | </table> |
|---|
| 156 | |
|---|
| 157 | <h2><a name="example">Example</a></h2> |
|---|
| 158 | |
|---|
| 159 | <blockquote><pre>#include <boost/integer.hpp> |
|---|
| 160 | |
|---|
| 161 | //... |
|---|
| 162 | |
|---|
| 163 | int main() |
|---|
| 164 | { |
|---|
| 165 | boost::int_t<24>::least my_var; |
|---|
| 166 | //... |
|---|
| 167 | } |
|---|
| 168 | </pre></blockquote> |
|---|
| 169 | |
|---|
| 170 | <h2><a name="demo">Demonstration Program</a></h2> |
|---|
| 171 | |
|---|
| 172 | <p>The program <a href="integer_test.cpp">integer_test.cpp</a> is a |
|---|
| 173 | simplistic demonstration of the results from instantiating various |
|---|
| 174 | examples of the sized type class templates.</p> |
|---|
| 175 | |
|---|
| 176 | <h2><a name="rationale">Rationale</a></h2> |
|---|
| 177 | |
|---|
| 178 | <p>The rationale for the design of the templates in this header includes:</p> |
|---|
| 179 | |
|---|
| 180 | <ul> |
|---|
| 181 | <li>Avoid recursion because of concern about C++'s limited |
|---|
| 182 | guaranteed recursion depth (17).</li> |
|---|
| 183 | <li>Avoid macros on general principles.</li> |
|---|
| 184 | <li>Try to keep the design as simple as possible.</li> |
|---|
| 185 | </ul> |
|---|
| 186 | |
|---|
| 187 | <h2><a name="alternative">Alternative</a></h2> |
|---|
| 188 | |
|---|
| 189 | <p>If the number of bits required is known beforehand, it may be more |
|---|
| 190 | appropriate to use the types supplied in <cite><a |
|---|
| 191 | href="../../boost/cstdint.hpp"><boost/cstdint.hpp></a></cite>.</p> |
|---|
| 192 | |
|---|
| 193 | <h2><a name="credits">Credits</a></h2> |
|---|
| 194 | |
|---|
| 195 | <p>The author of most of the Boost integer type choosing templates is <a |
|---|
| 196 | href="../../people/beman_dawes.html">Beman Dawes</a>. He gives thanks |
|---|
| 197 | to Valentin Bonnard and |
|---|
| 198 | <a href="../../people/kevlin_henney.htm"> Kevlin Henney</a> for sharing |
|---|
| 199 | their designs for similar templates. <a |
|---|
| 200 | href="../../people/daryle_walker.html">Daryle Walker</a> designed the |
|---|
| 201 | value-based sized templates.</p> |
|---|
| 202 | |
|---|
| 203 | <hr> |
|---|
| 204 | |
|---|
| 205 | <p>Revised May 20, 2001</p> |
|---|
| 206 | |
|---|
| 207 | <p>© Copyright Beman Dawes 1999. Permission to copy, use, modify, |
|---|
| 208 | sell and distribute this document is granted provided this copyright |
|---|
| 209 | notice appears in all copies. This document is provided "as |
|---|
| 210 | is" without express or implied warranty, and with no claim as to |
|---|
| 211 | its suitability for any purpose.</p> |
|---|
| 212 | </body> |
|---|
| 213 | </html> |
|---|
