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| 6 | <title>integer_traits: Compile-Time Limits for Integral Types</title> |
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| 7 | </head> |
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| 9 | <body bgcolor="#FFFFFF" text="#000000"> |
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| 10 | |
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| 11 | <h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align="center" width="277" height="86">Compile-Time Integral |
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| 12 | Type Limits</h1> |
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| 13 | |
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| 14 | <p> |
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| 15 | The C++ Standard Library <limits> header supplies a class template |
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| 16 | numeric_limits<> with specializations for each fundamental |
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| 17 | type.</p> |
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| 18 | <p> |
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| 19 | For integer types, the interesting members of std::numeric_limits<> are: |
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| 20 | <pre> static const bool is_specialized; // will be true for integers |
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| 21 | static T min() throw(); |
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| 22 | static T max() throw(); |
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| 23 | static const int digits; // for integers, # value bits |
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| 24 | static const int digits10; |
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| 25 | static const bool is_signed; |
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| 26 | static const bool is_integer; // will be true for integers</pre> |
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| 27 | For many uses, these are sufficient. But min() and max() are problematical because they are not constant expressions |
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| 28 | (std::5.19), yet some usages require constant expressions. |
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| 29 | <p> |
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| 30 | The template class <code>integer_traits</code> addresses this |
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| 31 | problem. |
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| 32 | |
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| 33 | |
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| 34 | <h2>Header <code><a href="../../boost/integer_traits.hpp">integer_traits.hpp</a></code> Synopsis</h2> |
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| 35 | |
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| 36 | <pre>namespace boost { |
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| 37 | template<class T> |
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| 38 | class integer_traits : public std::numeric_limits<T> |
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| 39 | { |
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| 40 | static const bool is_integral = false; |
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| 41 | }; |
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| 42 | |
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| 43 | // specializations for all integral types |
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| 44 | }</pre> |
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| 45 | |
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| 46 | |
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| 47 | <h2>Description</h2> |
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| 48 | |
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| 49 | Template class <code>integer_traits</code> is derived from |
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| 50 | <code>std::numeric_limits</code>. In general, it adds the single |
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| 51 | <code>bool</code> member <code>is_integral</code> with the |
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| 52 | compile-time constant value <code>false</code>. However, for all |
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| 53 | integral types <code>T</code> (std::3.9.1/7 [basic.fundamental]), |
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| 54 | there are specializations provided with the following compile-time |
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| 55 | constants defined: |
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| 56 | <p> |
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| 57 | <table border=1> |
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| 58 | <tr><th>member</th><th>type</th><th>value</th></tr> |
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| 59 | <tr><td><code>is_integral</code></td><td>bool</td><td><code>true</code></td></tr> |
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| 60 | <tr><td><code>const_min</code></td><td><code>T</code></td><td>equivalent |
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| 61 | to <code>std::numeric_limits<T>::min()</code></td></tr> |
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| 62 | <tr><td><code>const_max</code></td><td><code>T</code></td><td>equivalent |
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| 63 | to <code>std::numeric_limits<T>::max()</code></td></tr> |
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| 64 | </table> |
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| 65 | |
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| 66 | <p> |
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| 67 | |
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| 68 | <em>Note:</em> A flag <code>is_integral</code> is provided, because a |
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| 69 | user-defined integer class should specialize |
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| 70 | <code>std::numeric_limits<>::is_integer = true</code>, |
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| 71 | nonetheless compile-time constants <code>const_min</code> and |
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| 72 | <code>const_max</code> cannot be provided for that user-defined class. |
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| 73 | |
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| 74 | <h2> |
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| 75 | |
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| 76 | Test Program</h2> |
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| 77 | |
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| 78 | <p> |
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| 79 | |
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| 80 | The program <code><a href="integer_traits_test.cpp">integer_traits_test.cpp</a></code> |
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| 81 | exercises the <code>integer_traits</code> class. |
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| 82 | |
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| 83 | <h2>Acknowledgements</h2> |
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| 84 | |
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| 85 | Beman Dawes, Ed Brey, Steve Cleary, and Nathan Myers discussed the integer |
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| 86 | traits idea on the boost mailing list in August 1999. |
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| 87 | <hr> |
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| 88 | <a href="../../people/jens_maurer.htm"> |
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| 89 | Jens Maurer</a>, 2000-02-20 |
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