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89 lines
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<title>integer_traits: Compile-Time Limits for Integral Types</title>
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<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="center" width="277" height="86">Compile-Time Integral
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Type Limits</h1>
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<p>
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The C++ Standard Library <limits> header supplies a class template
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numeric_limits<> with specializations for each fundamental
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type.</p>
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<p>
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For integer types, the interesting members of std::numeric_limits<> are:
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<pre> static const bool is_specialized; // will be true for integers
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static T min() throw();
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static T max() throw();
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static const int digits; // for integers, # value bits
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static const int digits10;
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static const bool is_signed;
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static const bool is_integer; // will be true for integers</pre>
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For many uses, these are sufficient. But min() and max() are problematical because they are not constant expressions
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(std::5.19), yet some usages require constant expressions.
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<p>
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The template class <code>integer_traits</code> addresses this
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problem.
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<h2>Header <code><a href="../../boost/integer_traits.hpp">integer_traits.hpp</a></code> Synopsis</h2>
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<pre>namespace boost {
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template<class T>
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class integer_traits : public std::numeric_limits<T>
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{
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static const bool is_integral = false;
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};
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// specializations for all integral types
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}</pre>
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<h2>Description</h2>
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Template class <code>integer_traits</code> is derived from
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<code>std::numeric_limits</code>. In general, it adds the single
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<code>bool</code> member <code>is_integral</code> with the
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compile-time constant value <code>false</code>. However, for all
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integral types <code>T</code> (std::3.9.1/7 [basic.fundamental]),
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there are specializations provided with the following compile-time
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constants defined:
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<p>
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<table border=1>
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<tr><th>member</th><th>type</th><th>value</th></tr>
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<tr><td><code>is_integral</code></td><td>bool</td><td><code>true</code></td></tr>
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<tr><td><code>const_min</code></td><td><code>T</code></td><td>equivalent
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to <code>std::numeric_limits<T>::min()</code></td></tr>
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<tr><td><code>const_max</code></td><td><code>T</code></td><td>equivalent
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to <code>std::numeric_limits<T>::max()</code></td></tr>
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</table>
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<p>
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<em>Note:</em> A flag <code>is_integral</code> is provided, because a
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user-defined integer class should specialize
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<code>std::numeric_limits<>::is_integer = true</code>,
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nonetheless compile-time constants <code>const_min</code> and
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<code>const_max</code> cannot be provided for that user-defined class.
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<h2>
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Test Program</h2>
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<p>
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The program <code><a href="integer_traits_test.cpp">integer_traits_test.cpp</a></code>
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exercises the <code>integer_traits</code> class.
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<h2>Acknowledgements</h2>
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Beman Dawes, Ed Brey, Steve Cleary, and Nathan Myers discussed the integer
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traits idea on the boost mailing list in August 1999.
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<hr>
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<a href="../../people/jens_maurer.htm">
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Jens Maurer</a>, 2000-02-20
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