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