Boost C++ Libraries

The C++ Standard Library <limits> header supplies a class template numeric_limits<> with specializations for each fundamental type.

For integer types, the interesting members of std::numeric_limits<> are:

static const bool is_specialized;      // Will be true for integer types.
static T min() throw();                // Smallest representable value.
static T max() throw();                // Largest representable value.
static const int digits;               // For integers, the number of value bits.
static const int digits10;             // The number of base 10 digits that can be represented.
static const bool is_signed;           // True if the type is signed.
static const bool is_integer;          // Will be true for all integer types.

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.

The template class integer_traits addresses this problem.

namespace boost {
  template<class T>
  class integer_traits : public std::numeric_limits<T>
  {
  public:
     static const bool is_integral = false;
     //
     // These members are defined only if T is a built-in
     // integal type:
     //
     static const T const_min = implementation-defined;
     static const T const_max = implementation-defined;
  };
}

Template class integer_traits is derived from std::numeric_limits. The primary specialization adds the single bool member is_integral with the compile-time constant value false. However, for all integral types T (std::3.9.1/7 [basic.fundamental]), there are specializations provided with the following compile-time constants defined:

Note: The is_integral flag is provided, because a user-defined integer class should specialize std::numeric_limits<>::is_integer = true, while compile-time constants const_min and const_max are not provided for that user-defined class, unless boost::integer_traits is also specialized.

The program integer_traits_test.cpp exercises the integer_traits class.

Beman Dawes, Ed Brey, Steve Cleary, and Nathan Myers discussed the integer traits idea on the boost mailing list in August 1999.