integer traits (is_integer, is_integral)

Question

I need two traits concerning integers.

1. The first one would be like std::is_integral (or boost::is_integral), but usable with user defined types (for example a class wrapping an int, say int_wrapper): true if the type behaves like an integer and whose representation is like standard integral types (e.g. sizeof(T) * CHAR_BITS == std::numeric_limits<T>::digits if T is unsigned) But the definition of an integral type is very rigid, in that it consists of a list of these types. So specializing std::is_integral seems difficult, if not forbidden (though not stated explicitly I think): is_integral is a "primary" type trait (20.7.4.1, note 3: exactly one primary type trait is true for a type T, in my case int_wrapper has already is_class equal to true). What are the risks I take if I specialize this trait for int_wrapper? Do you know of a trait class (e.g. in Boost) that fit my needs?

2. The second trait I need is for types having integer semantics (with bits arithmetic operations, bit manipulations etc.). For example the mpz_class from GMP would satisfy this trait. Is std::numeric_limits<T>::is_integer appropriate for this trait? I read both that it is OK to specialize and set numeric_limits<T>::is_integer == true if T behaves like an integer, but also (in the C++ standard) that the terms "integral" and "integer" are synonymous (in which case we sould always have numeric_limits<T>::is_integer == is_integral<T>::value)

In conclusion, am I better of defining my own traits for my precise needs, or try extending standard ones?

Answer 1

It depends whether you want boost and other standard libraries to treat your class as integral. If so, you have no other way but specialize std/boost::is_integral<>. Otherwise make your own is_integral<> with its default implementation forwarding to std/boost::is_integral<> and specialize it for your integral wrapper.