To implement algorithms only in terms of incrementable types,
it is often necessary to determine the difference type that
corresponds to a particular incrementable type
.Accordingly,
it is required that if
WI is the name of a type that models the
weakly_incrementable concept (
[iterator.concept.winc]),
the type
iter_difference_t<WI>
be defined as the incrementable type's difference type
.
namespace std {
template<class> struct incrementable_traits { };
template<class T>
requires is_object_v<T>
struct incrementable_traits<T*> {
using difference_type = ptrdiff_t;
};
template<class I>
struct incrementable_traits<const I>
: incrementable_traits<I> { };
template<class T>
requires requires { typename T::difference_type; }
struct incrementable_traits<T> {
using difference_type = typename T::difference_type;
};
template<class T>
requires (!requires { typename T::difference_type; } &&
requires(const T& a, const T& b) { { a - b } -> integral; })
struct incrementable_traits<T> {
using difference_type = make_signed_t<decltype(declval<T>() - declval<T>())>;
};
template<class T>
using iter_difference_t = see below;
}