25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.2 Associated types [iterator.assoc.types] Incrementable traits [incrementable.traits]

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; }
Let be remove_cvref_t<I>.
The type iter_difference_t<I> denotes
  • incrementable_traits<>​::​difference_type if iterator_traits<> names a specialization generated from the primary template, and
  • iterator_traits<>​::​difference_type otherwise.
Users may specialize incrementable_traits on program-defined types.