cppdraft_translate/cppdraft/incrementable/traits.md

[incrementable.traits]

24 Iterators library [iterators]

24.3 Iterator requirements [iterator.requirements]

24.3.2 Associated types [iterator.assoc.types]

24.3.2.1 Incrementable traits [incrementable.traits]

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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 theweakly_incrementable concept ([iterator.concept.winc]), the typeiter_difference_t be defined as the incrementable type's difference type.

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namespace std {template struct incrementable_traits { }; templaterequires is_object_vstruct incrementable_traits<T*> {using difference_type = ptrdiff_t; }; templatestruct incrementable_traits: incrementable_traits { }; templaterequires requires { typename T::difference_type; }struct incrementable_traits {using difference_type = typename T::difference_type; }; templaterequires (!requires { typename T::difference_type; } &&requires(const T& a, const T& b) { { a - b } -> integral; })struct incrementable_traits {using difference_type = make_signed_t<decltype(declval() - declval())>; }; templateusing iter_difference_t = see below;}

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Let RI be remove_cvref_t.

The type iter_difference_t denotes

• (2.1)

incrementable_traits​::​difference_type if iterator_traits names a specialization generated from the primary template, and

• (2.2)

iterator_traits​::​difference_type otherwise.

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Users may specialize incrementable_traits on program-defined types.