[readable.traits] (original) (raw)

24 Iterators library [iterators]

24.3 Iterator requirements [iterator.requirements]

24.3.2 Associated types [iterator.assoc.types]

24.3.2.2 Indirectly readable traits [readable.traits]

To implement algorithms only in terms of indirectly readable types, it is often necessary to determine the value type that corresponds to a particular indirectly readable type.

Accordingly, it is required that if R is the name of a type that models the indirectly_readable concept ([iterator.concept.readable]), the typeiter_value_t<R> be defined as the indirectly readable type's value type.

template<class> struct cond-value-type { }; template<class T> requires is_object_v<T> struct cond-value-type<T> { using value_type = remove_cv_t<T>;};template<class T> concept has-member-value-type = requires { typename T::value_type; }; template<class T> concept has-member-element-type = requires { typename T::element_type; }; template<class> struct indirectly_readable_traits { };template<class T> struct indirectly_readable_traits<T*> : cond-value-type<T> { };template<class I> requires is_array_v<I> struct indirectly_readable_traits<I> { using value_type = remove_cv_t<remove_extent_t<I>>;};template<class I> struct indirectly_readable_traits<const I> : indirectly_readable_traits<I> { };template<has-member-value-type T> struct indirectly_readable_traits<T> : cond-value-type<typename T::value_type> { };template<has-member-element-type T> struct indirectly_readable_traits<T> : cond-value-type<typename T::element_type> { };template<has-member-value-type T> requires has-member-element-type<T> struct indirectly_readable_traits<T> { };template<has-member-value-type T> requires has-member-element-type<T> && same_as<remove_cv_t<typename T::element_type>, remove_cv_t<typename T::value_type>> struct indirectly_readable_traits<T> : cond-value-type<typename T::value_type> { };template<class T> using iter_value_t = see below;

Let be remove_cvref_t<I>.

The type iter_value_t<I> denotes

• indirectly_readable_traits<>​::​value_typeif iterator_traits<> names a specialization generated from the primary template, and
• iterator_traits<>​::​value_type otherwise.

Class template indirectly_readable_traits may be specialized on program-defined types.

[Note 1:

Some legacy output iterators define a nested type named value_typethat is an alias for void.

— _end note_]

[Note 2:

Smart pointers like shared_ptr<int> are indirectly_readable and have an associated value type, but a smart pointer like shared_ptr<void>is not indirectly_readable and has no associated value type.

— _end note_]