23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.2 Associated types [iterator.assoc.types]

23.3.2.2 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 type

iter_value_t<R>

be defined as the indirectly readable type's value type.

template<class> struct cond-value-type { };     // exposition only
template<class T>
  requires is_object_v<T>
struct cond-value-type<T> {
  using value_type = remove_cv_t<T>;
};

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<class T>
  requires requires { typename T::value_type; }
struct indirectly_readable_traits<T>
  : cond-value-type<typename T::value_type> { };

template<class T>
  requires requires { typename T::element_type; }
struct indirectly_readable_traits<T>
  : cond-value-type<typename T::element_type> { };

template<class T> using iter_value_t = see below;

Let

R_{I}

be remove_cvref_t<I>.

The type iter_value_t<I> denotes

(2.1)
indirectly_readable_traits< $R_{I}$ >::value_type if iterator_traits< $R_{I}$ > names a specialization generated from the primary template, and
(2.2)
iterator_traits< $R_{I}$ >::value_type otherwise.

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

[ Note

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

These types are not indirectly_readable and have no associated value types.

— end note

]

[ Note

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

]