std::experimental::ranges::value_type - cppreference.com

template< class I >
struct value_type {};

template< class T >
struct value_type<T*>;

template< class I >
    requires std::is_array<I>::value
struct value_type<I> : value_type<std::decay_t<I>> {};

template< class T >
struct value_type<const T> : value_type<std::decay_t<T>> {};

template< class T >
    requires requires { typename T::value_type; }
struct value_type<T>;

template< class T >
    requires requires { typename T::element_type; }
struct value_type<T>;

Computes the associated value type of the type I, if any. Users may specialize value_type for a program-defined type.

1) Primary template is an empty struct.

2) Specialization for pointers. If T is an object type, provides a member type type equal to std::remove_cv_t<T>. Otherwise, there is no member type.

3) Specialization for array types.

4) Specialization for const-qualified types.

5) Specialization for types that define a public and accessible member type value_type. If T::value_type is an object type, provides a member type type equal to T::value_type. Otherwise, there is no member type.

6) Specialization for types that define a public and accessible member type element_type (e.g., std::shared_ptr). If T::element_type is an object type, provides a member type type equal to std::remove_cv_t<typename T::element_type>. Otherwise, there is no member type.

Helper alias template

template< class T > 
using value_type_t = typename ranges::value_type<T>::type;

Notes

If a type contains both a value_type member and a element_type member, then the specializations (5) and (6) are ambiguous.

value_type is intended for use with Readable types such as iterators. It is not intended for use with ranges.

Example

See also