Std::extent (3) Linux Manual Page

std::extent – std::extent

Synopsis

Defined in header<type_traits>

    template <class T, unsigned N = 0>
    (since C++ 11)

        struct extent;

If T is an array type, provides the member constant value equal to the number of elements along the Nth dimension of the array, if N is in [0, std::rank<T>::value). For any other type, or if T is array of unknown bound along its first dimension and N is 0, value is 0.
Helper variable template

template< class T, unsigned N = 0 >                           (since C++17)

inline constexpr std::size_t extent_v = extent<T, N>::value;

Inherited from std::integral_constant

Member constants

value the number of elements along the Nth dimension of T
(public static member constant)
[static]

Member functions

converts the object to std::size_t, returns value
operator std::size_t (public member function)

operator() returns value
(public member function)
(C++14)

Member types

Type Definition
value_type std::size_t
type std::integral_constant<std::size_t, value>

Possible implementation

template<class T, unsigned N = 0>
struct extent : std::integral_constant<std::size_t, 0> {};

template<class T>
struct extent<T[], 0> : std::integral_constant<std::size_t, 0> {};

template<class T, unsigned N>
struct extent<T[], N> : std::extent<T, N-1> {};

template<class T, std::size_t I>
struct extent<T[I], 0> : std::integral_constant<std::size_t, I> {};

template<class T, std::size_t I, unsigned N>
struct extent<T[I], N> : std::extent<T, N-1> {};

Example

// Run this code

#include <iostream>

#include <type_traits>
int main()

{
    std::cout << std::extent<int[3]>::value << '\n'; //< default dimension is 0
    std::cout << std::extent<int[3][4], 0>::value << '\n';
    std::cout << std::extent<int[3][4], 1>::value << '\n';
    std::cout << std::extent<int[3][4], 2>::value << '\n';
    std::cout << std::extent<int[]>::value << '\n';
    const auto ext = std::extent<int[9]>{};
    std::cout << ext << '\n'; //< implicit conversion to std::size_t
    const int ints[] = {1, 2, 3, 4};
    std::cout << std::extent<decltype(ints)>::value << '\n'; //< array size
}

Output:

  3
  3
  4
  0
  0
  9
  4

std::extent (3) Linux Manual Page