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std::extent

Defined in header `<type_traits>`
template< class T, unsigned N = 0 > struct extent;		(since C++11)

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 an array of unknown bound along its first dimension and N is 0, value is 0.

The behavior of a program that adds specializations for extent or extent_v (since C++17) is undefined.

Helper variable template

template< class T, unsigned N = 0 >
inline constexpr std::size_t extent_v = extent<T, N>::value;

(since C++17)

Inherited from std::integral_constant

Member constants

value

[static]

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

Member functions

operator std::size_t	converts the object to std::size_t, returns `value` (public member function)
operator() (C++14)	returns `value` (public member function)

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

#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 
 
    [[maybe_unused]] int ary[][3] = { {1, 2, 3} };
 
    // ary[0] is type of reference of 'int[3]', so, extent 
    // cannot calculate correctly and return 0
    static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
    std::cout << std::extent<decltype(ary[0])>::value << '\n';
    // removing reference will give correct extent value 3
    std::cout << std::extent<std::remove_cvref_t<decltype(ary[0])>>::value << '\n';
}

Output:

is_array (C++11)	checks if a type is an array type (class template)
rank (C++11)	obtains the number of dimensions of an array type (class template)
remove_extent (C++11)	removes one extent from the given array type (class template)
remove_all_extents (C++11)	removes all extents from the given array type (class template)