std::extent

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

Contents

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)

Member types

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

[edit] 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::integral_constant<std::size_t, std::extent<T, N-1>::value> {};
 
template<class T, std::size_t N>
struct extent<T[N], 0> : std::integral_constant<std::size_t, N> {};
 
template<class T, std::size_t I, unsigned N>
struct extent<T[I], N> : std::integral_constant<std::size_t, std::extent<T, N-1>::value> {};

[edit] Example

#include <iostream>
#include <type_traits>
 
int main()
{
    std::cout << std::extent<int[3]>::value << '\n';
    std::cout << std::extent<int[3][4]>::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';
}

Output:

3
3
4
0
0

[edit] See also

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