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

Helper variable template

template< class T, unsigned N = 0 >
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::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> {};

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 
}

Output:

3
3
4
0
0
9
4

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)
(library fundamentals TS)
variable template alias of std::extent::value
(variable template)

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