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.

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::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

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

See also

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