| 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';
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
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) |
| (C++11) | removes one extent from the given array type (class template) |
| (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) |
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