std::extent
Defined in header <type_traits> |
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|---|---|---|
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:
3 3 4 0 0 9 4 0 3
See also
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(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) |
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https://en.cppreference.com/w/cpp/types/extent
