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std::extent(3) | C++ Standard Libary | std::extent(3) |
NAME¶
std::extent - std::extent
Synopsis¶
Defined in header <type_traits>
template< class T, unsigned N = 0> (since C++11)
struct extent;
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 > (since C++17)
inline constexpr std::size_t extent_v = extent<T, N>::value;
Inherited from std::integral_constant
Member constants¶
value the number of elements along the Nth dimension of T
[static] (public static member constant)
Member functions¶
operator std::size_t converts the object to std::size_t, returns
value
(public member function)
operator() returns value
(C++14) (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¶
// 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
[[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¶
is_array checks if a type is an array type
(C++11) (class template)
rank obtains the number of dimensions of an array type
(C++11) (class template)
remove_extent removes one extent from the given array type
(C++11) (class template)
remove_all_extents removes all extents from the given array type
(C++11) (class template)
2022.07.31 | http://cppreference.com |