std::extent
From cppreference.com
| 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 an array of unknown bound along its first dimension and N is 0, value is 0.
If the program adds specializations for std::extent or std::extent_v(since C++17), the behavior is undefined.
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
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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> {};
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Example
Run this code
#include <type_traits>
static_assert(
std::extent_v<int[3]> == 3 && // default dimension is 0
std::extent_v<int[3], 0> == 3 && // the same as above
std::extent_v<int[3][4], 0> == 3 &&
std::extent_v<int[3][4], 1> == 4 &&
std::extent_v<int[3][4], 2> == 0 &&
std::extent_v<int[]> == 0
);
int main()
{
const auto ext = std::extent<int['*']>{};
static_assert(ext == 42); // with implicit conversion to std::size_t
const int ints[]{1, 2, 3, 4};
static_assert(std::extent_v<decltype(ints)> == 4); // array size
[[maybe_unused]] int ary[][3] = {{1,