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 N^th 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.

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()
{
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
);

const auto ext = std::extent<int['*']>{};
std::cout << ext << '\n'; //< 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, 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]>);
static_assert(std::extent_v<decltype(ary[0])> == 0);
// removing reference will give correct extent value 3
static_assert(std::extent_v<std::remove_cvref_t<decltype(ary[0])>> == 3);
}

Output:

42