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.
Contents |
[edit]Helper variable template
template<class T, unsigned N =0> constexprstd::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> |
[edit]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>{}; |
[edit]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(){constauto ext = std::extent<int['*']>{}; static_assert(ext ==42);// with implicit conversion to std::size_t constint 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 of type reference to 'int[3]', so, the extent// cannot be calculated correctly and it returns 0 static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>); static_assert(std::extent_v<decltype(ary[0])>==0); // removing reference gives correct extent value 3 static_assert(std::extent_v<std::remove_cvref_t<decltype(ary[0])>>==3);}
[edit]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) |
(C++23) | a descriptor of a multidimensional index space of some rank (class template) |
