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 array of unknown bound along its first dimension and N is 0, value is 0.
| Contents | 
[edit] 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 ofT(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 <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 }
Output:
3 3 4 0 0 9 4
[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) | 


