std::extent - cppreference.com (original) (raw)

| | | | | ---------------------------------------------------- | | ------------- | | 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

• 1 Helper variable template
• 2 Inherited from std::integral_constant
  • 2.1 Member constants
  • 2.2 Member functions
  • 2.3 Member types
  • 2.4 Possible implementation
  • 2.5 Example
  • 2.6 See also

[edit] 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

| | the number of elements along the Nth dimension of T (public static member constant) | | -------------------------------------------------------------------------------------- |

Member functions

| | converts the object to std::size_t, returns value (public member function) | | ----------------------------------------------------------------------------------------------- | | | returns value (public member function) |

Member types

[edit] Possible implementation

template<class T, unsigned N = 0> struct extent : std::integral_constant<std::size_t, 0> {};   template 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

#include   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, 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

| | checks if a type is an array type (class template) [edit] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | obtains the number of dimensions of an array type (class template) [edit] | | | removes one extent from the given array type (class template) [edit] | | | removes all extents from the given array type (class template) [edit] | | | a descriptor of a multidimensional index space of some rank (class template) [edit] |