Defined in header <type_traits>
template< class T >
struct rank;
(since C++11)

If T is an array type, provides the member constant value equal to the number of dimensions of the array. For any other type, value is ​0​.

The behavior of a program that adds specializations for rank or rank_v (since C++17) is undefined.

Helper variable template

template< class T >
inline constexpr std::size_t rank_v = rank<T>::value;
(since C++17)

Inherited from std::integral_constant

Member constants

the number of dimensions of T or zero
(public static member constant)

Member functions

operator std::size_t
converts the object to std::size_t, returns value
(public member function)
returns value
(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>
struct rank : public std::integral_constant<std::size_t, 0> {};
template<class T>
struct rank<T[]> : public std::integral_constant<std::size_t, rank<T>::value + 1> {};
template<class T, std::size_t N>
struct rank<T[N]> : public std::integral_constant<std::size_t, rank<T>::value + 1> {};


#include <type_traits>
int main()
            std::rank<int>{} == 0
        &&  std::rank<int[5]>{} == 1
        &&  std::rank<int[5][5]>{} == 2
        &&  std::rank<int[][5][5]>{} == 3 );
    [[maybe_unused]] int ary[][3] = {{1, 2, 3}};
    // The reason of rank of "ary[0]" is calculated as 0
    static_assert(std::rank_v<decltype(ary[0])> == 0);
    // is that rank cannot deal with reference type. i.e. int(&)[3]
    static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
    // The solution is to remove reference type
    static_assert(std::rank_v<std::remove_cvref_t<decltype(ary[0])>> == 1);

See also

checks if a type is an array type
(class template)
obtains the size of an array type along a specified dimension
(class template)
removes one extent from the given array type
(class template)
removes all extents from the given array type
(class template)

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