std::rank

 

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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 std::rank or std::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

value [static]

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)
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>

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> {};

Example

Run this code

#include <type_traits>
 
int main()
{
    static_assert(
            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

is_array (C++11)	checks if a type is an array type (class template)
extent (C++11)	obtains the size of an array type along a specified dimension (class template)
remove_extent (C++11)	removes one extent from the given array type (class template)
remove_all_extents (C++11)	removes all extents from the given array type (class template)