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 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

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 <iostream>
#include <type_traits>
 
int main()
{
    std::cout << std::rank<int>{} << "\n\n";
 
    std::cout << std::rank<int[5]>{} << '\n';
    std::cout << std::rank<int[5][5]>{} << '\n';
    std::cout << std::rank<int[][5][5]>{} << '\n';
 
    [[maybe_unused]] int ary[][3]={ {1,2,3} };
    // The reason of rank of "ary[0]" is calculated as 0
    std::cout << std::rank<decltype(ary[0])>::value << '\n';
    // 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
    std::cout << std::rank<std::remove_cvref_t<decltype(ary[0])>>::value << '\n';
}

Output:

0
 
1
2
3
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)