std::rank

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.

If the program adds specializations for std::rank or std::rank_v(since C++17), the behavior is undefined.

Helper variable template

template< class T >
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

#include <type_traits>

static_assert(std::rank<int>{} == 0);
static_assert(std::rank<int[5]>{} == 1);
static_assert(std::rank<int[5][5]>{} == 2);
static_assert(std::rank<int[][5][5]>{} == 3);

int main()
{
    [[maybe_unused]] int ary[][3] = {{1, 2, 3}};

    // The rank of reference type, e.g., ary[0], that is int(&)[3], is 0:
    static_assert(std::rank_v<decltype(ary[0])> == 0);
    static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);

    // The solution is to remove the reference type.
    static_assert(std::rank_v<std::remove_cvref_t<decltype(ary[0])>> == 1);
}

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)
rank (C++26)	obtains the number of dimensions of reflected array type (function)

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