NAME¶

std::rank - std::rank

Synopsis¶

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

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.

Member constants¶

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

Member functions¶

operator std::size_t converts the object to std::size_t, returns value
(public member function)
operator() returns value
(C++14) (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 checks if a type is an array type
(C++11) (class template)
extent obtains the size of an array type along a specified dimension
(C++11) (class template)
remove_extent removes one extent from the given array type
(C++11) (class template)
remove_all_extents removes all extents from the given array type
(C++11) (class template)