# std::rank

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< cpp‎ | types

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

Run this code

    #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);
    }

### 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)

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