# std::is_unbounded_array

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Defined in header `<type_traits>` |  |
template< class T > struct is_unbounded_array; |  | (since C++20)

`std::is_unbounded_array` is an UnaryTypeTrait.

Checks whether `T` is an array type of unknown bound. Provides the member constant `value` which is equal to true, if `T` is an array type of unknown bound. Otherwise, `value` is equal to false.

The behavior of a program that adds specializations for `std::is_unbounded_array` or `std::is_unbounded_array_v` is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > inline constexpr bool is_unbounded_array_v = is_unbounded_array<T>::value; |  | (since C++20)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is an array type of unknown bound, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Possible implementation

    template<class T>
    struct is_unbounded_array: std::false_type {};

    template<class T>
    struct is_unbounded_array<T[]> : std::true_type {};

### Notes

Feature-test macro
`__cpp_lib_bounded_array_traits`

### Example

Run this code

    #include <iostream>
    #include <type_traits>

    #define OUT(...) std::cout << #__VA_ARGS__ << " : " << __VA_ARGS__ << '\n'

    class A {};

    int main()
    {
        std::cout << std::boolalpha;
        OUT( std::is_unbounded_array_v<A> );
        OUT( std::is_unbounded_array_v<A[]> );
        OUT( std::is_unbounded_array_v<A[3]> );
        OUT( std::is_unbounded_array_v<float> );
        OUT( std::is_unbounded_array_v<int> );
        OUT( std::is_unbounded_array_v<int[]> );
        OUT( std::is_unbounded_array_v<int[3]> );
    }

Output:

    std::is_unbounded_array_v<A> : false
    std::is_unbounded_array_v<A[]> : true
    std::is_unbounded_array_v<A[3]> : false
    std::is_unbounded_array_v<float> : false
    std::is_unbounded_array_v<int> : false
    std::is_unbounded_array_v<int[]> : true
    std::is_unbounded_array_v<int[3]> : false

### See also

is_array (C++11) | checks if a type is an array type (class template)
is_bounded_array (C++20) | checks if a type is an array type of known bound (class template)
extent (C++11) | obtains the size of an array type along a specified dimension (class template)

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