# std::is_empty

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< cpp‎ | types

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

If `T` is an empty type (that is, a non-union class type with no non-static data members other than bit-fields of size 0, no virtual functions, no virtual base classes, and no non-empty base classes), provides the member constant `value` equal true. For any other type, `value` is false.

If `T` is a non-union class type, `T` shall be a complete type; otherwise, the behavior is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > inline constexpr bool is_empty_v = is_empty<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is an empty class type , 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>

### Notes

Inheriting from empty base classes usually does not increase the size of a class due to empty base optimization.

std::is_empty<T> and all other type traits are empty classes.

### Example

Run this code

    #include <iostream>
    #include <type_traits>

    struct A {};

    struct B {
        int m;
    };

    struct C {
        static int m;
    };

    struct D {
        virtual ~D();
    };

    union E {};

    struct F {
        [[no_unique_address]] E e;
    };

    int main()
    {
        std::cout << std::boolalpha;
        std::cout << "A " << std::is_empty<A>::value << '\n';
        std::cout << "B " << std::is_empty<B>::value << '\n';
        std::cout << "C " << std::is_empty<C>::value << '\n';
        std::cout << "D " << std::is_empty<D>::value << '\n';
        std::cout << "E " << std::is_empty<E>::value << '\n';
        std::cout << "F " << std::is_empty<F>::value << '\n'; // the result is ABI-dependent

    }

Possible output:

    A true
    B false
    C true
    D false
    E false
    F true

### See also

is_class (C++11) | checks if a type is a non-union class type (class template)

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