W3cubDocs

/C++

std::is_empty

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

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

#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