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 to 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.

The behavior of a program that adds specializations for is_empty or is_empty_v (since C++17) 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

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)
returns value
(public member function)

Member types

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


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.


#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;
struct G {
    // C++ standard allow "as a special case, an unnamed bit-field with a width of zero 
    // specifies alignment of the next bit-field at an allocation unit boundary.
    // Only when declaring an unnamed bit-field may the width be zero."
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
    std::cout << "G " << std::is_empty<G>::value << '\n'; // unnamed bit-fields of width of 0

Possible output:

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

See also

checks if a type is a non-union class type
(class template)

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