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Showing content from https://en.cppreference.com/w/cpp/language/../string/../named_req/../types/is_empty.html below:

std::is_empty - cppreference.com

template< class T > struct is_empty;

(since C++11)

std::is_empty is a UnaryTypeTrait.

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 an incomplete non-union class type, the behavior is undefined.

If the program adds specializations for std::is_empty or std::is_empty_v, the behavior is undefined.

[edit] Template parameters [edit] Helper variable template

template< class T > 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 converts the object to bool, returns value
(public member function) returns value
(public member function) Member types [edit] 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.

[edit] Example

#include <iostream>
#include <type_traits>
 
struct A {};
static_assert(std::is_empty_v<A> == true);
 
struct B { int m; };
static_assert(std::is_empty_v<B> == false);
 
struct C { static int m; };
static_assert(std::is_empty_v<C> == true);
 
struct D { virtual ~D(); };
static_assert(std::is_empty_v<D> == false);
 
union E {};
static_assert(std::is_empty_v<E> == false);
 
struct F { [[no_unique_address]] E e; };
 
struct G
{
    int:0;
    // 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."
};
static_assert(std::is_empty_v<G>); // holds only unnamed bit-fields of zero width
 
int main()
{
    std::cout << std::boolalpha;
    std::cout << "F: " << std::is_empty_v<F> << '\n'; // the result is ABI-dependent
}

Possible output:

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 2015 C++11 the behavior was undefined if
T is an incomplete union type the base characteristic is
std::false_type in this case [edit] See also checks if a type is a non-union class type
(class template) [edit]

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