std::is_empty - cppreference.com

From 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.

Template parameters

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

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 {};
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:

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