NAME¶

std::is_empty - std::is_empty

Synopsis¶

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

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¶

T - a type to check

Helper variable template

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

Inherited from std::integral_constant

Member constants¶

value true if T is an empty class type, false otherwise
[static] (public static member constant)

Member functions¶

operator bool converts the object to bool, returns value
(public member function)
operator() returns value
(C++14) (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 {};
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:¶

F: true

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 the base characteristic is
T is an incomplete union type std::false_type in this case

See also¶

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