std::is_empty

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

std::is_empty is an 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), the base characteristic is std::true_type. For any other type, the base characteristic is std::false_type.

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

The behavior of a program that adds specializations for std::is_empty or std::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

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

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

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