< cpp‎ | types
Metaprogramming library
Type traits
Type categories
Type properties
(C++11)(until C++20*)
(C++11)(deprecated in C++20)
Type trait constants
Supported operations
Relationships and property queries
Type modifications
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)

(C++11)(until C++20*)(C++17)
Compile-time rational arithmetic
Compile-time integer sequences
Defined in header <type_traits>
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

T - a type to check

[edit] 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>

[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
    // 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

[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]