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std::is_empty

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

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 a non-union class type, T shall be a complete type; otherwise, the behavior is undefined.

The behavior of a program that adds specializations for is_empty or is_empty_v (since C++17) is undefined.

Template parameters

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

#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> == true ); // unnamed bit-fields of width of 0
 
int main()
{
    std::cout << std::boolalpha;
    std::cout << "F: " << std::is_empty_v<F> << '\n'; // the result is ABI-dependent
}

Possible output:

F: true