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std::is_empty

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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.

Contents

[edit] Template parameters

T - a type to check

[edit] Helper variable template

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

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>

[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

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

[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

(C++11)
 checks if a type is a non-union class type
(class template) [edit]
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