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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)
    See More

    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 > 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>

    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

    See also

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