From cppreference.com
template<classT>structis_empty;(since C++11)std::is_empty is a
.
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<classT>constexprboolis_empty_v=is_empty<T>::value;(since C++17)Inherited from
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_typebooltypestd::integral_constant<bool,value>Notes
Inheriting from empty base classes usually does not increase the size of a class due to
.
std::is_empty<T> and all other type traits are empty classes.
Example
Run this code
#include<iostream>#include<type_traits>structA{};static_assert(std::is_empty_v<A>==true);structB{intm;};static_assert(std::is_empty_v<B>==false);structC{staticintm;};static_assert(std::is_empty_v<C>==true);structD{virtual~D();};static_assert(std::is_empty_v<D>==false);unionE{};static_assert(std::is_empty_v<E>==false);structF{int:0;// C++ standard allows "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<F>);// holds only unnamed bit-fields of zero widthstructG{[[no_unique_address]]Ee;};intmain(){std::cout<<std::boolalpha;std::cout<<"G: "<<std::is_empty_v<G><<'\n';// the result is ABI-dependent}Possible output:
G: 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
C++11 the behavior was undefined if
T is an incomplete union type the base characteristic is
in this case See also
(C++11)
checks if a type is a non-union class type
(class template)
(C++26)
checks if reflected type is a class (but not union) type and has no non-static data members
(function)