std::is_empty
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 > constexprbool 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> |
[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 {staticint 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 ifT 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) |
