IsEmpty determines whether a type is a class (but not a union) that is empty. More...
#include <TypeTraits.h>
Additional Inherited Members | |
 Public Types inherited from mozilla::IntegralConstant< bool, IsClass< RemoveCV< T >::Type >::value &&__is_empty(RemoveCV< T >::Type)> | |
| typedef bool | ValueType |
| typedef IntegralConstant< bool, Value > | Type |
| Static Public Attributes inherited from mozilla::IntegralConstant< bool, IsClass< RemoveCV< T >::Type >::value &&__is_empty(RemoveCV< T >::Type)> | |
| static const bool | value |
Detailed Description
struct mozilla::IsEmpty< T >
IsEmpty determines whether a type is a class (but not a union) that is empty.
A class is empty iff it and all its base classes have no non-static data members (except bit-fields of length 0) and no virtual member functions, and no base class is empty or a virtual base class.
Intuitively, empty classes don't have any data that has to be stored in instances of those classes. (The size of the class must still be non-zero, because distinct array elements of any type must have different addresses. However, if the Empty Base Optimization is implemented by the compiler [most compilers implement it, and in certain cases C++11 requires it], the size of a class inheriting from an empty |Base| class need not be inflated by |sizeof(Base)|.) And intuitively, non-empty classes have data members and/or vtable pointers that must be stored in each instance for proper behavior.
static_assert(!mozilla::IsEmpty<int>::value, "not a class => not empty"); union U1 { int x; }; static_assert(!mozilla::IsEmpty<U1>::value, "not a class => not empty"); struct E1 {}; struct E2 { int : 0 }; struct E3 : E1 {}; struct E4 : E2 {}; static_assert(mozilla::IsEmpty<E1>::value && mozilla::IsEmpty<E2>::value && mozilla::IsEmpty<E3>::value && mozilla::IsEmpty<E4>::value, "all empty"); union U2 { E1 e1; }; static_assert(!mozilla::IsEmpty<U2>::value, "not a class => not empty"); struct NE1 { int x; }; struct NE2 : virtual E1 {}; struct NE3 : E2 { virtual ~NE3() {} }; struct NE4 { virtual void f() {} }; static_assert(!mozilla::IsEmpty<NE1>::value && !mozilla::IsEmpty<NE2>::value && !mozilla::IsEmpty<NE3>::value && !mozilla::IsEmpty<NE4>::value, "all empty");
Definition at line 425 of file TypeTraits.h.
The documentation for this struct was generated from the following file:
