Allows the size of an empty base subobject to be zero.

[edit]Explanation

The size of anyobject or member subobject is required to be at least 1 even if the type is an emptyclass type (that is, a class or struct that has no non-static data members),(unless with[[no_unique_address]], see below)(since C++20) in order to be able to guarantee that the addresses of distinct objects of the same type are always distinct.

However, base class subobjects are not so constrained, and can be completely optimized out from the object layout:

struct Base{};// empty class struct Derived1: Base{int i;}; int main(){// the size of any object of empty class type is at least 1    static_assert(sizeof(Base)>=1); // empty base optimization applies    static_assert(sizeof(Derived1)== sizeof(int));}

Empty base optimization is prohibited if one of the empty base classes is also the type or the base of the type of the first non-static data member, since the two base subobjects of the same type are required to have different addresses within the object representation of the most derived type.

A typical example of such situation is the naive implementation ofstd::reverse_iterator (derived from the empty basestd::iterator), which holds the underlying iterator (also derived fromstd::iterator) as its first non-static data member.

struct Base{};// empty class struct Derived1: Base{int i;}; struct Derived2: Base{    Base c;// Base, occupies 1 byte, followed by padding for iint i;}; struct Derived3: Base{    Derived1 c;// derived from Base, occupies sizeof(int) bytesint i;}; int main(){// empty base optimization does not apply,// base occupies 1 byte, Base member occupies 1 byte// followed by 2 bytes of padding to satisfy int alignment requirements    static_assert(sizeof(Derived2)==2*sizeof(int)); // empty base optimization does not apply,// base takes up at least 1 byte plus the padding// to satisfy alignment requirement of the first member (whose// alignment is the same as int)    static_assert(sizeof(Derived3)==3*sizeof(int));}

struct Empty{};// empty class struct X{int i;[[no_unique_address]] Empty e;}; int main(){// the size of any object of empty class type is at least 1    static_assert(sizeof(Empty)>=1); // empty member optimized out:    static_assert(sizeof(X)== sizeof(int));}

[edit]Notes

Empty base optimization is commonly used by allocator-aware standard library classes (std::vector,std::function,std::shared_ptr, etc) to avoid occupying any additional storage for its allocator member if the allocator is stateless. This is achieved by storing one of the required data members (e.g.,begin,end, orcapacity pointer for thevector) in an equivalent ofboost::compressed_pair with the allocator.

In MSVC, empty base optimization is not fully compliant with the standard requirements (Why is the empty base class optimization (EBO) is not working in MSVC?).

[edit]References

[edit]External links

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