Empty base optimization

Empty base optimization

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

Explanation

The size of any object or member subobject is required to be at least 1 even if the type is an empty class type (that is, a class or struct that has no non-static data members), 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:

#include <cassert>
 
struct Base {}; // empty class
 
struct Derived1 : Base {
    int i;
};
 
int main()
{
    // the size of any object of empty class type is at least 1
    assert(sizeof	          			          		
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