To ensure that the addresses of two different objects will be different. For the same reason, "new" always returns pointers to distinct objects. Consider:
class Empty { };
void f()
{
Empty a, b;
if (&a == &b) cout << "impossible: report error to compiler supplier";
Empty* p1 = new Empty;
Empty* p2 = new Empty;
if (p1 == p2) cout << "impossible: report error to compiler supplier";
}
There is an interesting rule that says that an empty base class need not be represented by a separate byte:
struct X : Empty {
int a;
// ...
};
void f(X* p)
{
void* p1 = p;
void* p2 = &p->a;
if (p1 == p2) cout << "nice: good optimizer";
}
This optimization is safe and can be most useful. It allows a programmer to use empty classes to represent very simple concepts without overhead. Some current compilers provide this "empty base class optimization".
