TITLE: Different object identities in multiple inheritance


PROBLEM: A reason to avoid conventional multiple inheritance


RESPONSE: Scott Meyers, "Effective C++", p 57,58.

"The minute you start to use multiple inheritance, you must tell yourself
for the fact that a single object can have more than one address, regardless
of whether you are using virtual or nonvirtual base classes. Here is an
example,

class A {...};
class B : public A {...};
class C : public A {...};
class D : public B, public C {...};


If you create an object of class D, that object may have up to five
different addresses, depending upon how its used:

D d;				// create object of class D

D* p1 = &d;			// p1 is address of D part of d

B* p2 = &d;			// p2 is address of B part of d

C* p3 = &d;			// p3 is address of C part of d

A* p4 = (B*) &d;		// p4 is address of A part of B part of d

A* p5 = (C*) &d;		// p5 is address of A part of C part of d

Each of the five pointers [p1 - p5 ] points to the same object - d - but
there is no guarantee that any two of the pointers will have the same value.
In fact, its a sure bet that the two different A parts of d will have
different addresses. If A were a virtual base class, then d might have only
four different addresses, but clearly that wouldn't help the matter of object
identity any."