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From: stanl@cbnewsm.ATT.COM (stanley.b.lippman)
Newsgroups: comp.lang.c++
Subject: virtual base classes
Message-ID: <8059@cbnewsm.ATT.COM>
Date: 27 Dec 89 20:19:54 GMT
Organization: AT&T Bell Laboratories
Lines: 77


*** Shankar Unni writes:

Consider the following situation:

    class A {public: int a_mem;};
    
    class B : virtual public A {};
    
    class C : public A {};
    
    class D : public B, virtual public C {};
    


                A   A       <<< ********** revised drawing
	       /    | 
     virtual  /     | 
	     /      | 
	    B       C
	     \     /
	      \   /  virtual
	       \ /
	        D


Questions:

   (a) How many "A"'s are there in "D"?

*****> two

   (b) If there is more than one, is the following access unambiguous, and
       if so, which "A" does it refer to:  "D::a_mem"

*****> no, it is ambiguous

My copy of cfront (2.0) seems to think that:

   (a) there is only *one* A in D.

*****> no, look again -- there are two, one within the C sub-object
       of D and the virtual OA instance:

struct A {	/* sizeof A == 4 */
int a_mem__1A ;
};

struct B {	/* sizeof B == 8 */
struct A *PA;
struct A OA;
};

struct C {	/* sizeof C == 4 */
int a_mem__1A ;  // members of A unrolled ...
};

struct D {	/* sizeof D == 16 */
struct A *PA;   // members of B unrolled ...
struct C *PC;
struct C OC;	// non-virtual instance of A within OC
struct A OA; 	// virtual instance of A from B
};

   (b) (this is therefore moot).

*****> cfront actually has a bug in that it is
       choosing the instance associated with the
       virtual object associated with B -- it should
       be ambiguous and will be fixed in 2.1

	d . PA-> a_mem__1A = 1 ;  // generated cfront


stan lippman
at&t bell laboratories
warren, new jersey