TITLE: Multiple inheritance and same signature virtual functions


JA = jamshid@ses.com (Jamshid Afshar), April 1, 1994
JK = James Kanze <kanze@us-es.sel.de>

[ The sections marked JA?? I think are comments also from Jamshid.
  I had trouble untangling the posting.                    -adc ]


JA:
          struct GunFighter {
                  virtual void draw() { cout << "Bang."; /*...*/; }
                  void get_mad() { draw(); }
          };

          struct Image {
                  virtual void draw() { cout << "Pretty"; /*...*/; }
          };

          class Cartoon : public Image, public GunFighter {};

          main() {
                  Cartoon c;
                  c.get_mad();
 	         // c.draw();   -- error: ambiguous
          }

JA??:

[...]  The above program should execute GunFighter::draw().
You should definitely report a compiler bug if it doesn't.  g++ 2.5.5
and Cfront 3.0.2 do the right thing.

JK:

Why should it prefer GunFighter::draw to Image::draw?  The fact that
the functions are virtual here should not really make a difference,
since there is no pointer or reference involved.  The compiler *knows*
the full type of the object in question.  Any decent compiler should
give an error.

JA:

I think you're missing something.  When I wrote that the above program
should execute GunFighter::draw(), I meant that c.get_mad() is called
in main() and it executes GunFighter::draw().  Do you disagree or were
you confused by the mentioning of the unrelated ambiguity error?

JK:

The problem occurs if I have a reference or a pointer to a Cartoon.
Should the compiler refuse to generate the call because the function
is ambiguous.  A class derived from Cartoon may have overridden
'draw', and it would not be ambiguous.  This is, I think, the
question.  Is such a program illegal (requiring a compiler
diagnostic), or simply undefined (and may be core dumping on
execution). 

JA:

If you have a pointer/reference or an object whose static type is
Cartoon, you cannot call draw() because it is ambiguous.  If you use a
pointer/reference to either base class, you can call draw() and it
chooses the draw() that is in that scope.

	Cartoon sam;
	sam.draw(); // error: ambiguous
	GunFighter& gf = sam;
	gf.draw();  // okay: "bang"

JA??:

Note, if you happened to define a Cartoon::draw() function, it would
override *both* GunFighter::draw() and Image::draw().  I think ARM
10.1.1 and 10.2 discuss this.  

JK:

If I understand the intent of section 10.2, calling 'draw' through a
pointer to GunFighter should get GunFighter::draw, and calling through
a pointer to Image should get Image::draw.  But it is not 100% clear,
since there is much reference to `derived::f', which in this case maps
to Cartoon::draw.  Cartoon::draw is unambiguously ambiguous.

JA:

I can't point to anything that explicitly says GunFighter::get_mad()
would call GunFighter::draw() (as opposed to being undefined), but the
ARM never implies that unrelated classes interfere with each others'
virtual functions.

	10.2: If a class `base' contains a virtual function vf, and a
	class `derived' derived from it also contains a function vf of
	the same type, then [...] the derived class function is said to
	override the base class function.


The January WP is a bit clearer -- it uses the word "declares" instead
of "contains".

I'm not sure what you mean by your last statement that Cartoon::draw
is ambiguous.  If you mean that calling draw() on a Cartoon
object/reference/pointer is an ambiguity error, then I agree.  If you
mean that declaring a function draw() in Cartoon is ambiguous, then
no, I disagree.  There's an example of this in 10.8c (commentary).
It's basically says:

	struct A { virtual void f(); };
	struct B { virtual void f(); };
	struct C : public A, public B { void f(); };
	C* pc = new C;
	B* pb = pc;
	A* pa = pa;

	// all the following invoke C::f()
	pa->f();
	pb->f();
	pc->f();