BUG: Function that Inherits Through Dominance Fails

MORE INFORMATION

Suppose a virtual-base class A is defined and classes B and C inherit from class A. Furthermore, a class E inherits from both B and C. The two classes, A and B, each provide a virtual user-defined conversion function to "int". The relationship may be represented by the following directed acyclic graph:

                         A int () }
                          ^      ^
                         /        \ 
                        /          \ 
                  B { int () }    C {}
                       ^           ^
                         \        / 
                          \      / 
                           E { }
				

The dominance rule requires that when you invoke the virtual user-defined conversion function to int from an instance of an object of type E, the version provided by B should be used. This conversion function may be invoked implicitly by assigning an instance of an object of type E to an int. It may also be invoked explicitly through the use of the "operator int" function.

As demonstrated by the following sample code, the compiler accurately identifies and invokes the correct user-defined conversion function when using the explicit call syntax. However, when attempting to make use of the implicit conversion, the compiler produces an error, citing an ambiguity in its search for the correct user-defined conversion function.

Sample Code

/* Compile options needed: /W3
*/ 
#include <iostream.h>

class A
{
public:
    virtual operator int () { return 2; }
};

class B : virtual public A
{
public:
    virtual operator int () { return 3; }
};

class C : virtual public A {};

class E : public B, public C {};

void main(void)
{
   int i;
   int error=0;

   E eObject;

   // Both of the invocations below should call B's
   // operator int ().

   if ((i = eObject) != 3)                // Fails to compile
      error = 1;

   if ((i = eObject.operator int()) != 3) // Compiles OK
      error = 1;

   if (!error)
      cout << "PASSED" << endl;
   else
      cout << "FAILED" << endl;
}