🚀 go-pugleaf

solomon@cs.wisc.edu (Marvin Solomon) writes:
: [gjetost:~/persist/CCversion] cat bug.c
: // Bug in g++ 2.2.2.
: // The call f(d) can be resolved either by
: //   Derived -> Derived& (trivial, ARM 13.2, p318)
: //      -> Base& (standard conversion 4.7)

Standard conversion applies only to accessible base classes
(ARM 4.7, p38).


: // or by
: //   Derived -> Base& (user-defined convertion Derived::operator Base&())
: // According to ARM 13.2, p. 318, paragraph [3]
: // "Of sequences not mentined in [2], those with only standard and
: // trivial conversions are better than all others."
: // Thus the user-defined conversion should not be used.
: 
: extern "C" void printf(...);
: 
: class Base {
: public:
: 	int i;
: 	Base(int ii) : i(ii) {};
: };
: 
: class Derived : private Base {
: public:
: 	Derived(int ii) : Base(ii) {}
: 	operator Base&();
: };
: 
: Derived::operator Base&() {
: 	printf("Derived %d -> Base\n", i);
: 	Base *b = new Base(100*i);
: 	return *b;
: }
: 
: void f(Base &b) {
: 	printf("f(%d)\n",b.i);
: }
: 
: main() {
: 	Derived d(99);
: 	f(d);

Base of d is not accessible to f(Base&) and hence, it uses
the user defined conversion.