Re: Accessing hidden members of a class (as in x.A::B::y instead of x.y)

"Alf P. Steinbach" <>
Sun, 27 Jul 2008 13:07:07 +0200
* James Kanze:

On Jul 26, 7:25 pm, Rajib <> wrote:

If I have the following code

class B {
        public: int b;

class C : public B {};

class E : public C, public B {};

int main() {
        E x;
        C y;
        x.C::B::b++; //test3.cpp:14: error: ?B? is an ambiguous base of ?E?
        return 0;

Why is this ambiguous? I thought the compiler would look up C
(and find it unambiguously) and then use that to look up C::B
(again finding it unambiguously).

So did I, and my first reaction is to say that you'd encountered
a compiler error. In the expression x.C::B::b++, B and b should
be looked up using qualified name lookup, which finds the B in
C, and the b in B. VC++ accepts this, and I seem to recall
other compilers in the past accepting it as well (but it is a
very distant past, so I'm not really sure). G++ complains as
written (which I think is an error),

No, it's probably correct; see my reply else-thread.

Comeau C/C++ (May 29 2008 09:37:15) for ONLINE_EVALUATION_BETA1
Copyright 1988-2008 Comeau Computing. All rights reserved.
MODE:strict errors C++ C++0x_extensions

"ComeauTest.c", line 12: error: base class "B" is ambiguous
       x.C::B::b++; //test3.cpp:14: error: ?B? is an ambiguous base of ?E?

but accepts x.C::b++.

This seems to be correct.

think that the consecrated solution here would be to cast this
to the target type: "static_cast< B* >( static_cast< C* >( &x )
)->b++" also works with g++.

See my reply else-thread.

Regardless, you don't want a hierarchy like this, since there is
no way to unambiguously refer to the B direct base of E (and
both g++ and VC++ warn about this).

Right, I forgot to mention that! :-)


- Alf

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