accessing base class in multiple inheritance

[Robert Swan]

Given:

struct a {
int db;
};

class b: public a{};
class c: public a{};

struct d: public b, public c {
int& dbb() {return b::db;}
int& dbc() {return c::db;}
};

class e: public d{};
class f: public d{};
class g: public d{};

class h:
public e,
public f,
public g {
void fine();
void not_fine();
};

I'd like to know why g++ accepts

void h::fine() {
e::dbb() = 1;
f::dbc() = 2;
}

but says that 'a' is an ambiguous base in

void h::not_fine() {
e::b::db = 1;
f::c::db = 1;
}

Thanks,
Robert

[Victor Bazarov]

Re: accessing base class in multiple inheritance

Robert Swan wrote:[color=blue]
> Given:
>
> struct a {
> int db;
> };
>
> class b: public a{};
> class c: public a{};
>
> struct d: public b, public c {
> int& dbb() {return b::db;}
> int& dbc() {return c::db;}
> };
>
> class e: public d{};
> class f: public d{};
> class g: public d{};
>
> class h:
> public e,
> public f,
> public g {
> void fine();
> void not_fine();
> };
>
> I'd like to know why g++ accepts
>
> void h::fine() {
> e::dbb() = 1;
> f::dbc() = 2;
> }
>
> but says that 'a' is an ambiguous base in
>
> void h::not_fine() {
> e::b::db = 1;
> f::c::db = 1;
> }[/color]

It is an ambiguous base. Qualifying names doesn't help. In the
end the compiler still tries to take "this" (h*) and convert it to
a*, which it can't due to ambiguity.

The only way to disambiguate is to tell which part of '*this' you
intend on using:

void h::not_fine() {
static_cast<e*> (this)->b::db = 1;
static_cast<f*> (this)->c::db = 1;
}

V

[Greg Schmidt]

Re: accessing base class in multiple inheritance

FWIW, VC7.1 seems to have no complaints about your example.

--
Greg Schmidt gregs@trawna.co m
Trawna Publications http://www.trawna.com/