Re: Why is this ambiguous
johnchx2@yahoo.com wrote:
Eric B wrote:
Here is the scenario:
template <typename T>
class Base
{
protected:
Notify(T& msg);
};
struct msgA {};
struct msgB {};
class Impl : public Base<msgA>, public Base<msgB>
{
void Do()
{
msgA a;
Notify(a); // <----- AMBIGUOUS call
}
}
The short answer is "because overload resolution doesn't choose between
names from different scopes." You have two versions of the function
Notify(), but since they live in different scopes (Base<msgA> and
Base<msgB>), overload resolution doesn't apply...and you're stuck with
the ambiguity.
The fix is easy: pull both names into a single scope with using
declarations, like this:
class Impl :
public Base<msgA>, public Base<msgB> {
using Base<msgA>::Notify;
using Base<msgB>::Notify;
void Do()
{
msgA a;
Notify(a);
}
};
Here is my current solution (simplified):
template <typename T>
class Base
{
protected:
int Notify(T& msg);
};
struct nulltypeB {};
struct nulltypeC {};
template < class A, typename B = nulltypeB, typename C = nulltypeC>
class Liaison : public A, public B, public C
{
protected:
// prevent implementation classes from having to use the command
// "using Base<MsgType>::Notify;"
// for each message they implement
template <typename T>
int Notify( T& msg )
{ return Base<T>::Notify( msg ); }
};
struct msgA {};
struct msgB {};
class Impl : public Liason < Base<msgA>, Base<msgB> >
{
void Do()
{
msgA a;
Notify(a);
}
};
Kind of a hack but better than having to write "using" everywhere.
Actually, the driving reason I went to this design is not to get rid of
the "using", but to get rid of an actual ambiguous call... Base
provides begin() and end() to iterate through its connectees. Since
these functions take no parameters, the calls to begin and end were
really and truly ambiguous. They had to be disambiguated like:
iter = pMyOb->Base<msgA>::begin();
I didn't particularly like that, so now with the new design, I can
write
iter = pMyOb->begin<msgA>();
(I have a templatized begin() and end() in Liason similar to Notify)
maybe not much simpler in this sample code but in the real code it
makes for much easier client code.
Eric
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