On Thu, Feb 17, 2011 at 12:22:53PM +0000, Jonathan Wakely wrote:
> [Moved from the gcc list]
>
> On 17 February 2011 11:03, Pascal Francq wrote:
> > Hi,
> > While compiling the following code, I got an error :
> >
> >
> > template<class C> class Super
> > {
> > public:
> > Â Â Â ÂSuper(void) {}
> > Â Â Â Âvoid Test(C*) {}
> > };
> >
> > class A
> > {
> > public:
> > Â Â Â ÂA(void) {}
> > };
> >
> > class A1 : public A
> > {
> > public:
> > Â Â Â ÂA1(void) : A() {}
> > };
> >
> > class A2 : public A
> > {
> > public:
> > Â Â Â ÂA2(void) : A() {}
> > };
> >
> > class Super2 : public Super<A1>, public Super<A2>
> > {
> > public:
> > Â Â Â ÂSuper2(void) {}
> > };
> >
> > void Test(void)
> > {
> > Â Â Â ÂSuper2 T;
> > Â Â Â ÂA1* ptr;
> > Â Â Â ÂT.Test(ptr);
> > }
> >
> >
> > The compiler gives me the following error for the Test() function:
> > Â Â Â Âerror: request for member âTestâ is ambiguous
> > Â Â Â Âerror: candidates are: Âvoid Super<C>::Test(C*) [with C = A2]
> > Â Â Â Âerror: Â Â Â Â Â Â Â Â Â Â Â Â Âvoid Super<C>::Test(C*) [with C = A1]
> > But here the call refers clearly to the second method. The error still appears
> > if A1 and A2 do not inherit from a same root class. If I replace the code with
> > an explicit call it works:
> > Â Â Â ÂT.Super<A1>::Test(ptr)
> > But this make the code less cleaner.
> >
> > Is this a problem related to a misunderstood concept from me, a wrong
> > implementation or a technical problem of g++ ?
>
>
> The ambiguity has nothing to do with templates, the code can be reduced to:
>
> class A1
> {
> public:
> void Test(A1*) {}
> };
>
> class A2
> {
> public:
> void Test(A2*) {}
> };
>
> class Super2 : public A1, public A2
> {
> };
>
> void Test(void)
> {
> Super2 T;
> A1* ptr;
> T.Test(ptr);
> }
>
> This is covered by 10.2 [class.member.lookup] in the C++ standard,
> which says that a name is ambiguous if found in more than one base
> class. That makes the lookup ill-formed, before overload resolution
> is attempted.
In addition: it is not necessary to use everywhere an explicit call.
You can import the function by adding "using"-definitions. With:
class Super2 : public A1, public A2
{
public:
using A1::Test;
using A2::Test;
};
you can call T.Test(ptr) directly, and C++ will do correct overload
resolution -- both with and without templates
HTH,
Axel
