Re: Strange behavior with templates and G++

[Axel Freyn <axel-freyn@gmx.de>]

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:
> >        Super(void) {}
> >        void Test(C*) {}
> > };
> >
> > class A
> > {
> > public:
> >        A(void) {}
> > };
> >
> > class A1 : public A
> > {
> > public:
> >        A1(void) : A() {}
> > };
> >
> > class A2 : public A
> > {
> > public:
> >        A2(void) : A() {}
> > };
> >
> > class Super2 : public Super<A1>, public Super<A2>
> > {
> > public:
> >        Super2(void) {}
> > };
> >
> > void Test(void)
> > {
> >        Super2 T;
> >        A1* ptr;
> >        T.Test(ptr);
> > }
> >
> >
> > The compiler gives me the following error for the Test() function:
> >        error: request for member ‘Test’ is ambiguous
> >        error: candidates are:  void Super<C>::Test(C*) [with C = A2]
> >        error:                          void 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:
> >        T.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