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In-Reply-To: <201102171203.53710.pascal@francq.info>
References: <201102171203.53710.pascal@francq.info>
Date: Thu, 17 Feb 2011 13:09:00 -0000
Message-ID: <AANLkTi=MyLQc-PkZ2CjPmbO9Lv1Oia8Dfvmz-SbNgHXa@mail.gmail.com>
Subject: Re: Strange behavior with templates and G++
From: Jonathan Wakely <jwakely.gcc@gmail.com>
To: Pascal Francq <pascal@francq.info>
Cc: gcc-help <gcc-help@gcc.gnu.org>
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[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:
> =A0 =A0 =A0 =A0Super(void) {}
> =A0 =A0 =A0 =A0void Test(C*) {}
> };
>
> class A
> {
> public:
> =A0 =A0 =A0 =A0A(void) {}
> };
>
> class A1 : public A
> {
> public:
> =A0 =A0 =A0 =A0A1(void) : A() {}
> };
>
> class A2 : public A
> {
> public:
> =A0 =A0 =A0 =A0A2(void) : A() {}
> };
>
> class Super2 : public Super<A1>, public Super<A2>
> {
> public:
> =A0 =A0 =A0 =A0Super2(void) {}
> };
>
> void Test(void)
> {
> =A0 =A0 =A0 =A0Super2 T;
> =A0 =A0 =A0 =A0A1* ptr;
> =A0 =A0 =A0 =A0T.Test(ptr);
> }
>
>
> The compiler gives me the following error for the Test() function:
> =A0 =A0 =A0 =A0error: request for member =91Test=92 is ambiguous
> =A0 =A0 =A0 =A0error: candidates are: =A0void Super<C>::Test(C*) [with C =
=3D A2]
> =A0 =A0 =A0 =A0error: =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0=
void Super<C>::Test(C*) [with C =3D A1]
> But here the call refers clearly to the second method. The error still ap=
pears
> if A1 and A2 do not inherit from a same root class. If I replace the code=
 with
> an explicit call it works:
> =A0 =A0 =A0 =A0T.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.