Re: Strange behavior with templates and G++

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On 17 February 2011 13:16, Axel Freyn wrote:
> 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

Right, in that case name lookup finds two overloads of "Test" without
looking in any base classes, then overload resolution selects the
right one.