request for member `…’ is ambiguous in g++

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Looks like your situation is like this:

struct A {
  void f();
};

struct B {
  void f(int);
};

struct C : A, B { };

int main() { 
  C c; 
  c.B::f(1); // not ambiguous
  c.f(1);    // ambiguous
}

The second call to f is ambiguous, because in looking up the name, it finds functions in two different base class scopes. In this situation, the lookup is ambiguous – they don’t overload each other. A fix would be to use a using declaration for each member name. Lookup will find names in the scope of C and don’t lookup further:

struct C : A, B { using A::f; using B::f; };

Now, the call would find two functions, do overload resolution, and find that the one taking int will fit. Carried over to your code, it would mean that you have to do something like the following

struct controller : ISource<const SConsolePacket&>, ISource<const SControlPacket&> {
  using ISource<const SConsolePacket&>::addListener;
  using ISource<const SControlPacket&>::addListener;
};

Now, the two names are in the same scope, and now they can overload each other. Lookup will now stop at the controller class, not diving further into the two base-class branches.

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