Re: My attempt at a small hierarchy. Why doesn't my code compile?

See example code below. The instantiation of a Concrete object (see
main function) makes the compiler (VS 2003) complain that Concrete is
an abstract class because MethodsA::callA() and MethodsB::callB() are
not defined.
 
Hopefully it's obvious what I'm trying to acheive. Can someone tell me
where I'm going wrong?
 
Thanks,
 
Pete
 
// Low level interface
struct MethodsA
{
   virtual ~MethodsA() {}
   virtual void callA() = 0;
};
 
// Another low level interface
struct MethodsB
{
   virtual ~MethodsB() {}
   virtual void callB() = 0;
};
 
// Higher level interface - the main base class of concrete objects
struct MainBase
   : public MethodsA,
        public MethodsB
{
};
 
// A suggested implementation of a first low level interface
// Can be used as a mixin to easily create concrete objects
class ImplOfAMixin : public MethodsA
{
   void callA() {}
};
 
// A suggested implementation of a second low level interface
// Can be used as a mixin to easily create concrete objects
class ImplOfBMixin : public MethodsB
{
   void callB() {}
};
 
// Concrete class
class Concrete
   : public MainBase, // "isa" main base
        public ImplOfAMixin, // pull in this mixin
        public ImplOfBMixin // and this mixin
{
};
 
int main()
{
   Concrete obj;
 
   return 0;
}