Double dispatch without knowing the full hierarchy

Question

I would like to implement the following thing in C++:

I would like to have a bunch of child classes of a single class with the ability to call a function that takes a pair of objects of any of these types. There is supposed to be a generic implementation that is called for mixed types or the base type and specialised implementations which get called if two objects of the same derived type are used as arguments.

As far as I know, this is a classic application of double dispatch. However, I have the following constraint:

It must be possible to derive new classes from the existing ones and add new pair-functions for these new classes without changing existing classes, for instance in an external library..

The approach I proposed in my last question is faulty, and the solution proposed there only works for types that are known at the time when the base class is written.

Any suggestion on how to implement this? Is that even possible?

Update: Code says more than a thousand words. The following approach works:

#include <iostream>

class B;

class A
{
public:
  virtual void PostCompose(A* other)
    {
      other->PreCompose(this);
    }
  virtual void PreCompose(A* other)
    {
      std::cout << "Precomposing with an A object" << std::endl;
    }
  virtual void PreCompose(B* other);
};

class B : public A
{
public:
  using A::PreCompose;
  virtual void PostCompose(A* other)
    {
      other->PreCompose(this);
    }
  virtual void PostCompose(B* other)
    {
      other->PreCompose(this);
    }
  virtual void PreCompose(B* other)
    {
      std::cout << "Precomposing with a B object" << std::endl;
    }
};

void A::PreCompose(B* other)
  {
    PreCompose((A*)other);
  }

int main()
{
  B b;
  A* p = &b;
  p->PostCompose(p); // -> "Precomposing with a B object"
}

but it requires knowledge of B when implementing A. Is there a better way?

Answer 1

Since the derived classes only need to detect if the parameter type matches the object type, you can just use a straightforward check.

virtual void foo( base *argument_base ) {
    if ( derived *argument = dynamic_cast< derived * >( argument_base ) ) {
        argument->something = pair_match_foo;
    } else {
        base_class::foo( argument_base );
    }
}