Question

I want to make a class with a member function that takes a reference to another class, where both classes are derived from abstract classes. I get a compiler error that the class Container is abstract because it doesn’t implement addElem().

class Ielem
{
public:
  virtual void action() = 0;
};

class Elem: public Ielem
{
public:
  void action() {};
  void extra() {};
};

class Icontainer
{
public:
  virtual void addElem(Ielem &elem) = 0;
};

class Container: public Icontainer
{
public:
  void addElem(Elem &elem) { elem.extra(); };
};

int main(int argc, char* argv[])
{
  Elem e;
  Container c;
  c.addElem(e);

  return 0;
}

It seems like this ought to work, because any reference to an Elem is also a reference to an Ielem. It compiles if I make Container::addElem take a reference to an Ielem. But then Container::addElem() can’t call Elem::extra() unless I use dynamic_cast, which isn’t available on the embedded compiler I’m using, or a regular cast, which isn’t type safe.

Suggestions?

Answer 1

The problem is simply that your virtual method doesn’t have the same signature as the concrete method which is intended to overload it; so the compiler sees it as a different function entirely and complains because you haven’t implemented void addElem(Ielem &elem). This is one solution, which you probably don’t want–

class Icontainer
{
public:
  virtual void addElem(Elem &elem) = 0;  //Ielem -> Elem
};

It depends on all your other constraints but I think what I would do–and what seems to conform to general design guidelines, e.g. Sutter & Alexandreascu, would be to create an intermediate abstract class with the full interface–

class Melem: public Ielem
{
public:
  // void action() {}; //Already have this form Ielem
  void extra() = 0;
};

and then

class Icontainer
{
public:
  virtual void addElem(Melem &elem) = 0;
};

class Container: public Icontainer
{
public:
  void addElem(Melem &elem) { elem.extra(); }; 
     //*Now* we're implementing Icontainer::addElem
};