Allow other base class to implement a virtual function

Question

Is it possible to do the following:
My base class has 3 pure virtual functions. My derived class implements 2 of these virtual functions and inherits from another class that implements the final 3rd virtual function.

My current code won't compile so I am thinking this is not valid? Although it would be great if I could somehow use this approach. Below is my practical application/use of this approach.

Any suggestions of a different approach I could use to achieve this functionality?

class ListBox
{
public:
    virtual void onScroll() = 0;
    virtual void foo() = 0;
    virtual void bar() = 0;
};

class DragScrollHandler
{
public:
    void onScroll()
    {
        ...
    }
};

class HorzListBox: public ListBox, public DragScrollHandler // could also do public HoverScrollHandler, etc.
{
public:
    void foo() override
    {
        printf("foo\n");
    }

    void bar() override
    {
        printf("foo\n");
    }

    HorzListBox()
        : ListBox(), DragScrollHandler()
    {

    }
};

Answer 1

ListBox and DragScrollHandler are a priori different concepts, so there is no kind of link across the multiple inheritance. I mean it is not automatic to set a link in between the virtual onScroll inherited in some way, and an implementation of onScroll on another unrelated (to the other) branch.

You can define it on HorzListBox to call the inherited implementation:

void onScroll() override {
   DragScrollHandler::onScroll();
}

This would fulfill the contract: implement the abstraction and use the inherited implementation.

But may be the best way (in your case) is to separate concepts and have a Scrollable:

class Scrollable {
public: 
  virtual void onScroll()=0;
};

class ListBox: virtual public Scrollable {
public:
  virtual void foo()=0;
};

class DragScrollHandler: virtual public Scrollable {
public:
  void onScroll() override {...}
};

class HorzListBox: public ListBox, public DragScrollHandler {
public:
  void foo() override {...}
};