Re: Why doesn't this multiple virtual inheritance code compile?

My intention is to
- Create an abstract base class "Shape" that must be an "Observer"
- Create an class "Square" that is a "Shape" and also an
"ObserverImp"

I thought I could do this like so:

struct Observer
{
 virtual void Notify() = 0;
};

struct ObserverImp : public Observer
{
 void Notify() {}
};

struct Shape : public virtual Observer
{
};

struct Square : public Shape, public ObserverImp
{
};

Shape* ShapeFactory()
{
 return new Square;
}

$ g++ -Wall -c test.cpp
test.cpp: In function 'Shape* ShapeFactory()':
test.cpp:21:14: error: cannot allocate an object of abstract type
'Square'
test.cpp:16:1: note: because the following virtual functions are
pure within 'Square':
test.cpp:3:16: note: virtual void Observer::Notify()
test.cpp:22:1: warning: control reaches end of non-void function

Q: How can I do this using c++?