covariant return types

[Comp1597]

The following example of covariant return types is on the faq: I do
feel that I understand this particular example.

I would like to know the general conditions necessary and sufficient
for covariant return types to work.
I've googled a bit but haven't found a newbie-friendly account.

Thanks.

class Shape {
public:
virtual ~Shape() { } // A virtual destructor
virtual void draw() = 0; // A pure virtual function
virtual void move() = 0;
...
virtual Shape* clone() const = 0; // Uses the copy constructor
virtual Shape* create() const = 0; // Uses the default
constructor
};

class Circle : public Shape {
public:
Circle* clone() const; // Covariant Return Types; see below
Circle* create() const; // Covariant Return Types; see below
...
};

Circle* Circle::clone() const { return new Circle(*this); }
Circle* Circle::create() const { return new Circle(); }