watch out for return value

virtual_function.cpp

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() = default;
~Circle() = default;
  void draw() {}
  void move() {}
  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();      }