How to replace pointers with references in C++?

Question

"I am sure there are tens of questions with the same title. Many of them are duplicate. Mine might be duplicate too, but I couldn't find any. So I try to make it very neat, short and simple."

I have an hierarchy like this:

class Shape {
public:
   virtual void virtualfunc()  { std::cout << "In shape\n"; }
};

class Circle: public Shape {
public:
   void virtualfunc()  { std::cout << "In Circle\n"; };
};

and when I use the classes with the help of pointer, the functions are called as I expected:

int main() {
   Shape shape_instance;
   Shape* ref_shape = &shape_instance ;
   Circle circle_instance;
   Circle* ref_circle = &circle_instance;

   ref_shape = dynamic_cast<Shape*> (ref_circle);
   ref_shape->virtualfunc();
}

Here the program calls the virtualfunc() of the derived class and the result is naturally : In Circle

Now, I want to get rid of the pointers, use references instead, and get the same result. So I make trivial modifications to main() to look like this:

int main() {
   Shape shape_instance;
   Shape& ref_shape = shape_instance;
   Circle circle_instance;
   Circle& ref_circle = circle_instance;

   ref_shape = dynamic_cast<Shape&>(ref_circle);
   ref_shape.virtualfunc();
}

But this time, the program calls the virtualfunc() of the base class and the result is : In Shape

I appreciate if you let me know which concept of the references I am missing and how to change the references in the main() to get the result I got in the pointer version.

thank you

Answer 1

References cannot be reseated. Once you initialize the reference in the initialization, it becomes an alias to the referred object and cannot be distinguished from it. The latter assignment:

ref_shape = dynamic_cast<Shape&>(ref_circle);

really means:

shape_instance = dynamic_cast<Shape&>(ref_circle);

You can, on the other hand, bind a new reference to the object (and you don't need the dynamic_cast, as the conversion from reference to derived to reference to base is implicit):

Shape & another_ref = ref_circle;
another_ref.virtualfunc();          // Dispatches to Circle::virtualfunc