Lecture 17

Dynamic method dispatch

e.g., 

class Oval {
  public Oval() {  }
  public String getShape () { return "Oval"; }
}

class Circle extends Oval {
  public Circle() {  }
  public String getShape() { return "Circle"; }
}

 class test {

  static void printShape(Oval x) {
     System.out.println (x.getShape());
  }

  public static void main (String [] args) {
     Oval x; 
     if (args[0].equals("circle") {
        x = new Circle();
     } else {
        x= new Oval();
     }

     printShape (x); // which class x is cannot be determined at complie time
  }
}


what is polymorphism?
what is overloading?
what is overriding?


To implement dynamic method binding



     Oval               vtable
    _______         ___________ 
    |      | -----> | getShape | --->
    |------|        |----------|
    |      |        |          |
    |      |        |          |
    |      |        |          |
    -------         -----------


  r1 := f 
  r2 := *r1
  r2 := * (r2 + (3-1) *4)
  call *r2       ^     ^
                 |     |________________ each entry occupies 4 bytes
                 |
                 3rd entry in vtable


multiple inheritance


     A        B
     ^        ^
      \      /
       \    /
        \  /
         \/
        
          C

class Dog {
    color
    Hairness
    print() {   }
}

class Guard {
    HoursOnDuty
    print () {   }
}

class GuardDog : public Guard, public Dog {
}


x = new GuardDog ()
x.print();  // which print() to call?


class GuardDog : public Guard, public Dog {

  Dog::print (); // in C++ we can specify which print() to inherit

}


class Guard {

   virtual print() {   }

}

class Dog {

   virtual print() {   }

}

class GuardDog {

   print() { ....}

}


Offset of Dog view is known at compile time

           GuardDog
 ----->  |---------|
Guard    |         |-----> vtable (of Guard)
view     |---------|
         |         |
         |         |
         |---------|
 ----->  |         |-----> vtable (of Dog)
 Dog     |---------|
 view    |         |
         |---------|
         |guarddog |
         | data    |
         |---------|



Shared multiple inheritance (Figure 10.9 at page 573)



"mix-in" inheritance: (Java)