C++ OOPs Part2

Polymorphism & Overloading

Polymorphism occurs when there is a hierarchy of classes and they are related by inheritance.

C++ polymorphism means that a call to a member function will cause a different function to be executed depending on the type of object that invokes the function.

Static linkage or Early binding or Compile Time Polymorphism

Consider the following example where a base class has been derived by other two classes:

class Shape {
   protected:
      int width, height;
   public:
      Shape( int a=0, int b=0)
      {
         width = a;
         height = b;
      }
      int area()
      {
         cout << "Parent class area :" <<endl;
         return 0;
      }
};
class Rectangle: public Shape{
   public:
      Rectangle( int a=0, int b=0):Shape(a, b) { }
      int area ()
      { 
         cout << "Rectangle class area :" <<endl;
         return (width * height); 
      }
};
class Triangle: public Shape{
   public:
      Triangle( int a=0, int b=0):Shape(a, b) { }
      int area ()
      { 
         cout << "Triangle class area :" <<endl;
         return (width * height / 2); 
      }
};
// Main function for the program
int main( )
{
   Shape *shape;
   Rectangle rec(10,7);
   Triangle  tri(10,5);

   // store the address of Rectangle
   shape = &rec;
   // call rectangle area.
   shape->area();

   // store the address of Triangle
   shape = &tri;
   // call triangle area.
   shape->area();
   
   return 0;
}

When the above code is compiled and executed, it produces the following result:

Parent class area
Parent class area

The reason for the incorrect output is that the call of the function area() is being set once by the compiler as the version defined in the base class. This is called static resolution of the function call, or static linkage – the function call is fixed before the program is executed. This is also sometimes called early binding because the area() function is set during the compilation of the program.

Dynamic linkage or Late binding or Run time Polymorphism.

But now, let’s make a slight modification in our program and precede the declaration of area() in the Shape class with the keyword virtual so that it looks like this:

virtual int area()
      {
         cout << "Parent class area :" <<endl;
         return 0;
      }

After this slight modification, when the previous example code is compiled and executed, it produces the following result:

Rectangle class area
Triangle class area

Virtual Function

A virtual function is a function in a base class that is declared using the keyword virtual. Defining in a base class a virtual function, with another version in a derived class, signals to the compiler that we don’t want static linkage for this function.<br /> <br /> What we do want is the selection of the function to be called at any given point in the program to be based on the kind of object for which it is called. This sort of operation is referred to as dynamic linkage, or late binding.<br /> <br /> <h3 style=” left=”” text-align:=””> Pure Virtual Functions:

class printData 
{
   public:
      void print(int i) {
        cout << "Printing int: " << i << endl;
      }

      void print(double  f) {
        cout << "Printing float: " << f << endl;
      }

      void print(char* c) {
        cout << "Printing character: " << c << endl;
      }
};

int main(void)
{
   printData pd;
 
   // Call print to print integer
   pd.print(5);
   // Call print to print float
   pd.print(500.263);
   // Call print to print character
   pd.print("Hello C++");
 
   return 0;
}