virtual base class..

Virtual Functions (I)
 Member function preceded by keyword ‘virtual’ in base class
and overridden in derived class
 If object of base class invokes virtual function, then copy of
base class is invoked and if derived class object invokes it,
then copy of derived class is invoked.
 Virtual functions are declared to specify late binding.
 When base class pointer points at derived class object, c++
determines which copy to be called depending upon the type
of the object at run time
 They are resolved at run time not at compile time
Virtual Functions (II)
 General rules while defining virtual function:
 Must be member of some class
 Accessed using object pointers
 Can be friend of another class
 Prototype of base class and derived class virtual function must
be identical
 No need to use keyword ‘virtual’ in definition if its is defined
outside the class
 Can not be a static member
EXAMPLES:
EX 1:
#include <iostream.h>

class B
{
public:
virtual void display()
{ cout<<"Content of base class.\n"; }
};

class D : public B
{
public:
       void display()
{ cout<<"Content of derived class.\n"; }
};

int main()
{
B *b;
D d;
// b->display();

b = &d;    /* Address of object d in pointer variable */
b->display();
return 0;

}
exp 2:

#include <iostream.h>
class B
{
public:
 virtual void display()      /* Virtual function */
{ cout<<"Content of base class.\n"; }
};

class D1 : public B
{
public:
void display()
{ cout<<"Content of first derived class.\n"; }
};

class D2 : public B
{
public:
void display()
{ cout<<"Content of second derived class.\n"; }
};

int main()
{
B *b;
D1 d1;
D2 d2;

/* b->display();  // You cannot use this code here because the function of base class is virtual. */

b = &d1;
b->display();   /* calls display() of class derived D1 */
b = &d2;
b->display();   /* calls display() of class derived D2 */


}
exp 3:
#include<iostream.h>
#include<conio.h>
class student
{            protected: int rn;
                public:
                       void getrn(int a)
                       {
                            rn=a;
                       }
                       void showrn()
                       {
                            cout<<"rollno="<<rn<<"\n";
                       }
};
class test:virtual public student
{
      protected:
      float part1,part2;
      public:
             void getmarks(float a,float b)
             {
                  part1=a;
                  part2=b;
                  }
                  void showmarks()
                  {
                       cout<<"marks 1"<<part1<<"marks2"<<part2;
                  }
};
class sports: virtual public student
{
      protected:
      float score;
      public:
             void getscore(float a)
             {
                  score=a;
                  }
                  void showscore()
                  {
                       cout<<"sports="<<score;
                  }
                  };
class result:public test,public sports
                  {
                        float total;
                        public:
                               void display()
                               {
                                    total=part1+part2+score;
                                    showrn();
                                    showmarks();
                                    showscore();
                                    cout<<"total="<<total;
                                    }
                               };
      int main()
      {
          result s;
          s.getrn(10);
          s.getmarks(40,30);
          s.getscore(20);
          s.display();
          getch();
          return 0;
          }