c++ - Why use base class pointers for derived classes

Question

class base{
    .....
    virtual void function1();
    virtual void function2();
};

class derived::public base{
    int function1();
    int function2();
};

int main()
{
    derived d;
    base *b = &d;
    int k = b->function1() // Why use this instead of the following line?
    int k = d.function1(); // With this, the need for virtual functions is gone, right?

}

I am not a CompSci engineer and I would like to know this. Why use virtual functions if we can avoid base class pointers?

Answer 1

The power of polymorphism isn't really apparent in your simple example, but if you extend it a bit it might become clearer.

class vehicle{
      .....
      virtual int getEmission();
 }

 class car : public vehicle{
      int getEmission();
 }

 class bus : public vehicle{
      int getEmission();
 }

 int main()
 {
      car a;
      car b;
      car c;
      bus d;
      bus e;

      vehicle *traffic[]={&a,&b,&c,&d,&e};

      int totalEmission=0;

      for(int i=0;i<5;i++)
      {
          totalEmission+=traffic[i]->getEmission();
      }

 }

This lets you iterate through a list of pointers and have different methods get called depending on the underlying type. Basically it lets you write code where you don't need to know what the child type is at compile time, but the code will perform the right function anyway.