CareerCup

1) user can call some function which is not defined in the base class
2) by the help of casting it is allowed

What else???????

It is illegal to point to an object of the base class with a pointer that is of the derived class type.

Base *b = new Derived(); // compiles properly

Derived object contains Base part and possibly additional things. When you write

Base *b = new Derived();

a Derived object is created and pointer b points to start of Base part. However when you write

Derived *d = new Base();

a Base object is created which has no knowledge about Derived part and don't know where d should point to. So this is invalid.

1. It's not allowed.
2. The problem: 'this' is implicitly passed to every non-static member function of the class. When Derived's member function tries to access Derived's data member through a pointer to a Base object something bad will happen. Derived's part just doesn't exist.
3. Another question is "what about virtual functions?"