Why can't reference to child Class object refer to the parent Class object?

Question

I was explaining OOP to my friend. I was unable to answer this question. (How shameful of me? :( )

I just escaped by saying, since OOP depicts the real world. In rea

Answer 1

Think "inheritance" = "specialisation", although this seems counter-intuitive at first. The set of "Childs" is a subset of the set of "Parents" (you see that your example is a bit misleading). It naturally follows that a variable that can 'hold' a "Child" cannot a hold an abitrary member of the "Parent" set because it may not be in it the "Child" subset. On the other way, a variable that can 'hold' a "Parent" can hold every member of the "Child" set.

There seems to be 2 ways of viewing inheritance. On the programming level, "Child" is a superset of "Parent" abilities, as Kornel said. But conceptually, "Child" is a specialisation of "Parent" thus representing only a subset of all possible "Parent". Think for example "Vehicle" and "Car": a Car is a special Vehicle. The set of all Cars is still a subset of all vehicles. You may even "do more" with a Car than with a general Vehicle (e.g. changing tires, fill in gasoline etc.) but it's still a Vehicle.