A Base Class pointer can point to a derived class object. Why is the vice-versa not true?

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If I tell you I have a dog, you can safely assume that I have a pet.

If I tell you I have a pet, you don’t know if that animal is a dog, it could be a cat or maybe even a giraffe. Without knowing some extra information you can’t safely assume I have a dog.

similarly a derived object is a base class object (as it’s a sub class), so it can be pointed to by a base class pointer. However, a base class object is not a derived class object so it can’t be assigned to a derived class pointer.

(The creaking you will now hear is the analogy stretching)

Suppose you now want to buy me a gift for my pet.

In the first scenario you know it is a dog, you can buy me a leash, everyone is happy.

In the second scenario I haven’t told you what my pet is so if you are going to buy me a gift anyway you need to know information I haven’t told you (or just guess), you buy me a leash, if it turns out I really did have a dog everyone is happy.

However if I actually had a cat then we now know you made a bad assumption (cast) and have an unhappy cat on a leash (runtime error).

ClassCastException Cat

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