What types of A and B is the simple assignment statement A equals B legal in C plus plus?

Answer 1

A and B can be any type, even different types, so long as one can be statically cast to the same type as the other. Thus A can be the same class as B, or A can be a base class of B. But if B is not derived from A, then B must implement a static cast to type A. If such a cast would make no logical sense, then A=B makes no sense either, and would therefore be an illegal assignment.

Illegal assignments are not a bad thing as it actively prevents people from doing silly things. Twisting the logic in order to make an assignment legal won't necessarily make the assignment logical. You might not be able to stop them, but you can at least hinder them.

Imagine if A were Apples and B were Bananas. Does the assignment Apples=Bananas make any sense? On the surface, you would haver to say no. But if they were both derived from a common base class, say Fruit, then the assignment Apples=Bananas could theoretically be statically downcast to Apples::Fruit=Bananas::Fruit, in which case it has the potential to be a legal assignment. That is, only the Fruit portion of A would be assigned the Fruit portion of B. However, the logic behind Apples=Bananas is somewhat ambiguous. If the end-user really wanted to perform this type of assignment then the end-user should explicitly cast the types themselves, rather than hide the implementation behind an implicit Apples=Bananas assignment.

As a rule of thumb, overloading an assignment operator should always produce a predictable and logical result. If the logic of an assignment is unclear even to to you, then imagine how confusing it would be to the end-user, or to someone else simply reading your code (bafflement and bewilderment spring to mind). So if there's ever any ambiguity, it's best avoided. You can't stop people trying to do illogical things with assignments, but actively encouraging them by including illogical operators yourself says more about you than it does about them. If in doubt, leave it out. Don't make it too easy.