Bjarne Stroustrup: You can program with a lot of free-standing classes. If I want a complex number, I write a complex number. It doesn’t have any virtual functions. It’s not meant for derivation. You should use inheritance only when a class hierarchy makes sense from the point of view of your application, from your requirements. For a lot of graphics classes it makes perfect sense. The oldest example in the book is the shape example, which I borrowed from Simula. It makes sense to have a hierarchy of shapes or a hierarchy of windows, things like that. But for many other things you shouldn’t plan for a hierarchy, because you’re not going to need one.
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Bill Venners: So the invariant justifies the existence of a class, because the class takes the responsibility for maintaining the invariant.
Bjarne Stroustrup: That’s right.
Bill Venners: The invariant is a relationship between different pieces of data in the class.
Bjarne Stroustrup: Yes. If every data can have any value, then it doesn’t make much sense to have a class. Take a single data structure that has a name and an address. Any string is a good name, and any string is a good address. If that’s what it is, it’s a structure. Just call it a struct. Don’t have anything private. Don’t do anything silly like having a hidden name and address field with get_name and set_address and get_name and set_name functions. Or even worse, make a virtual base class with virtual get_name and set_name functions, and override it with the one and only representation. That’s just elaboration. It’s not necessary.
As an interesting biographical note, Stroustrup is a Profesor at A&M; I had no idea. Anyhow, the rest of this interview is fantastic.