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The C++ Style Sweet Spot
A Conversation with Bjarne Stroustrup, Part I
by Bill Venners
October 13, 2003

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The other way people get into trouble is exactly the opposite. They believe that C++ should be an extremely high level language, and everything should be object-oriented. They believe that you should do everything by creating a class as part of a class hierarchy with lots of virtual functions. This is the kind of thinking that's reflected in a language like Java for instance, but a lot of things don't fit into class hierarchies. An integer shouldn't be part of a class hierarchy. It doesn't need to. It costs you to put it there. And it's very hard to do elegantly.

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.

So you can start with much simpler abstractions. Again the standard library can provide some examples: vector, string, complex number. Don't go to hierarchies until you need them. Again, one indication that you've gone too far with class hierarchies is you have to write casts all the time, casting from base classes to derived classes. In really old C++, you would do it with a C style cast, which is unsafe. In more modern C++, you use a dynamic cast, which at least is safe. But still better design usually leads you to use casting only when you get objects in from outside your program. If you get an object through input, you may not know what it is until a bit later, and then you have to cast it to the right type.

Bill Venners: What is the cost of going down either of those two paths, being too low-level or too enamored with object-orientation? What's the problem?

Bjarne Stroustrup: The problem with the C way is that if you write code C-style, you get C-style problems. You will get buffer overflows. You will get pointer problems. And you will get hard to maintain code, because you're working at a very low level. So the cost is in development time and maintenance time.

Going to the big class hierarchy is again, you write more code than you need to, and you get too much connection between different parts. I particularly dislike classes with a lot of get and set functions. That is often an indication that it shouldn't have been a class in the first place. It's just a data structure. And if it really is a data structure, make it a data structure.

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