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If you're overriding a method, you need the other documentation - the one that is rarely provided, or if provided is incomplete: what other methods in the ancestor class will call it - and you need to make sure you don't call them, to avoid an accidental infinite loop. You need to know what the state of the class will be - fully initialized, what invariants hold. You need to know what other methods are safe to call and when.
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Unless you missed the whole start of my email - i.e. the bit where I point out that it's not for performance reasons "only", or even primarily.
Following up on this, here's what Anders Hejlsberg has to say about it [1]: <quote> There are several reasons. One is performance. We can observe that as people write code in Java, they forget to mark their methods final. Therefore, those methods are virtual. Because they're virtual, they don't perform as well. There's just performance overhead associated with being a virtual method. That's one issue. A more important issue is versioning. There are two schools of thought about virtual methods. The academic school of thought says, "Everything should be virtual, because I might want to override it someday." The pragmatic school of thought, which comes from building real applications that run in the real world, says, "We've got to be real careful about what we make virtual." When we make something virtual in a platform, we're making an awful lot of promises about how it evolves in the future. For a non-virtual method, we promise that when you call this method, x and y will happen. When we publish a virtual method in an API, we not only promise that when you call this method, x and y will happen. We also promise that when you override this method, we will call it in this particular sequence with regard to these other ones and the state will be in this and that invariant. Every time you say virtual in an API, you are creating a call back hook. As an OS or API framework designer, you've got to be real careful about that. You don't want users overriding and hooking at any arbitrary point in an API, because you cannot necessarily make those promises. And people may not fully understand the promises they are making when they make something virtual. </quote> Herb Sutter also has an interesting article about virtual methods, which is somewhat related: he says not to make virtual functions public (or public functions virtual) [2]. Fabian [1] http://www.artima.com/intv/nonvirtual.html [2] http://www.gotw.ca/publications/mill18.htm =================================== This list is hosted by DevelopMentor® http://www.develop.com View archives and manage your subscription(s) at http://discuss.develop.com
