David Evans cs.virginia/~evans

1. Lecture 10: Using Object-Oriented Languages You can't think seriously about thinking without thinking about thinking about something. Seymour Papert Today’s corollary: You can’t think seriously about programming, without thinking about programming something. Start working in groups as described in the manifest. David Evans http://www.cs.virginia.edu/~evans CS655: Programming Languages University of Virginia Computer Science

2. Menu • Today’s Task • Position Paper Hints • Contravariance, covariance and ski-team room-mates • Language team sales pitches University of Virginia CS 655

3. Today’s Task • Show a high-level module design for your web browser • Depending on the language, this might have subclassing, subtyping and uses relationships. • Show a concrete code excerpt for redisplaying all the elements • Argue that your design (in conjunction with your implementation language) is superior to alternatives • Assume all languages have equally good development tools, well-trained programmers, etc. University of Virginia CS 655

4. Position Paper Hints • Average on Position Paper 2 = 0.89 (12 out of 25 were 1.0 or above) • Make sure to answer the assigned question • If it isn’t clear, ask • If you make general statements, back them up with real examples • Don’t turn in a laundry list – select and organize the important things • Be resourceful, cite your resources • Things in your references list should be cited in your text (e.g., Bounds checking can be added to C [Wahbe93] ...) University of Virginia CS 655

5. Contra/Co-Variance Athlete How can Girl override set_roomate? Covariance: set_roommate (Girl) set_roommate (Boy) Contravariance: set_roommate (Athlete) Novariance: set_roommate (Skier) (Eiffel) Skier set_roommate (Skier) (Sather) (C++ - but changing?, Java) Boy Girl Problem: s: skier; g: girl; b: boy; s := g; ... s.set_roommate (b); University of Virginia CS 655

6. What’s wrong with Meyer’s Rule? • Disallow polymorphic catcalls: s.f (t) where some subtype of type of s hides f or changes covariantly type of parameters of f. • Violates useful notion of subtyping: adding a new subtype breaks existing programs! University of Virginia CS 655

7. What does C++ do? • Can add covariant methods in subtype, but they overload the original method, instead of overriding it! • Example: class skier { virtual void set_roommate (class skier *); } class girl : public skier { void set_roommate (class girl *); // overloads! } // class boy similar skier *s; girl *g; boy *b; s = g; s->set_roommate (b); // No type error – calls skier::set_roommate! g->set_roommate (b); // Some compilers complain (but shouldn’t?) Good explanation is worth 1 position paper point. University of Virginia CS 655

8. Some C++ Facts • 1996 Draft Standard is 680 pages long • 50x more complex than Algol60! Is this progress? • Current open issues list of Core Language Issues contains 173 issues • Compare to Knuth’s 9 ambiguities in Algol 60 • Current open issues list for standard library (Revision 12) is 59 pages • Good luck! University of Virginia CS 655

9. Browser Design Pitches • Next time: John Viega on Automated Delegation solution to Multiple Inheritance • Think of and send me one good question. • Accomplish something interesting to talk about in your project meetings. University of Virginia CS 655