1. David Evans http://www.cs.virginia.edu/~evans
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.
Background
just got here last week
finished degree at MIT week before
Philosophy of advising students
don’t come to grad school to implement someone else’s idea
can get paid more to do that in industry
learn to be a researcher
important part of that is deciding what problems and ideas are worth spending time on
grad students should have their own project
looking for students who can come up with their own ideas for research
will take good students interested in things I’m interested in – systems, programming languages & compilers, security
rest of talk – give you a flavor of the kinds of things I am interested in
meant to give you ideas (hopefully even inspiration!) but not meant to suggest what you should work on
CS655: Programming Languages
University of Virginia
Computer Science
David Evans

2. University of Virginia CS 655
Menu
Today’s Task
Position Paper Hints
Contravariance, covariance and ski-team room-mates
Language team sales pitches
24 February 2019
University of Virginia CS 655

3. University of Virginia CS 655
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.
24 February 2019
University of Virginia CS 655

4. University of Virginia CS 655
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] ...)
24 February 2019
University of Virginia CS 655

5. University of Virginia CS 655
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);
24 February 2019
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!
24 February 2019
University of Virginia CS 655

7. University of Virginia CS 655
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.
24 February 2019
University of Virginia CS 655

8. University of Virginia CS 655
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!
24 February 2019
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.
24 February 2019
University of Virginia CS 655