Objects and Classes

1. Objects and Classes David Walker CS 320

2. Advanced Languages • advanced programming features • ML data types, exceptions, modules, objects, concurrency, ... • fun to use, but require special techniques to compile and optimize • today will be looking at how to compile objects and classes similar to those found in Java • Appel chapter 14.1-14.4

3. Object Tiger • Add class declarations to Tiger: • classes are a collection of • field (variable) declarations • method declarations • every class declaration gives a new name to a class • a class may inherit methods and fields from the class it extends • the class “object” sits at the top of the class hierarchy; it has no fields and no methods

4. An Object Tiger Class class name superclass that Vehicle inherits from let class Vehicle extends Object { var position := start method move (x : int) = (position := position + x) } in ... end field declaration method declaration

5. Another Object Tiger Class let class Vehicle extends Object { var position := start method move (x:int) = (position := position + x) } class Car extends Vehicle { var passengers := 0 method await(v:vehicle) = if (v.position < position) then v.move(position – v.position) else self.move(10) in ... end new field declaration new method declaration call to inherited method v’s “position” field current object’s “position” field

6. Yet Another Object Tiger Class let class Vehicle extends Object { var position := start method move (x:int) = position := position + x } class Car extends Vehicle { ... } class Truck extends Vehicle { method move (x:int) = if x <= 55 then position := position * x in ... end method override

7. Using the Classes let class Vehicle extends Object { ... } class Car extends Vehicle { ... } class Truck extends Vehicle {...} var t := new Truck var c := new Car var v : Vehicle := c in c.passengers := 2; c.move(60); v.move(70); c.await(t); end new object created subtyping allows a car to be viewed and used as a generic vehicle a car calls an inherited method subtyping allows a truck to be viewed and used as a generic vehicle

8. Implementing Object Tiger • Some key problems: • how do we access object fields? • both inherited fields and fields for the current object? • how do we access method code? • if the current class does not define a particular method, where do we go to get the inherited method code? • how do we handle method override?

9. Class Hierarchy • The class hierarchy is the graph of inheritence relationships in a program: • In a single-inheritence (SI) language, the graph is a tree • In a multiple-inheritence (MI) language, the graph is a dag • Multiple-inheritence languages are much trickier to implement than single-inheritence languages Object Vehicle Car Truck

10. Object Layout (SI) • Objects are laid out somewhat like records • each variable has a slot in the record • in order to implement field lookup we need to have a systematic way to find a given field • eg: v.position • v may be a generic vehicle or it may be a car or a truck • we need to put “position” in the same place in the record that implements vehicles, cars and trucks

11. Object Layout (SI) • Solution: extension on the right • lay out the inherited fields first in the same order as in the parent (SI => only 1 parent) • lay out the newly declared to the right

12. Object Layout (SI) class A extends Object { var a := 0 } class B extends A { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends B { var e := 0 }

13. Object Layout (SI) class A extends Object { var a := 0 } class B extends A { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends B { var e := 0 } A a

14. Object Layout (SI) class A extends Object { var a := 0 } class B extends A { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends B { var e := 0 } A B a a b c

15. Object Layout (SI) class A extends Object { var a := 0 } class B extends A { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends B { var e := 0 } A B C a a a b d c

16. Object Layout (SI) class A extends Object { var a := 0 } class B extends A { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends B { var e := 0 } A B C D a a a a b d b c c e

17. Static & Dynamic Methods • The result of compiling a method is some machine code located at a particular address • at a method invocation point, we need to figure out what code location to jump to • Java has static & dynamic methods • to resolve static method calls, we look at the static type of the calling object • to resolve dynamic method calls, we need the dynamic type of the calling object

18. during semantic analysis, the compiler knows: static type (class) of the object calling the method the list of methods in each class and determines the closest method (up the class hierarchy) with the given name and inserts instructions to pass object as self parameter a direct call to the known method Static Methods let class A extends Object { static method foo (x:int) = ... static method bar (x:int) = ... } class B extends A { static method foo (x:int) = ... } var a : A = new A var b : A = new B var c : B = new B in a.foo(3); (* calls foo in class A *) b.foo(3); (* calls foo in class A *) c.bar(3); (* calls bar in class A *) c.foo(3); (* calls foo in class B *)

19. Method called depends on object’s dynamic type During semantic analysis, may be unknown At run-time, we determine which code to jump to object stores a pointer to its method table (v-table) as well as its object vars At compile-time, we generate code to look up v-table in object extract method from table jump to method body Dynamic Methods let class A extends Object { method foo (x:int) = ... method bar (x:int) = ... } class B extends A { method foo (x:int) = ... } var a : A = new A var b : A = new B var c : A = if long-and-tricky-computation then a else b in c.foo(3)

20. Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () }

21. Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } A a A_f

22. Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } A B a a A_f A_f B_g

23. Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } A B C a a a A_f A_f A_f B_g C_g

24. Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } A B C D a a a a b A_f A_f A_f D_f B_g C_g C_g

25. Object Layout II (SI) class A extends Object { var a := 0; method f () } class B extends A { method g () } class C extends B { method g () } class D extends C { var b := 0 ; method f () } A B C D a a a a b D a A_f A_f A_f D_f b B_g C_g C_g

26. Multiple Inheritence • Multiple inheritence is trickier to implement than single inheritence because creating objects of a subclass from their subclass by “extension on the right” doesn’t work • if C inherits from both A and B, we can’t put A’s variables at the front and put B’s variables at the front of the object in the same place! • we need to do a global analysis to determine object layout

27. Object Layout (MI) class A extends Object { var a := 0 } class B extends Object { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends A,B,C { var e := 0 } A B C D a a a b b c c d d e

28. Object Layout (MI) class A extends Object { var a := 0 } class B extends Object { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends A,B,C { var e := 0 } A B C D • Determine object layout by: • global graph coloring! • a node for each field name • an interference edge between • names that coexist in the same • class (via inheritence or • otherwise) a a a b b c c d d e

29. Object Layout (MI) class A extends Object { var a := 0 } class B extends Object { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends A,B,C { var e := 0 } A B C D a a a b b c c d d wasted space in every object e

30. Object Layout II (MI) class A extends Object { var a := 0 } class B extends Object { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends A,B,C { var e := 0 } A B C D a b a a 1 a: 1 a: c d b 2 b: c 3 c: 1 1 a: a: d 4 d: 2 wasted space per class 2 b: d: e 5 e:

31. Object Layout II (MI) class A extends Object { var a := 0 } class B extends Object { var b := 0 var c := 0 } class C extends A { var d := 0 } class D extends A,B,C { var e := 0 } A B • To fetch a field using this • representation, we: • load the first field of the object • to get the class descriptor c • load M [c + fieldOffset] • Note: fieldOffset can be • precomputed using global • graph coloring as before a b c 1 1 a: a: 2 b:

32. A Problem • Global analyses are the bane of modern computing • many applications use dynamically linked libraries, mobile code, get patches to fix bugs, etc. • when we don’t have the whole program at compile-time, we can’t do global analyses! • solution (most of the time): • a smart custom linker • still tricky when new code is linked dynamically to code that is already running • hence, Java has single inheritence

33. Other OO Features • Down-casts and type tests • Java has casting mechanism “(C) x” to cast variable x to class C • at run time we look up x’s dynamic type and determine whether it is a subtype of C • these type casts are currently pervasive and a source of both inefficiency and errors • soon, Java and C# will be adding parametric polymorphism, a la ML, to make many of these unnecessary casts go away

34. Other OO Features • Protection mechanisms • to encapsulate local state within an object, Java has “private” “protected” and “public” qualifiers • private methods/fields can’t be called/used outside of the class in which they are defined • during semantic analysis (type checking), the compiler maintains this information in the symbol table for each class

35. Summary • Object-oriented languages provide new challenges for compiler writers • how to find fields and methods • how to make field and method access just as efficient as ordinary function call and variable lookup • lots of ongoing research in OO language implementation tackles these and other interesting questions