Lecture #19: More Special Effects---Exceptions and OOPExceptions and ContinuationsApproach I: Do NothingApproach II: Non-Standard ReturnApproach III: Stack manipulationApproach III: DiscussionApproach IV: PC tablesNew Topic: Dynamic Method Selection and OOPI. Fully Dynamic ApproachCharacteristics of Dynamic ApproachImplementing the Dynamic ApproachPros and Cons of Dynamic ApproachII. Straight Single Inheritance, Dynamic TypingImplementing the Smalltalk-like ApproachPros and Cons of Smalltalk ApproachSingle Inheritance with Static TypesImplementation of Simple Static Single InheritanceInterfacesInterface Implementation I: Brute ForceInterface Implementation II: Make Interface Values DifferentInterface Implementation II, IllustratedImproving Interface Implementation IIFull Multiple InteritanceImplementing Full Multiple Inheritance IImplementing Full Multiple Inheritance I (contd.)Implementing Full Multiple Inheritance IILecture #19: More Special Effects—Exceptions andOOPAdministrivia• First autograder ran Wednesday night. I’ll add more tests for thefinal run.• Test #2 on Tuesday, 14 April (in class).Last modified: Thu Apr 2 12:27:02 2009 CS164: Lecture #19 1Exceptions and Continuations• Exception-handling in programming languages is a very limited formof continuation.• Execution continues after a function call that is still active whenexception raised.• Java provides mechanism to return a value with the exception, butthis adds no new complexity.Last modified: Thu Apr 2 12:27:02 2009 CS164: Lecture #19 2Approach I: Do Nothing• Some say keep it simple; don’t bother with exceptions.• Use return code convention:Example: C library functions often return either 0 for OK or non-zero for various degrees of badness.• Problems:Last modified: Thu Apr 2 12:27:02 2009 CS164: Lecture #19 3Approach I: Do Nothing• Some say keep it simple; don’t bother with exceptions.• Use return code convention:Example: C library functions often return either 0 for OK or non-zero for various degrees of badness.• Problems:– Forgetting to check.– Code clutter.– Clumsiness: makes value-returning functions less useful.– Slight cost in always checking return codes.Last modified: Thu Apr 2 12:27:02 2009 CS164: Lecture #19 3Approach II: Non-Standard Return• First idea is to modify calls so that they look like this:call _fjmp OKcode to handle exceptionOK:code for normal return• To throw exception:– Put type of exception in some standard register or memory loca-tion.– Return to instructionafternormal return.• Awkward for the ia32 (above). Easier on machines that allow return-ing to a register+constant offset address [why?].• Exception-handling code decides whether it can handle the excep-tion, and does another exception return if not.• Problem: Requires small distributed overhead for every functioncall.Last modified: Thu Apr 2 12:27:02 2009 CS164: Lecture #19 4Approach III: Stack manipulation• C does not have an exception mechanism built into its syntax, butuses library routines:jmp_buf catch_point;void Caller () {if (setjmp (catch_point) == 0) {normal case, which eventuallygets down to Callee} else {handle exception}}void Callee () {...// Throw exception:longjmp (catch_point, 42);...}...Caller’sframe...otherframes...Callee’sframeCaller’sFP, SP,addr ofsetjmp call& otherscatch point:Last modified: Thu Apr 2 12:27:02 2009 CS164: Lecture #19 5Approach III: Stack manipulation• C does not have an exception mechanism built into its syntax, butuses library routines:jmp_buf catch_point;void Caller () {if (setjmp (catch_point) == 0) {normal case, which eventuallygets down to Callee} else {handle exception}}void Callee () {...// Throw exception:longjmp (catch_point, 42);...}...Caller’sframeCaller’sFP, SP,addr ofsetjmp call& otherscatch point:When longjmp called, re-store stack as indicated bycatch point and return tothe end of the setjmp call.Last modified: Thu Apr 2 12:27:02 2009 CS164: Lecture #19 5Approach III: Discussion• On exception, call to setjmp appears to return twice, with two dif-ferent values.• Does not require help from compiler,• But implementation is architecture-specific.• Overhead imposed on every setjmp call.• If used to implement try and catch, therefore, would impose coston every try.• Subtle problems involving variables that are stored in registers:– The jmpbuf typically has to store such registers, but– That means the value of some local variables may revert unpre-dictably upon a longjmp.Last modified: Thu Apr 2 12:27:02 2009 CS164: Lecture #19 6Approach IV: PC tables• Sun’s Java implementation uses a different approach.• Compiler generates a table mapping instruction addresses (programcounter (PC) values) to exception handlers for each function.• If needed, compiler also leaves behind information necessary to re-turn from a function (“unwind the stack”) when exception thrown.• To throw exception E:while (current PC doesn’t map to handler for E)unwind stack to last caller• Under this approach, a try-catch incurs no cost unless there is anexception, but• Throwing and handling the exception more expensive than other ap-proaches, and• Tables add space.Last modified: Thu Apr 2 12:27:02 2009 CS164: Lecture #19 7New Topic: Dynamic Method Selection and OOP• “Interesting” language feature introduced by Simula 67, Smalltalk,C++, Java: thevirtual function(to use C++ terminology).• Problem:– Arrange classes in a hierarchy of types.– Instance of subtype “is an” instance of its supertype(s).– In particular, inherits their methods, but can override them.– Adynamiceffect: Cannot in general tell from program text whatbody of code executed by a given call.• Implementation difficulty (as usual) depends on details of a lan-guage’s semantics.• Some things still static:– Names of functions, numbers of arguments are (usually) known– Compiler can handle overloading by inventing new names for func-tions. E.g., G++ encodes a function f(int x) in class Q asZN1Q1fEi,and f(int x, int y) as ZN1Q1fEii.Last modified: Thu Apr 2 12:27:02 2009 CS164: Lecture #19 8I. Fully Dynamic Approach• Regular Python has a completely dynamic approach to the problem:class A:x = 2; def f (self): return 42a = A (); b = A ()print a.x, a.f() # Prints 2 42a.x = lambda (self, z): self.w * za.f = 13; a.w = 5print a.x(3), a.f, a.w # Prints 15 13 5print b.x(3), b.f, b.w # Errorprint A.x # Prints 2A.x = lambda (self): 19A.f = 2A.v
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