Object-Oriented Software Construction

Transcription

1 1 Object-Oriented Software Construction Bertrand Meyer

2 2 Lecture 21: Typing issues, covariance

3 The America Conjecture 3 (Pierre America at TOOLS Europe 1990) One can have at most two of the three properties of: Static typing. Substitutivity. Covariance. Can we disprove the America conjecture?

4 Typing: A simple problem! 4 The basic operation of object-oriented computation: x.f (arg)

5 The typing problem 5 When, and how, do we know that: There is a feature applicable to OBJ and corresponding to f? arg is an acceptable argument for that feature?

6 Terminology 6 x OBJ x: Entity. OBJ: The object attached to x at some time during execution. f: Routine (One of the two forms of feature; the other is attributes.)

7 Typing vs. binding 7 What do we know about the feature to be called? Static typing: At least one Dynamic binding: The right one Example: my_aircraft.lower_landing_gear

8 Inheritance and typing 8 * deferred + effected ++ redefined * PLANE * AIRCRAFT lower_landing_gear* * COPTER BOEING AIRBUS lower_landing_gear+ B_737 B_747 A_320 lower_landing_gear++ B_747_400

9 Cost of correcting errors (Boehm) 9 LARGE PROJECTS SMALL PROJECTS Requirements Design Code Development Acceptance Operation test test

10 Typing rules (all locally checkable) 10 Declaration rule. Call rule. Attachment rule. (For the precise formulations see Eiffel: The Language.)

11 Declaration rule 11 Every entity must be declared as being of a certain type. For example: x: AIRCRAFT n: INTEGER ba1: BANK_ACCOUNT

12 Call rule 12 If a class C contains the call x.f there must be a feature of name f in the base class of the type of x, and that feature must be available (exported) to C.

13 Attachment rule 13 In an assignment x := y, or the corresponding argument passing, the base class of the type of y must be a descendant of the base class of the type of x.

14 For typing to be acceptable 14 No exception to type ee-rules ( casts ). Multiple inheritance Unconstrained genericity: class LIST [G]... Constrained genericity: class VECTOR [G > NUMERIC]...

15 For typing to be acceptable (cont d) 15 Assignment attempt: x?= y Contracts, to clarify the semantics and include constraints other than typing (e.g. numerical ranges). Covariance for routine arguments. Anchored declarations (like x) to avoid endless redeclarations.

16 Multiple inheritance 16 DOCUMENT MESSAGE MAILABLE_ DOCUMENT

17 A class hierarchy 17 DRIVER partner: DRIVER share (other: DRIVER) partner++ share++ TRUCKER BIKER partner++ share++ HEAVY_TRUCKER partner++ share++

18 Choosing a partner 18 class DRIVER feature end partner: DRIVER -- This driver s alternate share (other: DRIVER) is -- Choose other as alternate. require other /= Void do partner := other end...

19 A typical call 19 d1, d2: DRIVER... d1.share (d2)

20 Specializing 20 class TRUCKER inherit feature end DRIVER redefine end partner partner: TRUCKER -- This driver s alternate....

21 The need for covariance 21 class TRUCKER inherit feature DRIVER redefine partner, share end partner: TRUCKER -- This driver s alternate. share (other: TRUCKER) is -- Choose other as alternate. require other /= Void do end partner := other end

22 Direct covariance 22 DRIVER partner: DRIVER share (other: DRIVER) partner++ share++ TRUCKER BIKER partner++ share++ HEAVY_TRUCKER partner++ share++

23 Avoiding redefinition avalanche 23 Anchored declarations: class TRUCKER inherit DRIVER feature redefine end partner, share partner: TRUCKER -- This driver s alternate. share (other: like partner) is -- Choose other as alternate. require other /= Void do partner := other end end

24 24 End of lecture 22