About Double Dispatch

Transcription

1 Learning Object-Oriented Programming and Design with TDD About Double Dispatch Stéphane Ducasse W8S02

2 W8S02 2 / 31 Outline Some fun exercises Thinking about them Discovering double dispatch Stepping back

3 W8S02 3 / 31 Exercise 1 Given primitive addi(i,j) returns i + j primitive addf(f1,f2) returns f1 + f2 i.asfloat() returns a float

4 W8S02 4 / 31 Adding Integer and Float >>> >>> >>> >>> 3.3 Implement + But with Not a single explicit conditional No static type support

5 A First Hint W8S02 5 / 31 Two classes Integer and Float

6 Let Us See W8S02 6 / 31 Integer >> + anumber "fill me up :)" Float >> + anumber "fill me up :)"

7 Another Key Hint W8S02 7 / 31 When you execute a method you know that the receiver is from the class of the method!

8 Even More Hints W8S02 8 / 31 Remember the Boolean implementation Sending a message to an object is a choice operator

9 Let Us Get Started W8S02 9 / 31 Imagine that we add one method sumwithinteger: aninteger Integer >> + anumber Integer >> sumwithinteger: aninteger Float >> + anumber "fill me up :)"

10 Look Like An Easy Definition W8S02 10 / 31 Integer >> + anumber Integer >> sumwithinteger: aninteger ^ addi(self, aninteger) Float >> + anumber "fill me up :)"

11 How Do We Connect Them? W8S02 11 / 31 Integer >> + anumber ^ anumber sumwithinteger: self Integer >> sumwithinteger: aninteger ^ addi(self, aninteger) Float >> + anumber "fill me up :)"

12 On Float Too W8S02 12 / 31 Integer >> + anumber ^ anumber sumwithinteger: self Integer >> sumwithinteger: aninteger ^ addi(self, aninteger) Float >> + anumber Float >> sumwithinteger: aninteger "fill me up :)"

13 On Float Too W8S02 13 / 31 Integer >> + anumber ^ anumber sumwithinteger: self Integer >> sumwithinteger: aninteger ^ addi(self, aninteger) Float >> + anumber Float >> sumwithinteger: aninteger ^ addf(self, asfloat(aninteger))

14 Supporting W8S02 14 / 31 Integer >> + anumber ^ anumber sumwithinteger: self Integer >> sumwithinteger: aninteger ^ addi(self, aninteger) Float >> + anumber ^ anumber sumwithfloat: self Float >> sumwithinteger: aninteger ^ addf(self, asfloat(aninteger))

15 Supporting W8S02 15 / 31 Integer >> + anumber ^ anumber sumwithinteger: self Integer >> sumwithinteger: aninteger ^ addi(self, aninteger) > Integer >> sumwithfloat: afloat > ^ addf(afloat, asfloat(self)) Float >> + anumber ^ anumber sumwithfloat: self Float >> sumwithinteger: aninteger ^ addf(self, asfloat(aninteger)) > Float >> sumwithfloat: afloat > ^ addf(self, afloat)

16 W8S02 16 / 31 Ok now Relax! Take a pen and follow the calls to the following expressions Follow with your fingers if necessary :)

17 W8S02 17 / 31 Key Point Integer >> + anumber ^ anumber sumwithinteger: self Two choices/messages: one for + one for sumwithinteger:

18 Exercise2: How to Add Fraction? W8S02 18 / 31 f := Fraction num: 1 denum: 2. f num >>> 1 f denum >>> 2 f asfloat >>> 0.5 1/ /5 1/3 + 4/3

19 Introducing Fraction W8S02 19 / 31 Fraction >> + anumber ^ anumber sumwithfraction: self...

20 Introducing Fraction W8S02 20 / 31 Fraction >> + anumber ^ anumber sumwithfraction: self Fraction >> sumwithfraction: afrac...

21 Introducing Fraction W8S02 21 / 31 Fraction >> + anumber ^ anumber sumwithfraction: self Fraction >> sumwithfraction: afrac ^ Fraction num: (self num * afrac denum) + (afrac num * self denum) denum: afrac denum * self denum...

22 Taking Care of Integer and Float W8S02 22 / 31 Fraction >> + anumber ^ anumber sumwithfraction: self Fraction >> sumwithfraction: afrac ^ Fraction num: (self num * afrac denum) + (afrac num * self denum) denum: afrac denum * self denum Integer >> sumwithfraction: afrac... Float >> sumwithfraction: afrac...

23 Introducing Fraction W8S02 23 / 31 Fraction >> + anumber ^ anumber sumwithfraction: self Fraction >> sumwithfraction: afrac ^ Fraction num: (self num * afrac denum) + (afrac num * self denum) denum: afrac denum * self denum... Integer >> sumwithfraction: afrac ^ Fraction num: (self * afrac denum) + afrac num denum: afrac denum Float >> sumwithfraction: afrac ^ addf(self, afrac asfloat)

24 Full Code for Fraction W8S02 24 / 31 Fraction >> + anumber ^ anumber sumwithfraction: self Fraction >> sumwithfraction: afrac ^ Fraction num: (self num * afrac denum) + (afrac num * self denum) denum: afrac denum * self denum Fraction >> sumwithinteger: aninteger ^ Fraction num: (self num + aninteger * afrac denum) denum: afrac denum Fraction >> sumwithfloat: afloat ^ addf(self afloat, afloat) Integer >> sumwithfraction: afrac ^ Fraction num: (self * afrac denum) + afrac num denum: afrac denum Float >> sumwithfraction: afrac ^ addf(self, afrac asfloat)

25 W8S02 25 / 31 Ok Now Relax Take a pen and follow the calls to the following expressions Follow with your fingers if necessary :) 1/ /5 1/3 + 4/3

26 W8S02 26 / 31 Stepping Back We can add Fraction without changing any previous method Another example of "Sending a message is making a choice" We send two messages + to select Integer, Float, Fraction then the message sumwith... to reselect the correct definition in Integer, Float, Fraction Different kinds of messages Primary operations Double dispatching methods

27 W8S02 27 / 31 Double Dispatch Essence of Visitor Design Pattern (see Lecture) Double dispatch is a clear illustration of Do not ask, Tell OOP tenet Used really frequently for event, drawing,...

28 W8S02 28 / 31 When not using Double Dispatch No class to dispatch on! We need an different instance of dispatch to

29 W8S02 29 / 31 What about Overloading Double dispatch is also useful in statically typed languages Avoid overloading for double dispatch - some type systems do not work well

30 W8S02 30 / 31 Conclusion Powerful Modular Just sending an extra message to an argument and using late binding

31 A course by Stéphane Ducasse Reusing some parts of the Pharo Mooc by Damien Cassou, Stéphane Ducasse, Luc Fabresse Except where otherwise noted, this work is licensed under CC BY-NC-ND 3.0 France