	CS 535 Object-Oriented Programming Spring Semester, 2003 Double Dispatch
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Contents of Doc 13, Double Dispatch

Double Dispatch

Singleton - One Instance

References
Ralph Johnson’s Object-Oriented Programming & Design lecture notes, Polymorphism (Day 5) http://st-www.cs.uiuc.edu/users/cs497/lectures.html

VisualWorks Source Code


Doc 13, Double Dispatch Slide # 2

Double Dispatch

Multiplication Motivation
Integer * Integer

   Primitive integer * integer operation

Integer * Float

   Convert integer to float
   Use primitive float * float operation

Float * Integer

   Convert integer to float
   Use primitive float * float operation

Integer * Matrix

   Multiple each element of Matrix by integer

Matrix * Integer

   Multiple each element of Matrix by integer

Integer * Fraction

   Multiple Fraction numerator by integer

Actual operation depends on the type of both arguments

How to implement?


Doc 13, Double Dispatch Slide # 3

Double Dispatch

Integer>>+ aNumber 
   “We do not know what aNumber is”
   “Send message to aNumber telling it we are an integer”
   
   ^aNumber sumFromInteger: self

Float>>sumFromInteger: anInteger
   “Now we know the type of the receiver and sender
   Do the correct thing for this pair”
   ^anInteger asFloat + self
Float+ aNumber 
   "Answer a Float that is the result of adding the receiver to the argument.  
   The primitive fails if it cannot coerce the argument to a Float"
   <primitive: 41>
   ^aNumber sumFromFloat: self
Double>>sumFromInteger: anInteger
   ^anInteger asDouble + self
Doc 13, Double Dispatch Slide # 4

Example
3 + 2.5

Integer>>+ aNumber 
   
   ^aNumber sumFromInteger: self

Then

2.5 sumFromInteger: 3

Float>>sumFromInteger: anInteger
   ^anInteger asFloat + self

3.0 + 2.5

Float+ aNumber 
   <primitive: 41>
   ^aNumber sumFromFloat: self

primitve 41 adds 3.0 and 2.5 and returns 5.5


Doc 13, Double Dispatch Slide # 5

Triple Dispatching?
Why does Float>>sumFromInteger: send another message?

It has all the information it needs

It could perform the operation there, but it is easier to just call +


Doc 13, Double Dispatch Slide # 6

Adding a New Type Of Arithmetic Value
New type X
Add primary methods: + - / *

Add double displatching methods

sumFromDouble:
sumFromFloat:
sumFromFraction:
sumFromInteger:
sumFromX:

Same with quotentFrom, productFrom, differenceFrom

Existing Types
Add double displatching methods

sumFromX:
quotentFromX:
productFromX:
differenceFromX:


Doc 13, Double Dispatch Slide # 7

Shared Responsibilities
Sometimes an operation depends on the class of several objects

Arthmetic – depends on the types of both arguments

Displaying object - depends on type of object and windowing system


Doc 13, Double Dispatch Slide # 8

Singleton - One Instance

Sometimes need to insure only one instance of a Class

Smalltalk defineClass: #SingletonExample
   superclass: #{Core.Object}
   indexedType: #none
   private: false
   instanceVariableNames: ''
   classInstanceVariableNames: 'uniqueInstance '
   imports: ''
   category: 'CS535'

SingletonExample class methodsFor: 'instance creation'

current
   uniqueInstance ifNil: [uniqueInstance := self basicNew].
   ^uniqueInstance

new
   "Force all creation access through current to insure only one instance"

   self shouldNotImplement

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