Variable Size Objects

Damien Cassou, Stéphane Ducasse and Luc Fabresse http://stephane.ducasse.free.fr

Variable Size Instances?

Array new: 10
> #(nil nil nil nil nil nil nil nil nil nil)

Array new: 5
> #(nil nil nil nil nil)

Yes arrays can have different sizes

What You Will Learn

How to define variable size objects?
How to instantiate and access variable size objects?

Roadmap

How to define variable size objects?
How to instantiate and access variable size objects?

Variable Class Definition

Use message variableSubclass: instead of subclass:

Example

ArrayedCollection variableSubclass: #Array
	instanceVariableNames: ''
	classVariableNames: ''
	package: 'Collections-Sequenceable'

Example Variable Class

Example

Object variableSubclass: #StrangePoint
        instanceVariableNames: 'x y'
        classVariableNames: ''
        package: 'Collections-Sequenceable'

Instances of a variable class have a variable size (indexed) zone after named variables
Only one indexed instance variable per class (always the last one)

Roadmap

How to define variable size objects?
How to instantiate and access variable size objects?

Variable Class Instantiation and Index Access

Create instances with new: max
Access indexed values with at: and at:put:
First element starts at index 1
size returns the number of indexed instance variables

| a |
a := Array new: 4.
a at: 2 put: 'lulu'.
a at: 1
> nil
a at: 2 
> 'lulu'

Classes with Different Shape

Classes with named instance variables
- Counter has an instance variable named count
Classes with a variable/indexed instance variable
- Array has only an indexed instance variable
Classes with some named instance variables and one indexed instance variable

Refining the Variable Part

Indexed	Named	Definition Method	Examples
No	Yes	#subclass:...	Color
Yes	No	#variableSubclass:	AdditionalMethodState
Yes	No	#variableByteSubclass:	ByteString
Yes	No	#variableWordSubclass:	Bitmap

Some methods related to class types: isPointers, isBits, isBytes, isFixed, isVariable

Constraints

Classes defined using subclass: can have any kind of subclasses
Classes defined using variableSubclass: can only have variableSubclass: subclasses

What You Should Know

How to define variable size objects?
- define a variableSubclass: with an indexed instance variable
How to instantiate variable size objects?
- use new:
- use at: and at:put: to access indexed values

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