CS 4448 - Fall 1998
Object-Oriented Programming and Design
Talk 8.1
by
Scott Longmore

1. Introduction
2. Packages
3. Modules
4. Objects/Classes
5. OO Design
6. Summary

Perl -

• Philosophy - Laziness, Impatience, & Hubris
  
  
• OO Philosophy - Use OO design where applicable, avoid it where it does not apply
  
  
• Governed more by convention than syntax

• Does not enforce OO design, up to designer
  
  
• Provides tools to create OO design, however no specific syntax for OO elements

• Package is a namespace that keeps variables and methods separate from other packages i.e. modularity

• Fundamental block for modules and objects

• With few exceptions, there are no global variables in Perl

• Code always compiled in "Current Package"
  
  
• "Current Package" determined by package declaration, initial package is main

• Package declaration determines which symbol table is used

• Symbol table is a hash of package variable names

• Nested packages possible

• AutoLoading - if Package subroutine undefined, AUTOLOAD routine evaluated

• AutoLoading generally used for loading a definition or system call

III. Modules 

• Reusable package defined in a file with the same name ending in .pm extension

• Similar to a header file in C/C++

• Implemented through use/require command

• Similar to #include at beginning of file

• Module used to export symbols or object/class definition

• Modules to not implement access protection like C++, Ada, etc.
• Designer responsible for encapsulation and abstraction in design

Objects:

• Object interface not enforced

• Object is a referenced "thingy" which knows its package/class

• Referenced "thingy" becomes Object when bless command ties reference to package

• Blessing occurs within "constructor"
• No constructor syntax, "new" common convention

• No class syntax, class is simply a package

• Inheritance implemented via @ISA array

• @ISA contains parent package(s) to search methods

• Method inheritance implemented only

• Perl typeless, so instance variable implemented on the fly

• No method syntax, method is simply a subroutine

• Class method - expects class as 1^st arg

• Instance method - expects object as 1^st arg

• Dual nature method - differentiates object/class via ref command

• 2 forms of invocation: method class/instance list or class/instance->method(list)

• Parent method invocation via SUPER::

• DESTROY destructor method
  
  
• AUTOLOAD method can be used to interface instance variables without separate methods

Guidelines:

• Don't verify class within object - can break inheritance

• Use 2 arg form of bless i.e. bless $self $class

• Derived class knows about immediate base class not vice versa.

• When applicable use, containment (has-a) relationships instead of inheritance (is-a)

• Instance variables and inheritance
  
  
• Containment (has-a) relationships
  
  
• Overriding base class methods
  
  
• Implementation (uses-a) relationship
  
  
• Code reuse
  
  
• Class and Object context
  
  
• Constructor inheritance
  
  
• Delegation ("passes the buck") relationship

 

• Designer can establish own convention, however OO paradigm can easily be broken

Modularity: 

• Packages establish own variable and method namespace

• Not implemented, up to designer

• Method inheritance implemented via @ISA