Intermediate Perl - Day 3

Goals

• discuss OOP terminology
• build and use class methods
• use inheritance (is-a)
• use overrides
• use extensions
• describe objects in classes
• create instances (objects) with constructors
• modify instances with instance methods
• work with multiple objects

Some preliminary thoughts

1. Use OOP where it makes sense. Don't use it where it doesn't make sense.
2. "...perl isn't obsessed with enforced privacy... a Perl module would prefer that you stayed out of its living room because you weren't invited, not because it has a shotgun."
3. "...a module [shouldn't] pollute any namespace it wasn't asked to pollute."
4. "...generally you should treat the object as a black box. All access to the object should be obtained through the published interface via the provided methods."
5. it is critical that you document the interface to your module

some definitions

• a module is the basic unit of perl re-usability. It usually contains a package/class, is named after that entity, and ends in .pm
• a package is a namespace, usually found in a stand-alone module
• a class is a definition of an object, its properties and behaviors.
• an object is an instance (particular occurrence) of a class
• an instance method is a class-based subroutine/function that defines behaviors for an object.
• a class/static method is a class-based subroutine/function that does not apply to specific objects (constructors, aggregate functions, etc).

The difference between the class and the object (instance) may be reminiscent of the relationship between Plato's Ideal and Real.

Perl classes and OOP

• class methods class->method(); syntax (note that this differs from &function(); syntax.
• inheritance and multiple inheritance from base/parent classes
• overriding (not overloading, which is different and is handled by module)
• extension

Perl objects

• starting simply: blessing a reference into a class

  $dogname = 'Fido';
  $dogObj = \$dogname;
  bless($dogObj, Pets);
  

• Constructors
  The construction of a particular object is usually handled in an automated manner.

  package Pen;
  
  sub new {
  	#constructor
  	#USAGE: Pen->new;
  
  	#	the class is the first arg passed to a class function
  	my $class = shift; #@_ is assumed.
  
  	#	this will be cleaner when working with anonymous hashes and arrays
  	my $penType = 'not specified';
  	my $penRef = \$penType;
  
  	#	creates the (object) instance reference and return it.
  	bless $penRef, $class;
  
  }#end, constructor
  
  
  #thank you, drive through
  1;
  
  
  
  

• passing an argument to the constructor

  
  
  package Pen;
  
  sub new {
  	#constructor
  	#USAGE: Pen->new("type of pen");
  
  	#	the class is the first arg passed to a class function
  	my $class 	= shift; #@_ is assumed. remember that shift is destructive
  	my $penType	= shift; #now the passed arg
  
  	#	this will be cleaner when working with anonymous hashes and arrays
  	my $penRef = \$penType;
  
  	#	created the object reference and return it.
  	bless $penRef, $class;
  
  }#end, constructor
  
  
  #thank you, drive through
  1;
  
  

• Holding real object data: hashes instead of scalars

  
  package Pen;
  
  sub newValues {
  	#constructor
  	#USAGE $ref = Pen->newValues('pen type', 'pen color', 'ink color');
  
  	#	just doing this to keep your head straight 
  	my $class 		= $_[0];
  	my $penType		= $_[1];
  	my $penColor	= $_[2];
  	my $inkColor	= $_[3];
  
  	#	anonymous hash!
  	my $penRef = { 	penType => $penType, 
  					penColor => $penColor,
  					inkColor => $inkColor,
  					};
  
  
  	#	created the instance reference and return it.
  	bless $penRef, $class;
  
  }#end, constructor
  
  
  #thank you, drive through
  1;
  

Class methods v. Instance Methods

Homework

• destructors
• serialization and persistence of objects
• regex debugging, profiling

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