Perl Module Basics

use WeakRef  qw(weaken);

# constructor   returns new BOA::Logger objects

sub new {

   my ($pkg, $filename) = @_;

   # lookup $filename in %BOA::Logger::CACHE   if an entry exists, return it

   return $CACHE{$filename} if $CACHE{$filename};

   # initialize $self as a reference to an empty hash

   my $self = {};

   # store in %CACHE

   $CACHE{$filename} = $self;

   weaken($CACHE{$filename});

   # ... same as previous example ...

}

Inheritance

BOA::Logger is a simple module, but simple doesn't last. As more BOA developers 

start using BOA::Logger, requests for new features will certainly start piling up. Sat 

isfying these requests by adding new features to the module might be possible, but 

the effect on performance might be severe. One solution would be to create a new 

module called BOA::Logger::Enhanced that supported some enhanced features 

and just copy the code in from BOA::Logger to get started. This has an unpleasant 

consequence: The code would be harder to maintain since bugs would need to be 

fixed in two places at once.

There is a better way. Object oriented classes can be enhanced using inheritance.

Inheritance enables one module to be based on one (or more) classes known as parent

or base classes. The new derived class that inherits from the parent class is known as 

the child class. Here's an example module, called BOA::Logger::Enhanced, that 

inherits from BOA::Logger:

package BOA::Logger::Enhanced;

use BOA::Logger;

@ISA = qw(BOA::Logger);

By assigning  BOA::Logger  to the package variable @ISA, the module tells Perl that 

it is inheriting from the BOA::Logger class. This variable is pronounced  is a  and 

refers to the fact that a BOA::Logger::Enhanced object  is a  BOA::Logger object. 

Inheritance relationships are known as  is a  relationships.

45

45