Perl Module Basics

sub level {

   my ($self, $level) = @_;

   if (@_ == 2) {

      croak("Argument to level() must be a non negative integer!")

         unless $level =~ /^\d+$/;

      $self >{level} = $level;

   }

   return $self >{level};

}

It might seem convenient to allow users to simply access the hash keys directly:

$logger >{level} = 100;  # works, but not a good idea

The problem with this is that it breaks the encapsulation of your class. You are no 

longer free to change the implementation of BOA::Logger you can't change the 

class to use an array underneath or change the keys of the hash. Also, you can't per 

form any checking of the value set for an attribute. As a general rule, all access to 

an object oriented class should be through methods, either class methods or 

object methods.

Destructors

The non OO version of BOA::Logger had a useful feature that this version lacks 

it prints a message when the program exits. You can provide this by setting up a 

destructor for the class. Destructors are the opposite of constructors they are 

called when an object is no longer being used.

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 They can perform cleanup actions, 

such as closing file handles. To create a destructor, simply define a method called 

DESTROY.

sub DESTROY {

   my $self = shift;

   $self >write($self >{level}, "BOA log exited: " . localtime(time) . "\n");

   $self >{fh} >close() or die "Unable to close log file : $!";

}

Class Data

By now you know that BOA is a big ol' application. As such, there are many modules 

that will want to write to the same log file. With the example OO implementation, 

14. When the last variable holding a reference to the object goes out of scope, or at program 

exit whichever comes first

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