Using Perl to Create Reusable Web Applications

By Eugene Logvinov
2004-09-21

Object and Class Relationships

Before designing the object model it is a good idea to examine existing CPAN (Comprehensive Perl Archive Network) modules that could be useful. The principal issue is the relationship between a newly created class and a CPAN module class. The relationship could be: a) a standard class object is included in the newly created class ("has-a" relationship), and less frequently b) the newly created class inherits from the standard class ("is-a" relationship).

The code listing below is an example of a constructor for My class, which inherits from BaseClass. Moreover, My class contains AnotherClass object, which is private (the name begins with underline character -- a convention not enforced by Perl itself).

Listing 1. My class: implementing relationships

package My;

require BaseClass; #required if BaseClass is present in BaseClass.pm

@ISA=qw(BaseClass);

use AnotherClass;

sub new {

  my $package=shift;

  my $self=$package->SUPER::new($package); #create object in BaseClass

  $self->{_another_class_object}=new AnotherClass;

  $self;

}

Listing 2. AbstractCGI class: implementing specific API (CGI)

package My;

require BaseClass; #required if BaseClass is present in BaseClass.pm

@ISA=qw(BaseClass);

use AnotherClass;

sub new {

  my $package=shift;

  my $self=$package->SUPER::new($package); #create object in BaseClass

  $self->{_another_class_object}=new AnotherClass;

  $self;

}







package AbstractCGI;

sub new {

  my $package=shift;

  my $self={

    _cgi_method=>undef, #simple class data

    _query=>undef

  };

  bless $self, ref $package || $package;

  $self->_init;

  $self;

}

sub _init {die} #private method

sub get_param {die}

sub is_print_form_mode { shift->get_param('mode') eq 'form' }

sub is_print_entries_mode { shift->get_param('mode') eq 'entries' }

Tutorial Pages:
» Object Technologies and HTML Templates in CGI Programming
» Object and Class Relationships

First published by IBM DeveloperWorks

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