Package or Class Example in Perl

Packages or Classes in Perl (OOPS Concept)

Definition :
In Perl, a class is corresponds to a Package.
To create a class in Perl, we first build a package.
A package is a self-contained unit of user-defined variables and subroutines, which can be re-used over and over again.
They provide a separate namespace within a Perl program that keeps subroutines and variables from conflicting with those in other packages.

Rules to be followed for a module in Perl :
The module in perl in general terms means a namespace defined in a file. Certain modules are only collections of function. In perl the modules must follow the following guidelines:
- The file name of a module must the same as the package name.
- The general naming convention of naming a package is to begin with a capital letter, but not mandatory always.
- All the file name should have the extension of "pm".
- Last line of a package is 1 i.e., returning always 1 as mentioned in the example below.
- In case no object oriented technique is used the package should be derived from the Exporter class.
- Also if no object oriented techniques are used the module should export its functions and variables to the main namespace using the @EXPORT and @EXPOR_OK arrays.
The use directive is used to load the modules.

Creating/Defining Object :
To create an instance of a class (an object) we need an object constructor.
This constructor is a method defined within the package.

What is Bless : 
You create a hash object and bless the hash object to who ever calling the constructor (new method) of a package/class.

E.g.,
Blessing an object based on the parameters passed to the constructor method
my $self = {
        _firstName => shift,
        _lastName  => shift,
        _ssn       => shift,
    };
 bless $self, $class;

Blessing an empty object is also possible
my $self = {};
bless $self , $class

E.g.,
Package Person;
sub new {
    my $class = shift;
    my $self = {
        _firstName => shift,
        _lastName  => shift,
        _ssn       => shift,
    };
    bless $self, $class;
    return $self;
}
1;

Creating Instance of Object :
In order to access package level methods, you need to create object of that class and then start accessing package level variables/methods.

my $object = new Person( "David", "Johnson", 23234345);
In the above example, we are creating object of Person from main.pl, here we are passing required parameters and the constructor (new method) of Person class will return a formatted hash object.

You can even bless an empty hash object as well.        
In case of empty object, the following returns empty object
my $object = new Person();

E.g.,
use Person;
use Data::Dumper;
my $object = new Person( "David", "Johnson", 23234345);

print "\n Person Object is : " . Dumper($object);

Information Hiding : 
This should not allow the users from modifying the object data.
You should allow the users to access the core object using setter/getter methods, there by achieving hiding object data from outside the world.

Let's see the below example for better understanding :
Support.pm has defined some getter(setFirstName) and setter(getFirstName) for accessing firstName
main.pl is accessing the firstName using getter(setFirstName) and setter(getFirstName)

support.pm

#!/usr/bin/perl

package support;

use strict;
use warnings;
use Data::Dumper;

sub new
{
    my $class = shift;
    print "\nClass Name : " . $class;
    my $self = {
        _firstName => shift,
        _lastName  => shift,
        _ssn       => shift,
    };
    # Print all the values just for clarification.
    print "\nFirst Name is $self->{_firstName}";
    print "\nLast Name is $self->{_lastName}";
    print "\nSSN is $self->{_ssn}";
    bless $self, $class;
    return $self;
}

sub setFirstName {
    my ( $self, $firstName ) = @_;
    $self->{_firstName} = $firstName if defined($firstName);
    return $self->{_firstName};
}

sub getFirstName {
    my( $self ) = @_;
    return $self->{_firstName};
}

1;
  

main.pl

#!/usr/bin/perl
use support;

use strict;
use warnings;
use Data::Dumper;

my $object = new support( "David", "Johnson", 23234345);

# Get first name which is set using constructor.
my $firstName = $object->getFirstName();

print "\n\nBefore Setting First Name is : $firstName";

# Now Set first name using helper function.
$object->setFirstName( "Mohd" );

# Now get first name set by helper function.
$firstName = $object->getFirstName();

print "\nAfter Setting First Name is : $firstName\n";  

Output :

Class Name : support
First Name is David
Last Name is Johnson
SSN is 23234345

Before Setting First Name is : David
After Setting First Name is : Mohd