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
