Perl objects 101

Perl Objects 101
Simple OOP with Perl

Terms
Class - a template describing data and
behavior specific to the object
Object - a container for “some data” which is
associated with “some behavior”
Method - a function associated with a class
Attribute - private data associated with each
unique object

Perl Terms
Class - a package containing methods
Object - a reference with an associated class
Method - a subroutine which takes an object
or class as its first argument
Attribute - any sort of Perl data should work

Bless = magic
bless $reference, Classname

is any reference (scalar, hash, array,
function, file handle)
Classname is any package name.
$reference

The result is a blessed reference; an object.

Methods
Methods are subs defined in the class’s
package.
Call them using the dereferencing arrow:
$obj->method( $param1, $param2 );

The first parameter will be an object reference.

Accessing the data
Since the object is still a reference, the
underlying data are easily accessible.
$obj->{attr1} = 42;

No Protection
What you just saw is considered BAD.
Any code outside of the class can easily tinker
with the insides of the object.

Class Methods
Same syntax to call Class methods:
$my_class->method();

Except the package name is used instead of the
object reference.

Constructors
Constructors are class methods that return an
object.
The name is arbitrary, although new() is most
commonly used.
package MyClass;
sub new {
bless { attr1 => 42 }, ‘MyClass’;
}

Better Constructor
It’s first parameter will normally be the class
name.
package MyClass;
sub new {
my ( $class ) = @_;
bless { attr1 => 42 }, $class;
}

Inheritance
@ISA array
It is a package global.
package MyInheritedClass;
use vars qw( @ISA );
@ISA = qw( MyClass );
sub my_method2 {};

@ISA Continued
When an object or class method is called, Perl
gets the package and tries to find a sub in that
package with the same name as the method.
If found, it calls that.
If not, it looks up the @ISA array for other
packages, and tries to find the method in those.

UNIVERSAL
All classes implicitly inherit from class
“UNIVERSAL” as their last base class.

Inheriting Shortcut
This is tedious:
use vars qw(@ISA);
@ISA = qw(MyClass);

There is a shortcut:
use base ‘MyClass’;

Calling Inherited Methods
We can do:
sub method1 {
my ( $self ) = @_;
MyClass::method1($self);
}

That’s not OOish and won’t work if MyClass
doesn’t have a sub method1.

SUPER Pseudo-Class
The right thing would be to do:
sub method1 {
my ( $self ) = @_;
$self->SUPER::method1($self);
}

Inheriting Constructors
Normally, a base class constructor will bless the
reference into the correct class, so we just need
to do some initialization:
sub new {
my ( $class, %params ) = @_;
my $self = $class->SUPER::new(%params);
# do something with the params here...
return $self;
}

Mostly, such constructors are not needed.

Destructors
Perl has automatic garbage collection, so
mostly, destructors aren’t needed.
If they are, create a sub called DESTROY:
sub DESTROY {
my ($self) = @_;
# free some resources
}

Multiple Inheritance
Just put more stuff into @ISA
@ISA = qw(Class1, Class2);

or
use base qw(Class1 Class2);

Data Storage for Objects
Most objects use hashrefs.
Convenient to use.
No protection.
You can’t prevent code from tinkering.
Well, there is a thing called “Inside Out
Objects”, but we won’t cover that today.

Other ways of OOP
Many, many modules have been created to help
with this.
Currently, Moo/Mouse/Moose is The One True
Way
This is Perl, so you know that isn’t quite true.

Perl objects 101

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Perl objects 101