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Now that we have the constructor, let's see how can we create attributes in Core Perl OOP.

Before that, let's take a look at the example in Moose.

examples/Moose/person01/script/person.pl

use strict;
use warnings;
use v5.10;

use Person;

my $teacher = Person->new;

$teacher->name('Foo');

say $teacher->name;

We are calling the constructor here Person->new that returns an object we assign to $teacher then we are calling the accessor $teacher->name('Foo') using it as a setter by providing it a value and then using the same accessor as a getter $teacher->name (without passing a value) to fetch the current value of the attribute. Using the same method called name

The implementation in Moose was rather simple:

examples/Moose/person01/lib/Person.pm

package Person;
use Moose;

has 'name' => (is => 'rw');

1;

Core Perl OOP - attributes

In core Perl we need the following:

examples/oop/person01/lib/Person.pm

package Person;
use strict;
use warnings;

sub new {
    my ($class, %args) = @_;

    my $self = \%args;

    bless $self, $class;

    return $self;
}

sub name {
    my ($self, $value) = @_;
    if (@_ == 2) {
        $self->{name} = $value;
    }

    return $self->{name};
}

1;

We have the constructor slightly changed. We'll talk about that again a little bit later.

We also have the implementation of the accessor which is also a getter and a setter.

How does this work?

When we call the $techer->name('Foo'); perl will notice that $teacher is a blessed reference to a hash and that it was bless-ed into the Person name-space. If it wasn't a blessed reference, perl wouldn't know what to do with the arrow and the "name" after that and it would throw an exception: Can't call method ... on unblessed reference

Because it is bless-ed and because it is bless-ed into the Person name-space, perl will look for the "name" function in the Person name-space.

Once that function is found, perl will call that function with the parameters we passed to it, but it will also take the variable we had on the left-hand side of the arrow ($teacher in our case) and pass it as the first argument.

If we look at the implementation of name in the Person.pm file, we can see that the "name" function is expecting two parameters. The first is going to be assigned to the $self variable, the second is going to be assigned to the $value variable. The names of the variable are arbitrary, but it is quite accepted in the Perl community to use the variable $self as the first parameter, so that will hold the "current object" inside the implementation of the class. (This is similar to 'this' or 'self' in some other programming languages.)

In our example this "name" function is called once as a "setter" when we pass a value to it, and once as a "getter" when we don't pass any value. Becuase perl passes the object as the first parameter this means that when it is called as a "setter" we are actually going to get 2 parameters and when it is called as a "getter" we are going to get one parameter.

The first statement in the "name" subroutine assigns the parameters to some local variables. In the second statement we check if this time the function should act as a getter or as a setter? We check the number of parameters. If we got two parameters then this is a setter. In this case we take the content of $self, which as you might recall from the article about the constructor is just a reference to a hash, and assignt to the 'name' key the $value we have received.

This shows that the attributes of an object in Perl are just key/value pairs in a hash reference.

If we use the 'name' function as a 'getter', then we don't pass any value to it, which means $value will be undef, but more importantly @_ will only have one element. This we will skip the assignment and the only thing we do is to return the value of the 'name' key from our hash reference.

Checking if $value is undef

In some cases instead of checking if the user has passed exactly 2 parameters, people check if the $value is defined or not and based on this they decided if the function behaves as a setter or as a getter. It is probably inferior to hecking the number of parameters as that means we won't be able to set the attibute to become undef.

More that 1 parameter?

In this example we have not dealt properly with the case when a users passes more than 1 parameter. If we call $teacher->name('Foo', 'Bar') this will behave quite incorrectly. Perl will automatically pass the content of $teacher which means our code will act as a getter.

Actually we have not decied what should be the correct behavior in this case so I cannot really know if this is the "correct" behaviour or not. It will certainly be surprising to someone calling this method and not setting the attibute. A better behaviour might be to throw an exception either using die or using Carp::croak.

What should the setter return?

The 'getter' should return the value of the attribute, but what should the setter return? In our case the 'setter' returns the newly assigned value, but there are other options.