To bless or not to bless, that is my question!

Question

first post from a newbie-user. Every question I google seems to bring me here and I always get a great answer to what I'm looking for; so naturally this was my first stop when I began pondering the usage of blessing in Perl.

I've just gotten into Perl's OOP and just today read the post asking what bless does. I now understand that it references a scalar/hash/array to an object, 'attaching' it, if you will.

In most of the examples of classes I see in Perl, they don't seem to have properties like I'm used to seeing in other languages...

{ package Person;
    my $property = "This is what I'm talking about :)";

    sub new { ... }
    ...
}

So, I created a stupid class with a property to see what would happen. I gave the property the value 'NIL' right off the bat and then 'Not Nil!' in the constructor. Using a method LIST, I was able to print the property, and as I expected, it printed 'Not Nil!'

My question is, if properties work the same as I expected them to work (declared in the body) then why use bless at all? What is the added benefit of having that reference when you could simply create a scalar/hash/array as a property, or create whatever references you want as a property?

I hope I explained what I'm trying to ask well enough, very green with Perl :)

Answer 1

Well, that is not how you create classes in Perl.

Your $property variable is defined in package scope. Therefore, there will only one copy of it per class rather than each object having its own copy.

One might implement such a class using hash based objects the long and hard way as below:

#!/usr/bin/perl

package Person;

use strict; use warnings;

sub new {
    my $class = shift;
    my $self = {};
    bless $self => $class;

    my ($arg) = @_;
    for my $property ( qw( message ) ) {
        if ( exists $arg->{$property} ) {
            $self->$property($arg->{$property});
        }
    }
    return $self;
}

sub message {
    my $self = shift;
    return $self->{message} unless @_;
    my ($msg) = @_;
    $self->{message} = $msg;
}

package main;

my $person = Person->new({
    message => "This is what I'm talking about :)"
});

print $person->message, "\n";

Now, this gets tedious fast. So, there are modules that help you deal with this as well as helping you define your classes in way that is safe for inheritance.

Class::Accessor is one such utility module.

For programs where startup time is not an issue, you should consider Moose. With Moose, you can write the above as:

#!/usr/bin/perl

package Person;

use Moose;

has 'message' => (is => 'rw', isa => 'Str');

__PACKAGE__->meta->make_immutable;
no Moose;

package main;

my $person = Person->new({
    message => "This is what I'm talking about :)"
});

print $person->message, "\n";

You should read perldoc perltoot and Moose::Manual::Unsweetened for the standard way of doing things.

Answer 2

What you did with $property in this case is declared a variable in the "Person" package's scope. You change that inside (or outside using $Person::property) of the package, and any object that refers to it will see the updated variable, so it acts a lot like a "static attribute (Java)" without any real "private" scope. By convention, hidden things in Perl ("private" or "protected") are prefixed with an underscore, but of course this isn't enforced.

You don't actually make a new class, as you pointed out, with the "package" keyword; you can use "package" without OOP at all. That simply creates a separate "namespace".

The advantage of "blessing" a variable, almost always a hash reference from what I've seen, is that you can have methods, just like any other OOP language. Just remember to bless whatever you return in the new {} subroutine ("new" isn't actually a reserved word; just a convention). When you call a method on the "object" (a blessed data structure like a hashref), the first argument of the method is the data structure itself. So, if you have a hashref called $myobject, which is blessed to AwesomeClass, and you define a method in AwesomeClass called doSomethingAwesome, which needs to accept one variable, you would have to "shift" @_ (which is the argument list of the subroutine, or use $_[0]) to access the $myobject hashref. Python does something similar, and all languages pass the object reference to the method somehow. ("this" keyword in many, see also "thiscall" calling convention)

NB: I've seen lots Perl bashing in my time, which has only been a few years as a programmer. Perl is an awesome language that was made by a very smart linguist (Larry Wall) and has a fanatical following -- more fanatical at one time than Ruby, perhaps, but not as much as David Koresh). Perl does things very differently than lots of languages but if you look at code golf entries on this site and others, you can clearly see that much can be accomplished with very little Perl (no guarantees about code legibility, especially for newcomers!)

Answer 3

The value of bless'ing an object is getting to use the methods from a particular package.

package MyClass;
sub answer { my ($self)=@_; return $self->{foo} * 42; }

package main;
my $object1 = { foo => 1, bar => "\t" };
my $object2 = bless { foo => 2, bar => "\t" }, "MyClass";

$ref1 = ref $object1;        #  'HASH'
$ref2 = ref $object2;        #  'MyClass'

$answer1 = $object1->answer;     # run time error
$answer2 = $object2->answer;     # calls MyClass::answer, returns 2 * 42 = 84

Answer 4

Ugh... Sinan's answer is entirely too learned for my taste, at least past 12am :)

So I'll give a shorter and somewhat less Perly one, just for variety's sake.

Your question is not really about Perl as far as I can tell, and can be just as easily ased in another form: "Why bother using C++ and OOP in it when C already has structs?"

In other words, you seem to be asking what the point of using OOP paradigm is.

The answer is of course that it helps solving certain software engineering problems easier than pure procedural programming. Emphasis on certain - OOP is not a panacea for every problem, any more than ANY technique/approach/paradigm is.

Using OOP (in a form of packages as classes and blessed hashes as objects in Perl) allows you to enjoy the benefits of inheritance, polymorphism and other OOPyish mumbo-jumbo which you are probably already fairly familiar with from you non-Perl OOP experience.

Can you do 100% of what you'd have done with a blessed object with a pure data structure? Absolutely. Would 100% of it be just as easy/short/readable/maintainable code as you can achieve using objects? Most likely not, though it depends on how well your OOP code actually leverages the benefits that OOP provides (BTW, I have encountered supposedly OOP code (Perl and not) which wasn't really taking any advantage of OOP paradigm and could have been made easier to read and understand had it been stripped of its OOP chrome).