[Mpls-pm] Dumb Question...Module Method - Dual Name

[Ian Malpass]

On Sun, 23 Oct 2005, Gary Vollink wrote:

> O.K., I think I finally have an idea of what is going on, and I'd like
> to thank each of you for introducing various ways to get it done.

Ah, perl. Bless your multiple cat-skinning methodologies.

> I definately need to read more about the differences between 'Instance
> method', 'class method' and 'function'... but for now, I understand
> enough to get what I need, done...

# Class method
Example->foo( 'bar' ) 
# Example::foo gets 'Example', 'bar' as its arguments.

# Instance/object method
$eg = Example->new();
$eg->foo( 'bar' );
# Example::foo gets (reference to) $eg, 'bar' as its arguments.

# Subroutine (probably equivalent to function, really)
Example::foo( 'bar' );
# Example::foo gets 'bar' as its argument.

So the difference between an instance and a class method is that the 
former is called on a blessed reference (object), and gets a copy of the 
object in its arguments, and so the method has access to other data stored 
in the object. The class method gets the class name as a string as its 
first argument instead[0].

A class method is then just a method that doesn't try to access any object 
data. You can call a class method on an object quite happily, but not vice 
versa. (As far as perl is concerned, methods are methods - there is no 
explicit differentation between the two, merely practical 
differentiation.)

Read perldoc perltoot, and then read Object Oriented Perl by Damian 
Conway[1]. Or skip perltoot and just go to OOP. It's very good.

> So I understand OO theory, I've just never had to deal with it in a
> day-to-day basis.  Yet, if I want to become the all-powerful Perl Guru
> than I know I can be... I'm going to have to get a lot more practical
> experience with it.  :-)

The major problem with/strength of OOP is that it's rather lax OO 
(especially in terms of encapsulation, although Conway discusses ways to 
enforce some encapsulation in his book). Many OO purists disparage perl's 
OO. Many OOPists laud its laid back approach. If you want to learn or 
apply OO theory, perl might not be the best language to choose.

Speaking as someone with no formal CS training, I like OOP :)

Ian

P.S. I didn't mention it in my mail about AUTOLOADing, but one thing I 
often do to try to make sure I don't do anything untoward is have some 
sort of list of allowed methods, and have AUTOLOAD complain if it's not 
one of those. You could probably leverage @EXPORT for that purpose.

[0] A common way to do object creation is

   sub new {
     my $proto = shift;
     my $class = ref $proto || $proto;
     return bless {}, $class;
   }

Thus you can do $eg->new() as well as Example->new(). If you call new() as 
an object method, $proto is the object, and calling ref $proto will give 
you its class name. If you call it as a class method, $proto is the class 
name, and ref $proto is undef (and false), so you get the value of $proto 
instead. I don't know that I've ever created an object using an object 
method, though.

[1] <http://www.amazon.com/gp/product/1884777791>

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The soul would have no rainbows if the eyes held no tears.

Ian Malpass
<ian at indecorous.com>