From: Avi Bryant <avi@...4.com>
Date: 2001-07-24T06:01:46+09:00
Subject: [ruby-talk:18386] Re: More newbie questions

On Tue, 24 Jul 2001, Matt wrote:

> On Tue, 24 Jul 2001, Dave Thomas wrote: 
> > Matt <matt@greenviolet.net> writes:
> > 
> > > cgi.out { cgi.p { cgi.rfc1123_date (Time.now) } }
> > 
> > rfc1123_date is a class method of CGI, as opposed to an instance
> > method. This means you'd call it using
> > 
> >   CGI.rfc1123_date(Time.now)
> > 
> > Here you're using the class (CGI) as the receiver of the message, as
> > opposed to an instance of the class (cgi in your example).
> 
> OK. That worked! Now I'm confused (again) when to use the class and
> when to use an instance. For example take rfc1123_date. Why wasn't it
> inherited by the instance? I admit to not knowing enough about OO, but
> the only other language I've ever really tried was Perl's OO. And I
> hated it. ;)

Classes are themselves objects, and have a completely different set of
methods from their instances.

For example, CGI.new calls the "new" method on the CGI class object, which
returns a new CGI instance.  However, "new" is not defined for instances:
cgi = CGI.new
cgi2 = cgi.new #this doesn't work

Class methods are often used for general utility functions that don't have
to do with a particular instance - since it wouldn't matter *which*
instance of cgi you called rfc1123_date on, it's made a class method
instead.  Class methods have other uses, too, of course.

There are a number of ways to define class methods - the most common is
probably this one:

class Foo
	def bar
		#this is an instance method
	end

	def Foo.bar
		#this is a class method
	end
end

Class methods are actually a special case of Ruby's singleton
methods; there's probably an index entry for that term in the pickaxe
book, but I can't give you a page number right now.

Cheers,
Avi