From: sera@... (Francis Hwang)
Date: 2003-05-20T11:50:16+09:00
Subject: Re: examples for my OSCON talk

"Chris Pine" <nemo@hellotree.com> wrote in message news:<02b701c31c8d$c6b0fc40$6401a8c0@MELONBALLER>...
> I know it's wrong to ask the mailing list for help on your homework, but
> this isn't *really* homework...
> 
> My talk is "Understanding Ruby's Object Model", but I'd like to start (and
> maybe end) with some really cool examples about the cool things you can do
> in Ruby relating to classes and modules being objects.

Not sure if this counts as cool, but it was useful to me: I've
programmed an object-relational mapping framework in Ruby where fields
are stored as data structures, to be retrieved at runtime. They're
class methods, so let's say you have a User class:

class User < DomainObject
	def User.fields
		# return an array of Field objects
	end
end

.... so then you can have code like

class ObjectCreator
	def verify (valueHash, aClass)
		# for each field defined for this domain object class, give it a
chance to
		# throw an exception if there's an invalid value
		aClass.fields.each { |field|
			field.verify (valueHash[field.name])
		}
	end
end

"aClass" in the code above is a metaclass: You don't pass in an
instance, but the class itself. (You'd call "verify (valueHash,
User)", not "verify (valueHash, johnDoe)".)

The neat thing about this is that you can attach class-specific data
and use it without knowing at runtime what class you're dealing with.
You can't do this in Java without jumping through some really really
nasty hoops: It binds class methods at compile-time. (Java class
methods aren't really class methods at all; they're more like global
methods that are bound to a particular namespace.)

> Also, a question:  what's the difference between a class's singleton class
> and a metaclass?  If I'm not mistaken, these are the same in Ruby, but I
> mean conceptually:  what is a metaclass?

Traditionally a singleton is any class which is guaranteed to have
only one instance. It's useful but also easy to trip over. (See
http://c2.com/cgi/wiki?SingletonPattern for a big discussion of this.)

Ruby also uses the word "singleton" to describe any instance whose
methods have been explicitly set to vary from the standard class
definition. For example:

myStrangeArray = [ 0, 1, 2 ]
def myStrangeArray.length
	999
end

(I don't know where this usage comes from or if it's unique to Ruby;
it tripped me up when I first came across it.)

Francis