From: Christoph Rippel <crippel@...>
Date: 2001-03-04T11:43:11+09:00
Subject: [ruby-talk:11961] RE:  singleton classes?

> From: nosuzuki@e-mail.ne.jp [mailto:nosuzuki@e-mail.ne.jp]On Behalf Of
> Johann Hibschman
> Sent: Saturday, March 03, 2001 12:40 PM
> To: ruby-talk ML; ruby-talk@netlab.co.jp
> Subject: [ruby-talk:11945] singleton classes?
> 
> 
> Hi,
> 
> Could someone either explain singleton classes to me or point me to a
> good explanation?  The one from the Ruby reference manual just
> confuses.  Where are these things useful and what, precisely, are
> their semantics?
> 
> I've used them once, so far, when I wanted to effectively add a method
> to class Class that only applied to one of my classes.  I did this to
> implement a little abbreviation for defining methods within my class.
Since there is difference between ``Class method'' and 
``instance method'' you need to be a little bit carefull.
Effectively a singelton method of the class object  

A = Class.new 

# is the same as a class method of the class A -

def A.print_name 
	p "my name is #{self}"
end

A.print_name  #  => "my name is A"

# thus if you define 

class B < A
end

# you also inherited the class method #print_name - i.e.

B.print_name # => "my name is B"

will work. Note class Class is  special since you cannot 
subtype class Class. If you want to add ``instance methods''
somewhat automatically you can think about using the 
`include Module'' mechanism.


Christoph