From: Sean O'Halpin <sean.ohalpin@...>
Date: 2006-07-11T23:10:39+09:00
Subject: Re: Definition of methods: self

I agree with Robert - avoid @@class_variables wherever possible. The
problem is the way @@class_variables are looked up - they are shared
within a class hierarchy. Also, class << LotteryDraw does not
introduce a new class definition scope (even though you may think it
looks like it should).

The following examples demonstrate the way lookup for class variables
works in the context you're using:

class A
  @@a = 42
end

# add a class variable to Object - without this, the next puts @@a
would cause an error
@@a = 24
class << A
  puts @@a # this refers to the toplevel Object @@a, not A's
end

class A
  puts @@a # this does refer to A's @@a
end

p @@a

__END__
24
42
24

(By the way, if you had run ruby with -w you would have got the warning:
  class variable access from toplevel singleton method
)

Also, the order in which you define @@class_variables matters. Look at
this example:

@@a = 24 # add a class variable to Object
class A
  @@a = 42
  # because @@a already exists in the class hierarchy (A inherits from Object)
  # this assignment updates it
end

class << A
  puts @@a # there is only one @@a now
end

class A
  puts @@a
end

p @@a

__END__
42
42
42


My advice is steer clear of them!

Regards,

Sean