From: Christoph Rippel <crippel@...>
Date: 2001-03-13T13:40:06+09:00
Subject: [ruby-talk:12541] Re: class variables

"Dave Thomas" <Dave@PragmaticProgrammer.com> wrote in message
news:m266hf9jf7.fsf@zip.local.thomases.com...
[...]
> > A class  variable is shared  among all  objects of a  class, and it  is
also
> > accessible to the class methods that we'll describe later. There is only
one
> > copy of a particular class variable  for a given class. Class variable
names
> > start with two ``at'' signs, such as ``@@count''. Unlike global and
instance
> > variables, class variables  must be initialized before they  are used.
Often
> > this initialization  is just a  simple assignment in  the body of  the
class
> > definition.
> >
> > It says 'one copy for a given class' not for `class hieararchy'.
>
> I agree: we don't really say either way in the book. To make it
> clearer that Ruby classes inherit class variables, I'll add some
> wording to page 214 at the next printing.

In general nice it might be nice if your book could point out in
a  comprehensive fashion the somewhat confusing differences
between a

-- variable local to an object
-- class variable
-- variable local to the Class
-- variable local to the Meta Class

(the visibility rules are particularly confusing IMO)

class A
attr_accessor       :var
@@class_var    =   "class variable"
@var_of_A        =   "var of A"
def class_var
     @@class_var
end
end

a = A.new
p   a.var                       # =>  nil   - not initialized yet
a.var   = "initialized"
p   a.var                     # =>  "initialized"

class << A
attr_accessor      :var_of_A
def  class_var= (other)
      @@class_var = other
end
def  class_var
        @@class_var
 end
@var_of_A       =   "var of meta A"
end

class  B < A
end

b = B.new
p b.class_var           # => "class variable"
p B.class_var           # => "class variable"

B.class_var=  "changed"
p  a.class_var           #  => "changed"

p B.var_of_A            # =>  nil  -  not initialized
B.var_of_A =  "var of B"

p B.var_of_A            # =>  "var of B"
p A.var_of_A           # => "var of A"

metaA = ObjectSpace._id2ref(
class << A
id
end
)
def metaA.var_of_meta
     @var_of_A
end

p metaA.var_of_meta    # => "var of meta A"


Christoph