From: Gary Wright <gwtmp01@...>
Date: 2007-03-26T09:17:19+09:00
Subject: Re: class methods and instance variables in ActiveRecord::Base


On Mar 25, 2007, at 2:45 PM, zig wrote:

> OK, I think I've got it now.  Class methods of the class have access
> to class variables, class instance variables, but not instance
> variables.  Instance methods of the class have access to class
> variables, instance variables, but not class instance variables.
> Class variables are accessible to both, but neither class instance
> variables nor instance variables are.  Therefore class instance
> variables are not the same thing as class variables.


It is also important to realize that class variables ('@@xyz') are
resolved lexically.  They are not resolved relative to 'self' but
instead relative to the surrounding lexical scope (top-level, file,
module blocks, class blocks).

This is very different than instance variables ('@xyz') which are
always resolved relative to 'self'.

class A
   @@example = 1
   puts @@example      # 1, lexically associated with A

   def self.example
     @@example
   end
end

puts @@example        # error, lexically associated with top-level
                       # but not defined yet.

def A.top_level_example
   @@example           # top level @@example!!!!
end

@@example = 2
puts A.top_level_example  # 2, gets @@example relative to top-level
puts A.example            # 1, gets @@example relative to A


Gary Wright