From: Gary Wright <gwtmp01@...>
Date: 2007-12-17T05:07:35+09:00
Subject: Re: Mixins and class variables


On Dec 16, 2007, at 10:25 AM, Brubix wrote:
> I can't figure out how to set class variables from class methods
> inherited from a module ?
>
> The following doesn't work as I expected (I spare you my others
> pathetic attempts):
>
> module MyModule
> 	def self.extended(base)
> 		base.setClassVar(base.name.downcase)
> 	end
> 	def setClassVar(value)
> 		puts "self is #{self.name}"
> 		puts "@@classVar=#{value}"
> 		@@classVar = value
> 	end
> 	def getClassVar
> 		@@classVar
> 	end
> end
>
> class Class1
> 	extend MyModule
> end
> class Class2
> 	extend MyModule
> end
>
> puts "classVar for Class1 is " + Class1.getClassVar
> puts "classVar for Class2 is " + Class2.getClassVar
>
> It gives:
> self is Class1
> @@classVar=class1
> self is Class2
> @@classVar=class2
> classVar for Class1 is class2    # Shouldn't be class1 ?!
> classVar for Class2 is class2

The New Haven Ruby Brigade spent an hour or so one night trying
to figure out the scoping rules for class variables.

The key to understanding them is to realize that instance variables
are dynamical scoped relative to self while class variables
are *lexically* scoped (i.e. the scoping is defined by the
syntactic structure of the code).

In your example, '@@classvar' is lexically located within MyModule.
That means it will *always* be associated with MyModule regardless
of the value of self at the time setClassVar or getClassVar are
executed.

Here is another situation that illustrates this difference:

class A
   @@cvar = 'class A'
   def cvar
     @@cvar
   end
end

puts A.new.cvar               # => class A
puts A.class_eval { @@cvar }  # => NameError, @@cvar undefined

The class_eval version fails because @@cvar in the block is
not lexically within the class A/end block.  It is *dynamically*
in the scope of class A, but not lexically within the scope of
class A.  In this case, @@cvar is resolved to the top-level
scope which is associated with Object.

MonkeeSage's suggestion to use Module#class_variable_get/set
forces the variables to be resolved relative to the receiver
and thus bypasses the lexical scoping rules for class variables.

This is another situation where the syntactic similarity of the
instance variable sigil ("@") and the class variable sigil ("@@")
causes people to think they behave in the same manner.  They don't.


Gary Wright