From: Tony Mobily <merc@...>
Date: 2006-02-20T23:22:04+09:00
Subject: Class instance variables, class variables and constants

Hello,

Thanks to the code in attach, I've figured out what is now completely  
obvious to me:

CLASS VARIABLE (@@something)
- Inherited by subclasses
- SAME ALLOCATION: If it's changed in Child, then it will change in  
Parent

CONSTANTS (Something)
- Inherited by subclasses
- PRIVATE ALLOCATION: If it's changed in Child, it will NOT change in  
parent

CLASS INSTANCE VARIABLES (@something)
- NOT Inherited by subclasses. They will simply be nil. Scope = Class
- DIFFERENT ALLOCATION: If it's changed in Child, it will NOT change  
in parent

I am sending this email to the mailing list so that if some Ruby  
newbie has the same problem with variable scope, they might find this  
email (and find it useful).

David A. Black  wrote:

"I think Matz is planning to change class variables in 2.0 so that they
are class/module scoped rather than hierarchy scoped."

But wouldn't it make class variables (@@something) the same as class  
instance variables (@something)?

Here's the code I used to figure things out:

-----------------------------------------------------

#!/usr/local/bin/ruby -w

class Outer
         @@class_var=10
         GlobalVar=100
         @civ=1000

         class Second < Outer

                 @@class_var=11
                 GlobalVar=101
                 @civ=1001

                 def Second::class_var
                         @@class_var
                 end

                 def Second::class_var=(what)
                         @@class_var=what
                 end

                 def Second::civ
                         @civ
                 end

                 def Second::civ=(what)
                         @civ=what
                 end


         end

         def Outer::class_var
                 return @@class_var
         end

         def Outer::class_var=(what)
                 @@class_var=what
         end

         def Outer::civ
                 @civ
         end

         def Outer::civ=(what)
                 @civ=what
         end



end

class Third < Outer

         def self::class_var
                 return @@class_var
         end

         def Third::class_var=(what)
                 @@class_var=what
         end

         def Third::class_var
                 @@class_var
         end

         def Third::class_var=(what)
                 @@class_var=what
         end

         def Third::civ
                 @civ
         end

         def Third::civ=(what)
                 @civ=what
         end
end


puts "Outer"
p Outer::class_var
p Outer::GlobalVar
p Outer::civ
puts "Outer::Second"
p Outer::Second::class_var
p Outer::Second::GlobalVar
p Outer::Second::civ
puts "Third"
p Third::class_var
p Third::GlobalVar
p Third::civ
puts
puts

Outer::Second::class_var=13
Outer::Second::GlobalVar=103
Outer::Second::civ=1003

puts "Outer"
p Outer::class_var
p Outer::GlobalVar
p Outer::civ
puts "Outer::Second"
p Outer::Second::class_var
p Outer::Second::GlobalVar
p Outer::Second::civ
puts "Third"
p Third::class_var
p Third::GlobalVar
p Third::civ
puts
puts


Third::class_var=14
Third::GlobalVar=104
Third::civ=1004

puts "Outer"
p Outer::class_var
p Outer::GlobalVar
p Outer::civ
puts "Outer::Second"
p Outer::Second::class_var
p Outer::Second::GlobalVar
p Outer::Second::civ
puts "Third"
p Third::class_var
p Third::GlobalVar
p Third::civ
puts
puts

# Just checking...
p Third::Second::class_var
p Third::Second::GlobalVar
p Third::Second::civ