Ruby class variable, class attribute(class instance variable), class constant tips

class Class_Attribute
  @var = 1 #class attribute or class instance variable
  
  def initialize
    @var = 2 #instance attribute
  end
 
  def report
    @var # instance attribute, not the class attribute
  end

  def Class_Attribute.report
    @var # class attribute
  end
end

class Child_A < Class_Attribute
  @var = 3
end

puts Class_Attribute.report        #=> 1
puts Class_Attribute.new.report    #=> 2
puts Child_A.report                #=> 3
puts Child_A.new.report            #=> 2

class Class_Constant
  VAR = [ 'a' ]
  VAR2 = [ 'b' ]
 
  def self.report
    VAR2[0]
  end
end

class Child_C < Class_Constant
  VAR2 = [ 'c' ]
end

puts Class_Constant::VAR[0]     #=> 'a'
puts Class_Constant::VAR2[0]    #=> 'b'
puts Class_Constant.report      #=> 'b'
puts Child_C::VAR2[0]           #=> 'c'
puts Child_C.report             #=> 'b'
puts Child_C::VAR[0]            #=> 'a'

from David A. Black:

   @@avar = 1
  class A
    @@avar = "hello"
  end
  puts @@avar  # => hello

  A.class_eval { puts @@avar }  # => hello

If you’re just looking to store some data in a variable which is unique to your class, but accessible from instances or externally, why not just use instance variables?

class A
  @foo = "bar"
  class << self; attr_reader :foo; end
end
nil
A.foo #=> "bar"

references:http://www.oreillynet.com/ruby/blog/2007/01/nubygems_dont_use_class_variab_1.html