Ruby Language Variable Scope and Visibility Class Variables
Example
Class variables have a class wide scope, they can be declared anywhere in the class. A variable will be considered a class variable when prefixed with @@
class Dinosaur
@@classification = "Like a Reptile, but like a bird"
def self.classification
@@classification
end
def classification
@@classification
end
end
dino = Dinosaur.new
dino.classification
# => "Like a Reptile, but like a bird"
Dinosaur.classification
# => "Like a Reptile, but like a bird"
Class variables are shared between related classes and can be overwritten from a child class
class TRex < Dinosaur
@@classification = "Big teeth bird!"
end
TRex.classification
# => "Big teeth bird!"
Dinosaur.classification
# => "Big teeth bird!"
This behaviour is unwanted most of the time and can be circumvented by using class-level instance variables.
Class variables defined inside a module will not overwrite their including classes class variables:
module SomethingStrange
@@classification = "Something Strange"
end
class DuckDinosaur < Dinosaur
include SomethingStrange
end
DuckDinosaur.class_variables
# => [:@@classification]
SomethingStrange.class_variables
# => [:@@classification]
DuckDinosaur.classification
# => "Big teeth bird!"
