With the code below, I want to print the statement, then add the subclass created into @animals array every time a new subclass is created. I'd like to know how I can create an instance of Animal and use its variable @animals.
class Animal attr_accessor :animals def initialize @animals = [] end def self.inherited(subclass) puts "a new subclass of #{subclass} was created" Animal.animals << subclass end end class Dog < Animal end class Cat < Animal end dog = Dog.new cat = Cat.new 
class Animal class << self attr_reader :animals, :animal_classes end @animals = [] @animal_classes = [] def initialize Animal.animals << self end def self.inherited(subclass) Animal.animal_classes << subclass end end class Dog < Animal end class Cat < Animal end class Armadillo < Animal end dog = Dog.new cat1 = Cat.new cat2 = Cat.new puts "Animals: #{Animal.animals.inspect}" # => Animals: [#<Dog:0x007ffe6d0acaf8>, #<Cat:0x007ffe6d0acaa8>, #<Cat:0x007ffe6d0aca58>] puts "Animal Classes: #{Animal.animal_classes.inspect}" # => Animal Classes: [Dog, Cat, Armadillo] 
Use a class variable:
class Animal @@animals = [] def self.inherited(subclass) @@animals << subclass end end ... if I understand what you're trying to do.
inheritedmethod makes use of a class variableAnimal.animalsrather than an instance variableAnimal.new.animals, which you defined in the initializer.Animalclass?Animal? The former would list 2 dogs and 1 cat if you instantiated them as such, while the latter would always just have 2 elementsDog, Catregardless of whether either was ever instantiated.