From "Well Grounded Rubyist" by David A. Black, Listing 5.6: Keeping track of car manufacturing statistics with class variables.

5.6_class_variables.rb

puts "\n"
# --> Keeping track of car manufacturing statistics with class variables <-- 

class Car
	@@makes = []  #1
	@@cars = {}
	@@total_count = 0
	
	attr_reader :make  #2
	
	def self.total_count  #3
		@@total_count
	end

	def self.cars  #3
		@@cars
	end
	
	def self.add_make(make)  #4
		unless @@makes.include?(make)
			@@makes << make
			@@cars[make] = 50  # ?????
		end
	end
	
	def initialize(make)
		if @@makes.include?(make)
			puts "Creating a new #{make}!"
			@make = make  #5
			@@cars[make] += 1  #6
			@@total_count += 1
		else
			raise "No such make: #{make}."  #7
		end
	end
	
	def make_mates  #8
		@@cars[self.make]
	end
end

#1
# defining class variables:
# @@make is an array.
# @@cars is a hash, whose keys are makes of cars, and the corresponding 
# values to the keys are counts of how many of each make there are.
# @@total_count is how many cars have been created.

#2
# reader attribute 'make' enables us to ask each car what its make is.
# value of 'make' is set when a car is created.
# and there's no writer attribute because we don't want code outside the 
# class changing the make of cars that already exist.

#3
# to provide access to @@total_count, we define the total_count method.

Car.add_make("Honda")
Car.add_make("Toyota")
Car.add_make("Nissan")

h1 = Car.new("Honda")
h2 = Car.new("Honda")
t1 = Car.new("Toyota")
t2 = Car.new("Toyota")
t3 = Car.new("Toyota")

puts "\nTotal Toyotas are: "
p t3.make_mates
#t2 = Car.new("Ford")

puts "\nTotal cars are: #{Car.total_count}"
puts "@@cars: "
p Car.cars