finalizer.rb

class Thing

  # The number of instances of this class that are currently in existence.
  @@count = 0

  # Print out how many thing there are.
  def self.print_count
    puts "There are now #{@@count} things."    
  end
  
  def initialize
    # Tally one more thing.
    @@count += 1
    Thing.print_count
    
    # When this object is destroyed, tally one fewer thing.
    ObjectSpace.define_finalizer(self) do 
      @@count -= 1
      Thing.print_count
    end
  end

end

puts "Creating things."
10.times { Thing.new }
puts "Exiting."