class A
  def self.count
    @count
  end
  
  def self.count=(v)
    @count = v
  end
  
  def initialize
    self.class.count += 1
  end
end

class B < A
  @count = 0
end

class C < A
  @count = 0
end

B.new
C.new
C.new

B.count #=> 1
C.count #=> 2