@ and @@ blog

at-and-at-at-class.rb

class SampleClass
  # (#1) class-level setter and getter for a
  def self.a=(value)
    @@a = value
  end
  def self.a
    @@a
  end

  # (#2) instance-level setter and getter for a  
  def a=(value)
    @@a = value
  end
  def a
    @@a
  end

  # (#3) class-level setter and getter for b
  def self.b=(value)
    @b = value
  end
  def self.b
    @b
  end

  # (#4) instance-level setter and getter for b
  def b=(value)
    @b = value
  end
  def b
    @b
  end

  # (#5) class- and instance-level attr_accessors
  class << self
    attr_accessor :c
  end
  attr_accessor :c
end

class SampleSubClassA < SampleClass; end
class SampleSubClassB < SampleClass; end