an example comparing *instance variable*, *class instance variable*, and *singleton class instance variable*.

class_and_singleton_instance_variables.rb

class Foo
  def i_method
    @foo = ''
  end

  @bar = ''

  class << self
    @baz = ''

    def c_method
      @bar2 = ''
    end
  end
end

Foo.singleton_class.instance_variables
# => [:@baz]

Foo.instance_variables
# => [:@bar]

Foo.c_method
Foo.instance_variables
# => => [:@bar, :@bar2]

a = Foo.new
a.instance_variables
# => []
a.i_method
a.instance_variables
# => [:@foo]