Ruby: インスタンス変数とクラス変数 ref: http://qiita.com/lnznt/items/4f71e57c9bd1a520e49a

file1.txt

$ ruby sample-ivar.rb 
foo は呼ばないでください.
foo は呼ばないでください.
foo は呼ぶなッ！！3 度目！！
--------
foo は呼ばないでください.
foo は呼ばないでください.
foo は呼ぶなッ！！3 度目！！

file3.txt

$ ruby sample-cvar.rb 
foo は呼ばないでください.
foo は呼ばないでください.
foo は呼ぶなッ！！3 度目！！
--------
foo は呼ぶなッ！！4 度目だ！！
foo は呼ぶなッ！！5 度目だ！！
foo は呼ぶなッ！！6 度目だ！！

file5.txt

$ ruby sample.rb 
[PM, D, M, MM, C, B, A, M0, Object, Kernel, BasicObject]
:D                # メソッド getv の出力
"I am 'PM'"       # メソッド foo の出力

file6.txt

   D -> C -> B -> A -> M0 -> M -> MM -> PM   # クラス変数参照の順番

sample-cvar.rb

class C
  def foo
    @@count ||= 0
    @@count += 1

    if @@count < 3
      puts "foo は呼ばないでください."
    else
      puts "foo は呼ぶなッ！！#{@@count} 度目！！"
    end
  end
end

c1 = C.new
c1.foo
c1.foo
c1.foo

puts '-' * 8

c2 = C.new
c2.foo
c2.foo
c2.foo

sample-ivar.rb

class C
  def foo
    @count ||= 0
    @count += 1

    if @count < 3
      puts "foo は呼ばないでください."
    else
      puts "foo は呼ぶなッ！！#{@count} 度目！！"
    end
  end
end

c1 = C.new
c1.foo
c1.foo
c1.foo

puts '-' * 8

c2 = C.new
c2.foo
c2.foo
c2.foo

sample.dot

digraph {
    rankdir=LR;
    node [shape = box];
    node [color = blue] A B C D;
    node [color = orange, peripheries = 2] M0 M MM PM;
    D    [style = filled, fillcolor = cyan];

    D -> C [color = blue];
    C -> B [color = blue];
    B -> A [color = blue];

    PM -> D  [color = green];
    D  -> M  [color = green];
    M  -> MM [color = green];
    MM -> C  [color = green];
    C  -> B  [color = green];
    B  -> A  [color = green];
    A  -> M0 [color = green];
}

sample.rb

module M0
  @@v = :M0
  def foo ; "I am 'M0'" ; end
end

module MM
  @@v = :MM
  def foo ; "I am 'MM'" ; end
end

module M
  include MM
  @@v = :M
  def foo ; "I am 'M'" ; end
end

module PM
  @@v = :PM
  def foo ; "I am 'PM'" ; end
end

class A
  include M0
  @@v = :A
  def foo ; "I am 'A'" ; end
end

class B < A
  @@v = :B
  def foo ; "I am 'B'" ; end
end

class C < B
  @@v = :C
  def foo ; "I am 'C'" ; end
end

class D < C
  include M
  prepend PM
  @@v = :D

  def foo ; "I am 'D'" ; end

  def getv
    @@v
  end
end

d = D.new
p d.class.ancestors #=> [PM, D, M, MM, C, B, A, M0, Object, Kernel, BasicObject]
p d.getv
p d.foo