Rubyでクラス変数の使い方の例

example1.rb

#!/usr/bin/env ruby

class Parent
  def self.puts_var
    puts self.class_variable_get(:@@CLASS_VAR)
  end

  def puts_var
    puts self.class.class_variable_get(:@@CLASS_VAR)
  end
end

class Child1 < Parent
  @@CLASS_VAR = 'Child1'
end

class Child2 < Parent
  @@CLASS_VAR = 'Child2'
end

Child1.puts_var     # => Child1
Child1.new.puts_var # => Child1
Child2.puts_var     # => Child2
Child2.new.puts_var # => Child2

example2.rb

#!/usr/bin/env ruby

class Parent
  # 差分はここだけ。これがあると、子クラス同士の影響を受けてしまう
  @@CLASS_VAR = 'Parent'

  def self.puts_var
    puts self.class_variable_get(:@@CLASS_VAR)
  end

  def puts_var
    puts self.class.class_variable_get(:@@CLASS_VAR)
  end
end

class Child1 < Parent
  @@CLASS_VAR = 'Child1'
end

class Child2 < Parent
  @@CLASS_VAR = 'Child2'
end

Child1.puts_var     # => Child2
Child1.new.puts_var # => Child2
Child2.puts_var     # => Child2
Child2.new.puts_var # => Child2