"class instance variable" 和 “class variable”的区别

• class variables are available in class methods and instance methods, but class instance variable is only available in class methods:
  ruby 代码

  class Test
    @@cla_var = 1
    @inst_var = 1
  
    def self.cla_var
      @@cla_var
    end
    def cla_var
     @@cla_var
    end
    def self.inst_var
      @inst_var
    end
  
    def inst_var
       #here @inst_var is treated as the instance variable not class instance variable
       @inst_var
    end
   end

• Class variables are shared by children of the class in which they are first defined, but  class instance variables not, every child has it's own copy of class instance variable(Rumor has it that ruby1.9 will change this).
  ruby 代码

  class Parent
     @@cla_var = 1
     @inst_var = 2
  
      def self.cla_var
          @@cla_var
      end
      def self.inst_var
          @inst_var
      end
  
      def self.cla_var=(cla_var)
          @@cla_var = cla_var
      end
      def self.inst_var=(inst_var)
          @inst_var = inst_var
      end
  
  end
  class Child1 < Parent
  end
  class Child2 < Parent
  end
  
  #class variables are shared by parent and all the children
  >> Parent.cla_var
  => 1
  >> Child1.cla_var
  => 1
  >> Child2.cla_var
  => 1
  >> Child1.cla_var = 3
  => 3
  >> Parent.cla_var
  => 3
  >> Child2.cla_var
  => 3
  
  #every children has it's own copy of class instance variables
  >> Parent.inst_var
  => 2
  >> Child1.inst_var
  => nil
  >> Child2.inst_var>> Child1.inst_var = 4
  => 4
  >> Parent.inst_var
  => 2
  >> Child2.inst_var
  => nil
  
  => nil