ruby中的类变量与类实例变量

阅读更多

首先，在ruby1.8中类变量是所有子类和父类共享的，可以看下面的代码：

class IntelligentLife  
  @@home_planet = nil
  
  def self.home_planet
    @@home_planet
  end
  def self.home_planet=(x)
    @@home_planet = x
  end
  #...
end
class Terran < IntelligentLife
  @@home_planet = "Earth"
end

class Martian < IntelligentLife
  @@home_planet = "Mars"
end


p IntelligentLife.home_planet
p Terran.home_planet
p Martian.home_planet

可以看到结果是相同的，都是"Mars".这是因为父类的类变量是被整个继承体系所共享的.

在这里我们如果想要得到我们所需要的结果，我们就要使用类实例变量，因为类实例变量是严格的per-class。而不是被整个继承体系所共享。

class IntelligentLife  
  @home_planet = nil
  
 class << self
    attr_accessor :home_planet
  end
end
class Terran < IntelligentLife
  @home_planet = "Earth"
end

class Martian < IntelligentLife
  @home_planet = "Mars"
end


p IntelligentLife.home_planet
p Terran.home_planet
p Martian.home_planet

而在1.9中，类变量不再被继承体系所共享。也就是和java中的静态变量差不多了。

下面的代码在1.9中运行.

class IntelligentLife  
  @@home_planet = "bobo"
  def self.home_planet
    @@home_planet
  end
end
class Terran < IntelligentLife
  @@home_planet = "Earth" 
end

class Martian < IntelligentLife
  @@home_planet = "Mars"
  def self.home_planet
    @@home_planet
  end
end


p IntelligentLife.home_planet
p Terran.home_planet
p Martian.home_planet

可以看到这时的类变量才是真正的类变量，也就是说是per-class的了.