让父类的方法访问子类的常量 [英] Have a parent class's method access the subclass's constants

问题描述

例如：

class Animal

 def make_noise
    print NOISE
 end

end

class Dog < Animal
    NOISE = "bark"
end

d = Dog.new
d.make_noise # I want this to print "bark" 

我如何完成上述任务？当前它说

How do I accomplish the above? Currently it says

uninitialized constant Animal::NOISE

推荐答案

我认为您并不是真的想要一个常量。我认为您想在该类上使用实例变量：

I think that you don't really want a constant; I think that you want an instance variable on the class:

class Animal
  @noise = "whaargarble"
  class << self
    attr_accessor :noise
  end
  def make_noise
    puts self.class.noise
  end
end

class Dog < Animal
  @noise = "bark"
end

a = Animal.new
d = Dog.new
a.make_noise  #=> "whaargarble"
d.make_noise  #=> "bark"
Dog.noise = "WOOF"
d.make_noise  #=> "WOOF"
a.make_noise  #=> "whaargarble"

但是，如果您确定要使用常量，则为：

However, if you are sure that you want a constant:

class Animal
  def make_noise
    puts self.class::NOISE
    # or self.class.const_get(:NOISE)
  end
end

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