类变量和继承。子类特定变量？

Question

我在子类化和类变量方面有点挣扎。我期待子类的类方法调用设置的类变量特定于该子类，但我看到它们是在超类中设置的。

例如，如果我运行以下代码：

class Obj
    @@can_say = []
    def self.says word
        @@can_say.push(word)
    end

    def self.listens_to calling
        define_method calling do
            "I say #{@@can_say}"
        end
    end
end

class Dog < Obj
    says "woof"
    says "bark bark"
    listens_to "goodboy"
end

class Cat < Obj
    says "meaow"
    says "purr"
    listens_to "lord"
end

cat = Cat.new
dog = Dog.new

puts("Dog: #{dog.goodboy}")
puts("Cat: #{cat.lord}")

我明白了：

ruby/test$ ruby test.rb 
Dog: I say ["woof", "bark bark", "meaow", "purr"]
Cat: I say ["woof", "bark bark", "meaow", "purr"]

我期待：

ruby/test$ ruby test.rb 
Dog: I say ["woof", "bark bark"]
Cat: I say ["meaow", "purr"]

有没有办法做到这一点？

Answer 1

我找到了一种方法：

class Obj
    class << self
        attr_accessor :can_say
    end

    def self.says word
        if @can_say.nil?
            @can_say = Array.new
        end
        @can_say.push(word)
    end

    def self.listens_to calling
        define_method calling do
            "I say #{self.class.can_say}"
        end
    end
end

class Dog < Obj
    says "woof"
    says "bark bark"
    listens_to "goodboy"
end

class Cat < Obj
    says "meaow"
    says "purr"
    listens_to "lord"
end

cat = Cat.new
dog = Dog.new

puts("Dog: #{dog.goodboy}")
puts("Cat: #{cat.lord}")

如果我能以某种方式摆脱：

if @can_say.nil?
    @can_say = Array.new
end