Class variables and inheritance. Subclass specific variables?

Question

I am struggling a bit with subclassing and class variables. I was expecting the class variables set by a class method call of the subclass to be specific to that subclass, but I see them being set in the super class.

For example, if I run the following code:

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}")

I get:

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

I was expecting:

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

Is there a way to achieve that?

This is a super common misconception about @@ vars ... see stackoverflow.com/questions/1251352/… — max pleaner
– max pleaner, Commented Apr 16, 2018 at 2:44
From the docs: "Class variables are shared between a class, its subclasses and its instances." — Stefan
– Stefan, Commented Apr 16, 2018 at 8:04

599644 · Accepted Answer · 2018-04-16 00:53:56Z

0

I kind of found a way:

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}")

Now if I can somehow get rid of:

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

answered Apr 16, 2018 at 0:53

599644

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3 Comments

Subash Over a year ago

you can use this (@can_say ||= []) << word to get rid of that, it is just saying the same thing but in a single line, it'll just create an array if not found or append to the existing one if found

599644 Over a year ago

Thanks @Subash, that's helpful to know! Is there any other way to somehow declare and initialize the @cas_say beforehand?

Subash Over a year ago

(@can_say ||= []) this line here without appending word is doing the initializing of the empty array

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