Create a new instance of parent's class variable for each child class

Question

I have this parent class:

class ListBase
  @@list = []

  def self.list
    @@list
  end
end

I'm trying to create two child classes like so:

class Child1 < ListBase
end

class Child2 < ListBase
end

I was under the impression that each of these child classes will have their own @@list class variable. However, I get this:

Child1.list.push(1)
Child2.list.push(2)
Child1.list # => [1, 2]
Child2.list # => [1, 2]

which means the child classes share the @@list from the parent class.

How can I create a separate class variable for each of the child classes without repeating?

Check: https://stackoverflow.com/questions/1251352/ruby-inherit-code-that-works-with-class-variables — Marcin Kołodziej, Nov 09 '18 at 04:39
I'm curious why you want to do that. Class *instance* variables are typically used for what you describe, as their scope is confined to the class in which they are created. — Cary Swoveland, Nov 09 '18 at 05:30
Actually, your question "How can I create a separate class variable for each of the child classes without repeating?" is contradictory. A class variable is **for sharing** among the child classes, and you are trying not to share it. — sawa, Nov 09 '18 at 08:34

score 1 · Answer 1 · answered Nov 09 '18 at 06:21

You are using class variable through class method (getter/reader).
So you can override that method in derived classes and use own class variable for every derived class.
Not fun, but that how class variables works, they are shared within a class and derived classes as well.

class ListBase
  def self.list
    @@base_list ||= []
  end
end

class ListOne
  def self.list
    @@one_list ||= []
  end
end

class ListTwo
  def self.list
    @@two_list ||= []
  end
end

ListOne.list << 1
ListTwo.list << 2

puts ListOne.list
# => [1]
puts ListTwo.list
# => [2]

score 1 · Accepted Answer · answered Nov 09 '18 at 08:29

1

As @CarySwoveland comments, in your use case, you should use a class instance variable:

class ListBase
  @list = []

  def self.list
    @list
  end
end

Not even sure why you thought of using a class variable for your use case.

answered Nov 09 '18 at 08:29

sawa

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1

Thanks, yes once you point it out, it became pretty clear that I actually ought to be using a class instance variable. – Ashish Acharya Nov 09 '18 at 12:32

Create a new instance of parent's class variable for each child class

2 Answers2