Ruby: Inherit code that works with class variables

Question

The situation: I have multiple classes that should each hold a variable with a configuration hash; a different hash for each class but the same for all instances of a class.

At first, i tried like this

class A
  def self.init config
    @@config = config
  end

  def config
    @@config
  end
end

class B < A; end
class C < A; end

But soon noticed that it wouldn't work that way because @@config is held in the context of A, not B or C, thus:

B.init "bar"
p B.new.config  # => "bar"
p C.new.config  # => "bar" - which would be nil if B had it's own @@config

C.init "foo"
p B.new.config  # => "foo" - which would still be "bar" if C had it's own @@config
p C.new.config  # => "foo"

I thought of using it like this:

modules = [B, C]
modules.each do |m|
  m.init(@config[m.name])
end
# ...
B.new  # which should then have the correct config

Now, it's clear to me why that happens, but I'm not sure about the reason for it being like this.

Couldn't it work the other way too, holding the class variable in the context of the subclass?

What i also found irritating was the fact that self is always the subclass even when called 'in' the superclass. From this, I first expected the code from the superclass is "executed in the context of" the subclass.

Some enlightenment about this would be greatly appreciated.

On the other hand, I likely have to accept it works that way and that I have to find another way to do this.

Is there a "meta" way to do this? (I tried with class_variable_set etc. but with no luck)

Or maybe is the whole idea of that 'init' method flawed in the first place and there's some other "pattern" to do this?

I could just make @@config a hash, holding all the configs and always pick the right one, but I find that a little awkward.. (isn't inheritance there to solve this kind of problem? ;)

Answer 1

The @@variables aren't class variables. They are class hierarchy variables, i.e. they are shared between the entire class hierarchy, including all subclasses and all instances of all subclasses. (It has been suggested that one should think of @@variables more like $$variables, because they actually have more in common with $globals than with @ivars. That way lies less confusion. Others have gone further and suggest that they should simply be removed from the language.)

Ruby doesn't have class variables in the sense that, say, Java (where they are called static fields) has them. It doesn't need class variables, because classes are also objects, and so they can have instance variables just like any other object. All you have to do is to remove the extraneous @s. (And you will have to provide an accessor method for the class instance variable.)

class A
  def self.init config
    @config = config
  end

  def self.config # This is needed for access from outside
    @config
  end

  def config
    self.class.config # this calls the above accessor on self's class
  end
end

Let's simplify this a bit, since A.config is clearly just an attribute_reader:

class A
  class << self
    def init config
      @config = config
    end

    attr_reader :config
  end

  def config
    self.class.config
  end
end

And, in fact, A.init is just a writer with a funny name, so let's rename it to A.config= and make it a writer, which in turn means that our pair of methods is now just an accessor pair. (Since we changed the API, the test code has to change as well, obviously.)

class A
  class << self
    attr_accessor :config
  end

  def config
    self.class.config
  end
end

class B < A; end
class C < A; end

B.config = "bar"
p B.new.config  # => "bar"
p C.new.config  # => nil

C.config = "foo"
p B.new.config  # => "bar"
p C.new.config  # => "foo"

However, I can't shake the feeling that there is something more fundamentally iffy about the design, if you need this at all.