Question

I’m trying to define a static variable and methods in a module that will be extended/used by numerous classes. The following example demonstrates:

module Ammunition
  def self.included(base)    
    base.class_eval("@@ammo = [bullets]") 
  end

  def unload
    p @@ammo #<-- doesn't work
  end  
end

class Tank
  include Ammunition
  @@a += [shells]
end

class Airplane
  include Ammunition  
  @@a += [missiles, photon_torpedoes]
end

Tank.new.unload
Airplane.new.unload

This doesn’t work because ammunition doesn’t know how to evaluate @@ammo in the context of the class for some reason (I original thought the module would behave just like an include file). I would have to copy ‘unload’ to each class, which I’m doing right now, but I want to DRY it up b/c I have many other methods to add to the module.

Suggestions? The reasonable solution would be to evaluate ‘unload’ in the context of the class and not the module (but how to do this in Ruby?)

Thanks!

Answer 1

class variables can work strangely, and this use shows that off. What is the scope of @@ammo? Ammunition or does Tank have its own copy of it? It turns out that @@ammo is scoped by the module, and the classes that include it can simply access it.

module Ammunition
  def self.included(base)    
    base.class_eval do
      puts "@@ammo was: #{defined?(@@ammo) ? @@ammo.join(',') : 'nil'}"
      @@ammo = ['bullets']
      puts "@@ammo is now: #{@@ammo}"
      puts '---'
    end
  end

  def unload
    @@ammo
  end  
end

class Tank
  include Ammunition
  @@ammo += ['shells']
end

class Airplane
  include Ammunition  
  @@ammo += ['missiles', 'photon_torpedoes']
end

puts "Tank unloaded: #{Tank.new.unload.join(', ')}"
puts "Airplane unloaded: #{Airplane.new.unload.join(', ')}"

This produces:

@@ammo was: nil
@@ammo is now: bullets
---
@@ammo was: bullets,shells
@@ammo is now: bullets
---
Tank unloaded: bullets, missiles, photon_torpedoes
Airplane unloaded: bullets, missiles, photon_torpedoes

When Tank includes the module, it sets @@ammo from nil to an array with bullets in it. When Airplane includes the module, it overwrites the ammo value we just set.

---

Here is what you want to do

module Ammunition
  def self.included(base)    
    base.class_eval do
      include Ammunition::InstanceMethods
      extend  Ammunition::ClassMethods
      @ammo = ['bullets']
    end
  end

  module ClassMethods
    def ammo
      @ammo
    end
  end

  module InstanceMethods
    def unload
      self.class.ammo.join(',')
    end
  end
end

class Tank
  include Ammunition
  @ammo += ['shells']
end

class Airplane
  include Ammunition  
  @ammo += ['missiles', 'photon_torpedoes']
end

puts "Tank unloaded: #{Tank.new.unload}"
puts "Airplane unloaded: #{Airplane.new.unload}"

Classes can have instance variables, and their scope is easier to understand. And separating your module into instance and class methods allow you to provide functionality to both. This snippet generates the following output

Tank unloaded: bullets,shells
Airplane unloaded: bullets,missiles,photon_torpedoes