Ruby mixin - accessing module instance variables in class instances

Question

I'm pretty new to Ruby and seems that I'm still confused with mixin and including modules in classes. I would like to be able to access instance variables (@) defined in modules in classes. I have the following piece of code:

module ModuleB
  attr_reader :b

  def ModuleB.initialize(browser)
    puts "initialize from ModuleB"
    @browser = browser
    @b = 5
  end
end

module ModuleA
  attr_reader :a
  include ModuleB

  def ModuleA.initialize(browser)
    ModuleB.initialize(browser)
    puts "initialize from ModuleA"
    @browser = browser
    @a = @b
  end

  def action_1
    @a = @b + 1
    return @a
  end
end

class ClassA
  include ModuleA
  def initialize(browser)
    ModuleA.initialize(browser)
    @browser = browser
    puts 'initialize - method in ClassA'
    @c = @a
    @d = @b
    puts "a = #{@a}"
    puts "b = #{@b}"
    puts "c = #{@c}"
    puts "d = #{@d}"
  end

end

s = 'hello'
instA = ClassA.new(s)
puts instA.action_1

And here's the output that I get:

initialize from ModuleB
initialize from ModuleA
initialize - method in ClassA
a =
b =
c =
d =
mixin_example2.rb:23:in `action_1': undefined method `+' for nil:NilClass (NoMethodError)
        from mixin_example2.rb:46:in `<main>'

Seems like @a and @b are uninitialized. Additional thing, is that I cannot use '+' operator in action_1 method. What did I miss?

Sorry if I repeated problem that might have already been raised but I didn't find an answer so far.

This seems like it should be an inheritance pattern not a module pattern. Using inheritance makes this smoother more functional and more obvious — engineersmnky
– engineersmnky, Commented May 26, 2017 at 12:54
mixin inheritance is inheritance, though. In fact, include simply creates a class and makes that class the superclass. — Jörg W Mittag
– Jörg W Mittag, Commented May 26, 2017 at 13:10
@engineersmnky, in fact what I'm trying to do (not shown in this piece of code) is to get multiple inheritance using modules. — Piotr Gesicki
– Piotr Gesicki, Commented May 27, 2017 at 4:09

Jörg W Mittag · Accepted Answer · 2017-05-26 13:09:44Z

2

Instance variables belong to instances (which is why they are called "instance variables"). You have three objects of interest here, every single one of which has its own @b which has nothing to do with any of the other @bs of the other objects: ModuleA has its own @b, as does ModuleB, and instA also has its own @b.

Here is a more sensible implementation of what you are trying to do:

module ModuleB
  def initialize(browser)
    puts "initialize from ModuleB"
    @browser = browser
    @b = 5
  end
end

module ModuleA
  include ModuleB

  def initialize(browser)
    super
    puts "initialize from ModuleA"
    @a = @b
  end

  def action_1
    @a = @b + 1
  end
end

class ClassA
  include ModuleA

  def initialize(browser)
    super
    puts 'initialize - method in ClassA'
    @c = @a
    @d = @b
    puts "a = #@a"
    puts "b = #@b"
    puts "c = #@c"
    puts "d = #@d"
  end
end

s = 'hello'
instA = ClassA.new(s)
# initialize from ModuleB
# initialize from ModuleA
# initialize - method in ClassA
# a = 5
# b = 5
# c = 5
# d = 5
#=> #<ClassA:0x007f8b5f12e110 @a=5, @b=5, @browser="hello", @c=5, @d=5>

puts instA.action_1
# 6

answered May 26, 2017 at 13:09

Jörg W Mittag

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9 Comments

engineersmnky Over a year ago

Excellent answer although in my opinion still less obvious than a direct class to class inheritance pattern. Although I understand your point regarding inclusion's superclass injection.

Piotr Gesicki Over a year ago

Thank you very much @Jorg. That's very clean code that I needed. However, I don't get your comment that there are 3 instances of @b in the code above: one for ModuleA, one for ModuleB and one for ClassA. If I instantiate object from ClassA how do I access @b from modules that are included? Isn't it just a single @b accessible on ClassA instance from superclass?

Piotr Gesicki Over a year ago

Also, one more comment but I'll probably create new question: if classA in fact inherited from 2 modules (let's say there was additional ModuleC) through includes, how could both superclass initialize methods be called?

Jörg W Mittag Over a year ago

@PiotrGesicki: Instance variables belong to objects. Each object has its own set of instance variables that are completely private to that particular object. The instance variable @foo of object a is completely separate from the instance variable @foo of object b. In your code, there are three different objects, all of which have an instance variable @b: ModuleA, ModuleB, and instA all have an instance variable named @b. But they are three different objects. They have nothing to do with each other. The fact that you set instance variable @b of ModuleA to some value …

Jörg W Mittag Over a year ago

… has absolutely no effect whatsoever on the instance variable @b of object ModuleB or the instance variable @b of object instA.

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