Question

I want to know how the instance variables of a Ruby module behaves across multiple classes which ‘mix’ it ‘in’. I wrote a sample code to test it:

# Here is a module I created with one instance variable and two instance methods.
module SharedVar
  @color = 'red'
  def change_color(new_color)
    @color = new_color
  end
  def show_color
    puts @color
  end
end

class Example1
  include SharedVar
  def initialize(name)
    @name     = name
  end
end

class Example2
  include SharedVar
  def initialize(name)
    @name     = name
  end
end

ex1 = Example1.new("Bicylops")
ex2 = Example2.new("Cool")

# There is neither output or complains about the following method call.
ex1.show_color
ex1.change_color('black')
ex2.show_color

Why it doesn’t work? And Could someone explain what will the actual behavior of @color across multiple Example$ instances?

Answer 1

In Ruby modules and classes are objects, so it’s possible to set instance variables for them.

module Test
  @test = 'red'
  def self.print_test
    puts @test
  end
end

Test.print_test #=> red

Your mistake is thinking that the variable @color is the same for:

module SharedVar
  @color
end

and

module SharedVar
  def show_color
    @color
  end
end

which is not.

In the first example, the instance variable belongs to the SharedVar object and in the second example the instance variable belongs to the object you include the module in.

Another explanation by self pointer. In the first example the self pointer is set to the module object SharedVar, so typing @color will refer to the object SharedVar and there’s no connection with another object. In the second example, the method show_color can be called only on some object, i.e. ex1.show_color, so the self pointer will point to ex1 object. So in this case the instance variable will refer to ex1 object.