From: Rick DeNatale <rick.denatale@...>
Date: 2008-02-13T22:37:17+09:00
Subject: Re: How to add a instance variable through a mixin?

On 2/12/08, Robert Klemme <shortcutter@googlemail.com> wrote:
> You can do
>
> module Foo
>    attr_accessor :var
>
>    def initialize(*a,&b)
>      @var = 10
>    end
> end
>
> And then
>
> irb(main):015:0> class Bar
> irb(main):016:1>   include Foo
> irb(main):017:1> end
> => Bar
> irb(main):018:0> Bar.new.var
> => 10
>
> Or
>
> irb(main):026:0> class Oink
> irb(main):027:1>   include Foo
> irb(main):028:1>   def initialize
> irb(main):029:2>     super
> irb(main):030:2>     @x = 10
> irb(main):031:2>   end
> irb(main):032:1> end
> => nil
> irb(main):033:0> Oink.new.var
> => 10

You have to be careful here, since it relies on invoking super in the
initialize method, and since the initialize method in the module
doesn't do this, it can break under the right conditions:

class A
  attr_reader :a_var

  def initialize
    @a_var = :a_var
  end

end

module M
  attr_reader :m_var
  def initialize
    @m_var = :m_var
  end
end

class B < A
  include M
end

B.new.instance_variables # => ["@m_var"]


Note that the A instance didn't get an @a_var instance variable.

The solution here is to invoke the superclass' initialize, which also
works for subclasses which don't have an initialize method themselves:

class A
  attr_reader :a_var

  def initialize
    @a_var = :a_var
  end

end

module M
  attr_reader :m_var
  def initialize
    super
    @m_var = :m_var
  end
end

class B < A
  include M
end

class C
  include M
end

B.new.instance_variables # => ["@a_var", "@m_var"]
C.new.instance_variables # => ["@m_var"]

But this is hard to do in general when the initialize methods take
different parameters.

module M
  attr_reader :m_var
  def initialize(*a, &b)
    super
    @m_var = :m_var
  end
end

class D
  include M
  def initialize(d_val)
    @d_var = d_val
    super
  end
end

D.new(10).instance_variables # =>
# ~> -:13:in `initialize': wrong number of arguments (1 for 0) (ArgumentError)
# ~> 	from -:13:in `initialize'
# ~> 	from -:30:in `initialize'
# ~> 	from -:35:in `new'
# ~> 	from -:35

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/