From: Simon Strandgaard <0bz63fz3m1qt3001@...>
Date: 2003-06-25T19:58:14+09:00
Subject: Re: protect parents from children

On Wed, 25 Jun 2003 20:47:23 +0900, ts wrote:

>>>>>> "S" == Simon Strandgaard <0bz63fz3m1qt3001@sneakemail.com> writes:
> 
> S> I fell into these pitfalls yesterday.. that a child was modifying a parent!
> 
>  You make a confusion : the child was not modifying the parent.
> 
>  An instance variable belongs to an object (i.e. B.new in your case).
> 
> S> class A
> [...]
> S> B.new.test1
> S> B.new.test2
> 
>  add this
> 
>    p B.new.value
> 
>  and you'll have [1, 2, 3]
> 
> 
> Guy Decoux

Me don't understand.   If I add debug-print's to selftest then I see that
the parent *has* changed  ??   Can you enligthen me ?

> ruby tst.rb
Watch out, we have been modified
[1, 2, 3, 4]
Watch out, we have been modified
[5, 6]
>

To me it looks like the parent has been modified by the child.


> expand -t2 tst.rb
class A
  def initialize
    @value = [1, 2, 3]
  end
  attr_reader :value
  def selftest
    if @value.size == 3
      puts "OK"
    else
      puts "Watch out, we have been modified"
      p @value
    end
  end
end

class B < A
  def test1
    objs = value
    objs << 4
    selftest
  end
  def test2
    @value = [5, 6]
    selftest
  end
end

B.new.test1
B.new.test2
>


--
Simon Strandgaard