From: Martin DeMello <martindemello@...>
Date: 2003-06-25T20:38:45+09:00
Subject: Re: protect parents from children

Simon Strandgaard <0bz63fz3m1qt3001@sneakemail.com> wrote:
> class A
>  def initialize
>    @value = [1, 2, 3]
>  end

Okay, class A defines an instance variable @value and initializes it to
[1,2,3].

>  attr_reader :value
>  def selftest
>    if @value.size == 3
>      puts "OK"
>    else
>      puts "Watch out, we have been modified"
>      p @value
>    end
>  end
> end

and tests to see if @value has changed since its initialization.

> class B < A

B inherits from A, and therefore inherits the methods initialize and
test, which have not been overridden, and the instance variable @value.

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

and adds two new methods, available to objects of class B, but not of
class A.

However, the terms 'parent' and 'child' refer to classes, whereas @value
is an instance variable. You're modifying an object of class B, which
contains an attribute @value that B just happens to have inherited from
its superclass, A. The values of instance variables are part of the
object, not the class.

Furthermore, the whole point of inheritance is that you can override
methods defined in the base class, so in that sense, a base class can
and usually does change the bits it inherited from the parent (which is
not the same thing as changing the parent).

martin