From: "Aleksi Niemelä" <aleksi.niemela@...>
Date: 2000-07-05T20:14:34+02:00
Subject: [ruby-talk:03846] Private variables can be changed outside?

Now I managed to drive myself almost insane. I thought private variables
could not be changed outside class. Anyway, the source code below does
change.

The important routine is hopefully clearly marked:

def change_object_burp_or_fish_burp_outside_fish(text="")
  @burp = "surprise" + text
end

The '@burp' could have different 'self' depending where it has been called
from. When you call it directly from the top level the @burp is an instance
variable of the object main (which prints in debugger #<Object:
@burp="burp">). When it's called from Fish#change_from_outside, self is
referring to an object #<Fish: @burp="blop">.

I don't know what to think. I think it's natural which object's @burp we're
accessing. But it's not natural we can access a private variable (of our
child class). If there's no bug in here, this opens up a huge hole. As every
class descents from Object, every class *can* access every other's instance
variables!

This is accomplished by 
1) creating a method Object#spy, which access the wanted instance variable
2) a method Class#spy, to the class we wanted to inspect, which caals just
created Object#spy
3) and then by firing a call to Class#spy, which in turn calls Object#spy
with proper 'self', pointing to Class-object.

So we can mess whatever object we want to. Now, if somebody just provides a
method to ask object's instance variables :P.

	- Aleksi

Ps. taking out the comment before 'private' didn't change the run on Ruby
1.4.5.

class Fish
#  private
  @burp
  public
  def initialize( burp ="blop" )
    @burp = burp
  end
  def internal_burp
    puts "Fish says   '#{@burp}'"
  end
  def change_from_outside
    change_object_burp_or_fish_burp_outside_fish
  end
end

@burp = "burp"

def main_burp
  puts "Object says '#{@burp}'"
end

def change_object_burp_or_fish_burp_outside_fish(text="")
  @burp = "surprise" + text
end

####

fish = Fish.new

fish.internal_burp
main_burp

puts "\nNow we call fish to change itself"
fish.change_from_outside

fish.internal_burp
main_burp

puts "\nNow we call self to change ourselves"
change_object_burp_or_fish_burp_outside_fish(" again")

fish.internal_burp
main_burp

#################
Outputs:
Fish says   'blop'
Object says 'burp'

Now we call fish to change itself
Fish says   'surprise'
Object says 'burp'

Now we call self to change ourselves
Fish says   'surprise'
Object says 'surprise again'