From: Brian Candler <B.Candler@...>
Date: 2003-07-07T07:01:59+09:00
Subject: Re: Freezing an arbitary object

On Mon, Jul 07, 2003 at 06:53:20AM +0900, Aryeh M. Frierdman wrote:
> Thanks for the answers so far but "none" of them actually do what I want.
> What I want to do is declare some instance/var/whatever as being invariant

Well, you should know what you mean by "whatever"

- local variables: cannot be frozen, since they don't belong to an object
  and are not an object themselves. They can always be reassigned to point
  to another object. Sorry, that's a Ruby fact of life.

- instance variables: are frozen when the object containing them is frozen

  class Foo
    attr_accessor :x
  end
  a = Foo.new
  a.x = 99
  a.freeze
  a.x = 100   # TypeError: can't modify frozen object

> and any attemt to vary it will result in an error.  I am convinced their
> has to be some generic way to do this *without* the caller to the "test"
> being "aware" that they are asking for it to be froozen.  The int example
> I gave was only that an example it could of very well been a String,
> MyClass, EvilDictator, etc.

Taking String as an example:

  foo = "hello"
  foo.freeze
  foo << "x"     # Error: object referenced by foo is frozen
  foo = "bye"    # Not error: foo now references a completely different object

And it can be done at a distance:

  def my_test_function(x)
    x.freeze
  end

  foo = "a"
  my_test_function(foo)
  foo << "b"     # Error: object has been frozen

But like I say, local variable "foo" cannot be frozen. I hope that's clear
enough...

Brian.