From: Paul Brannan <pbrannan@...>
Date: 2002-05-11T01:35:19+09:00
Subject: Re: freeze and exception safety

On Sat, May 11, 2002 at 01:13:35AM +0900, Morris, Chris wrote:
> > Is there an exception-safe way that I can freeze sub-objects?
> 
> What about this? Do a deep-clone of your Container, attempt to freeze
> everything in the clone, then replace the original with the clone. If an
> exception happens before the final assignment, don't do anything, the
> half-frozen clone goes away.

An interesting idea.  Not particularly speedy, but much more safe:

  class Foo
    def freeze
      x_copy = @x.deep_clone # The container must implement deep_clone
      x_copy.each { |x| x.freeze }
      x_copy.freeze
      super
      @x = x_copy
    end
  end

Since assignment cannot raise an exception, this should work.  However,
consider this:

  class Foo
    attr_reader :x

    def initialize
      @x = Container.new
    end

    def freeze
      # see above
    end
  end

  f = Foo.new
  x = f.x
  f.freeze
  p x.frozen? #=> false

There is no good way to replace the original object, because someone
else might always be holding a reference to it.  Is there a good way
around this that doesn't require the co-operation of the user, and will
work generically with any container?

Paul