From: Guillaume Marcais <guslist@...>
Date: 2003-07-07T23:14:43+09:00
Subject: Re: Freezing an arbitary object

Side question:

is there anyway to make the warning an error?

Guillaume.

On Mon, 2003-07-07 at 04:36, Robert Klemme wrote:
> 
> "Aryeh M. Frierdman" <aryeh@m-net.arbornet.org> schrieb im Newsbeitrag
> news:0N0Oa.44153$5h.15219260@news4.srv.hcvlny.cv.net...
> > 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
> > 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.
> 
> Apparently you want two things:
> 
> - freeze an instance
> - make an object reference unchangeable
> 
> It's important to not confuse these two issues.  The first is a property
> of an instance while the latter is a property of an object reference.  In
> Ruby there are only references.  There is no distinction between pointers,
> references and values like one might know from C++.  And there's a subtle
> difference between C++ references and Ruby references: C++ references
> can't be changed and always refer to the instance they were initialized
> with, Ruby references can be changed.  Thus Ruby references do have a
> little similarity with C++ pointers - but not too much, since pointer
> arithmetic is missing.
> 
> The first is achieved by doing obj.freeze as you determined correctly,
> while the latter is not possible with local references as Brian pointed
> out.  The closest you can get is to declare a constant (first letter
> uppercase) and then get a warning on reassignment like in:
> 
> irb(main):001:0> This_is_const = "foo"
> "foo"
> irb(main):002:0> This_is_const.freeze
> "foo"
> irb(main):003:0> This_is_const << "foo"
> TypeError: can't modify frozen string
>         from (irb):3:in `<<'
>         from (irb):3
> irb(main):004:0> This_is_const = "bar"
> (irb):4: warning: already initialized constant This_is_const
> "bar"
> irb(main):005:0>
> 
> Regards
> 
>     robert
> 
>