[ruby-talk:21696] Re: manipulating "immutable" objects such as Fixnum from within callbacks & al...

> Guillaume Cottenceau <[e-mail]> writes:
>
> > > You do understand that because Fixnums are immutable, that the
instance that
> > > ObjContainer.o refers to is actually a *different* object after the
doit_()
> > > method processes it, right? That is to say, if you were to compare the
> > > object id of ObjContainer.o before and after calling doit_(), you'd
get
> > > different ids.
> >
> > Of course.
>
> And I'd like to add that this issue is mostly an implementation trick to
> have faster numbers manipulations. From your programs you will seldomly
> take advantage of this "feature", on contrary you will shoot yourself
> sometimes when you don't remember this basic thing ;p.

It's more than an implementation trick--it's a desirable feature.
Programmers expect numbers to have copy semantics, not reference semantics,
regardless of their internal representation. That's why, even though floats
are internally stored as references, the following is correct:

x = 1.0
y = x
y += 1
p y
  ==> 2.0
p x
  ==> 1.0

The same goes for bignums, which are also represented as pointers.