Re: OUString is mutable?

On Wed, Oct 3, 2012 at 11:19 AM, Lubos Lunak <l.lunak@suse.cz> wrote:
> On Monday 01 of October 2012, Michael Stahl wrote:
> > On 01/10/12 11:40, Noel Grandin wrote:
> > > On 2012-10-01 10:15, Stephan Bergmann wrote:
> > > > While that argument is irrelevant in a C/C++ context, immutability is
> > > > also an important concept when reasoning about multi-threaded code.
> > > > Therefore, the distinction between OUString and OUStringBuffer IMO
> > > > does make sense after all.
> > >
> > > I don't see how the design helps you in a multithreaded context.
> > > If you share an OUString instance between two threads, either thread
> > > could assign to it, replacing it's contents, and invalidating what the
> > > other thread sees.
> >
> > but if you share 2 copies of an OUString instance in 2 threads, they
> > could access the one underlying buffer concurrently without issue, and
> > when they want to point their copy at a different value, they can do
> > that independently of the other -- again, OUString works in exactly the
> > same way as a pointer to an immutable object here, if you copy the
> > pointer you can dereference it without issue and point your copy to a
> > different object; you only need a lock if you don't copy the pointer and
> > share a single pointer that one thread may update (which depending on
> > the situation may be necessary, say you don't just want to save memory
> > but actually always want the threads to point to the same value).
>
>  Either I don't get what immutable actually means in this context (in which
> case I'd like to be enlightened), or you are talking about things that
> actually have nothing to do with immutability. OUString is not the same like
> a pointer to an immutable object, because this object contains a reference
> count

No it does not... the underlying rtl_uString (the part that _is_
immutable) is ref-counted
but OUString itself is not.

iow: a given OUString may be changed to point to a rlt_ustring
instance or another, but cannot mess with the one it points to

Norbert