RE: [boost] Proposal: statefull objects

From: Fernando Cacciola (fcacciola_at_[hidden])
Date: 2001-08-28 12:52:23

----- Original Message -----
From: Douglas Gregor <gregod_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Tuesday, August 28, 2001 1:35 PM
Subject: Re: [boost] Proposal: statefull objects

> On Tuesday 28 August 2001 11:50, you wrote:
> > ----- Original Message -----
> > From: Peter Dimov <pdimov_at_[hidden]>
> > To: <boost_at_[hidden]>
> > Sent: Tuesday, August 28, 2001 12:11 PM
> > Subject: Re: [boost] Proposal: statefull objects
> >
> > > From: "Fernando Cacciola" <fcacciola_at_[hidden]>
> > >
> > > > (At least, let me know if the trick I used to remove the requirement
of
> > > > default-constructible is appropriate)
> > >
> > > Unfortunately not, it ignores T's alignment requirements AFAICS.
> >
> > This is what I was worry about, precisely. Thanks for looking at it.
> >
> > But I'am confused then...
> > It is guaranteed that if the array of chars is allocated with new[] it
has
> > the appropriate aligment, but, from what you say, it appears that this
is
> > not the case when the array has automatic storage. Is that so?
> >
> > Hmm. Seems that you are right, pity!  I've realized that allocator<>
works
> > because it always uses malloc(), so the guarantee is fulfilled. (of
course!
> > If optional<T> is created in the stack there's no way to align 'storage'
> > properly!)
> >
> > Well, I think that optional<> is still usefull even if T is required to
> > have a default constructor, but then it is not as nice as I thougth.
> >
> > (but I'll keep finding a way remove that requirement without allocating
in
> > the heap).
>
> I was recently working on a variant type that was intended to be created
on
> the stack. I believe it is possible to do, though it isn't trivial :).
What I
> did was use the alignment_of metafunction in the type traits library to
get
> the alignment. Then I searched through a list of fundamental types to find
> the fundamental type that has the same alignment (there is an assumption
> here: that no structure has an alignment greater than that of a
fundamental
> type in the system). Once I found the fundamental type (call it
> "alignment_type"), my storage layout was:
>
> union {
>   char           data[sizeof(T)];
>   alignment_type align;
> };
>
> This gives us a character array large enough to hold a T, and at the
> alignment of T that is equivalent to the alignment of alignment_type.
>
> I can upload the code to do this if anyone is interested. The rest of the
> variant type is unfinished, but this part seems to work well enough :)
>
This sounds exactly what I need.
Please upload the file or send it directly to me in an email attachment.
Thanks!

Fernando Cacciola
Sierra s.r.l.
fcacciola_at_[hidden]
www.gosierra.com