> -----Original Message-----
> From: Travis Vitek [mailto:[EMAIL PROTECTED]
> Sent: Wednesday, June 18, 2008 11:04 AM
> To: [email protected]
> Subject: RE: preconditions for aligned_union
>
>
>
> >Eric Lemings wrote:
> >
> >> Travis Vitek wrote:
> >>
> >>
> >> I'm looking at the standard, and it appears that the following is
> >> legal...
> >>
> >> struct incomplete_t;
> >>
> >> std::aligned_union<0, void, incomplete_t>::type
> >> aligned_t;
> >>
> >> [...]
> >>
> >> Now I could implement aligned_union<> to ignore incomplete
> >> types (they have no alignment requirement), but this might
> >> cause problems if someone tried to use the result.
> >
> >How so? If someone uses only such types in the list (probably a very
> >rare use case), a zero result should be expected. If alignable types
> >are mixed with such non-alignable types, the result would be as if no
> >such non-alignable types were given.
>
> What if none of the provided types have an alignment requirement or
> size? What should the alignment be? What should the size be?
Undefined?
>
> >
> >So I'd say just ignore non-alignable types.
>
> So you see no problem with the following code failing at
> runtime due to
> misalignment or insufficient space?
>
> struct incomplete_t;
>
> // supposed to be 'suitable for use as uninitialized
> // storage for any object whose type listed and will
> // be at least Len bytes
> std::aligned_union<1, incomplete_t>::type aligned_buf;
>
> struct incomplete_t
> {
> long double val;
> // [...]
> };
>
> void save_value (const long double& v)
> {
> ((incomplete_t*)&aligned_buf)->val = v;
> }
Yeah, that could be problematic...if the standard actually
allows it.
>
> I'm pretty sure that the standard assumes we will use
> std::alignment_of<> and std::aligned_storage<> to implement
> std::aligned_union<>. If that is the case, then the
> requirement for the
> type to be complete would be implied.
The only other possible course of action that I see is to check
for incomplete and other non-alignable types at compile-time
and generate a diagnostic. Not sure how you would do that
with type traits but, if its possible, that might be more
"well-behaved".
Brad.
