[boost] Re: type_with_alignment and is_pod

From: David Abrahams (dave_at_[hidden])
Date: 2003-12-03 17:00:47

Douglas Paul Gregor <gregod_at_[hidden]> writes:

> On Wed, 3 Dec 2003, David Abrahams wrote:
>> Douglas Paul Gregor <gregod_at_[hidden]> writes:
>> > type_with_alignment is supposed to give back a POD type. When we create
>> > other types in type_with_alignment that may be returned (say, the aligned
>> > structs for the GCC-specific version), we should be marking them POD via
>> > BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_pod,...,true).
>>
>> Obviously you didn't include all the possible numbers:
>>
>> BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_pod,::boost::detail::max_align,true)
>> BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_pod,::boost::detail::lower_alignment<1> ,true)
>> BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_pod,::boost::detail::lower_alignment<2> ,true)
>> BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_pod,::boost::detail::lower_alignment<4> ,true)
>> BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_pod,::boost::detail::lower_alignment<8> ,true)
>> BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_pod,::boost::detail::lower_alignment<10> ,true)
>> BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_pod,::boost::detail::lower_alignment<16> ,true)
>> BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_pod,::boost::detail::lower_alignment<32> ,true)
>
> Hmmm, that's not right. The user never gets lower_alignment<N> back, they
> get lower_alignment<N>::type. But we can't do that, because those could be
> built-in types.
>
> The right way is probably to do a
> BOOST_TT_AUX_BOOL_TRAIT_IMPL_SPEC1(is_pod,T,true) for all T's in
> BOOST_TT_ALIGNMENT_STRUCT_TYPES. I can't fix this now, but should get a
> chance later, when I some compiler other than GCC hanging around.
>
>> Why not just use a partial specialization where it's available?
>
> Yeah, we can do that, too, by specializing is_pod on has_one_T<T>.
>
>> > I did this for GCC (so I find it odd it's broken there)
>>
>> It's only broken with gcc-2.95.x and maybe only on Cygwin.
>
> It's fine on GCC 2.95.3 under Linux (according to the regression tests, at
> least)

Not unless they ran since I checked in my type_with_alignment changes
this morning.

> and it doesn't make sense that the change you made would have fixed
> any GCC: GCC won't ever pick one of the has_one_T structures unless
> you've found a really screwy alignment (i.e., not 1, 2, 4, 8, 16, or
> 32).  

But maybe it broke a GCC?

> Perhaps alignment_of is broken in Cygwin, so it's giving some
> nonsense alignment?

I dunno.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com