Travis Vitek wrote:
Martin Sebor wrote:
[...]
I'm probably missing something but is aligned_storage only specified
for alignment of powers of 2? (It looks to me as though those are the
only alignments we support.)
Yes. I probably need to do something here, but I'm not sure what. Table
51 says
_Align shall be equal to alignment_of<T>::value for some type
T or to `default-alignment'
This essentially says that I need to support all valid alignment values
on the platform. Fortunately, for the time being the power of two
restriction is okay. The only functionality I have available to me for
doing alignment (on the tested platforms) is __declspec(align(#)) on
Microsoft and __attribute__ ((aligned(#))) on gcc-4.3. Both of these
support functions require that the alignment value be a power of two.
The Microsoft compiler has the 8192 limit.
An upper limit is acceptable (it should be mentioned in one of
the appendices to the spec). I'm more interested in alignments
that aren't powers of 2. It's not clear to me why restricting
the template to powers of 2 is okay or why it's difficult not
to support all of them up to the limit.
The draft shows a 'typical implementation' of aligned_storage that uses
the new alignas keyword, but alignas doesn't appear to be supported
anywhere.
That's probably because they didn't want to favor any existing
implementation over others.
The following code is very similar to that typical
implementation, but it does not compile because value passed to the
aligned attribute has to be a literal, not a constant expression.
That's what you for using a crappy compiler ;-) It compiles
with gcc. See
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19163
but watch out for:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36625
We should open an enhancement request with Microsoft.
Martin
template <std::size_t _Size,
std::size_t _Align = __rw_default_alignment<_Size>::value>
struct __rw_aligned_storage
{
typedef struct {
char _C_align [_Size] __attribute__ ((aligned(_Align)));
} type;
};
Travis
Martin
See http://tinyurl.com/5kmvdy for reference.
Martin
