On Fri, May 22, 2015 at 03:59:47PM +0100, Jonathan Wakely wrote:
> >>+ alignas(alignof(_Tp2)) unsigned char _M_storage[sizeof(_Tp)];
> >
> >Is alignof(_Tp2) always the same as alignof(_Tp2::_M_t) on all targets
> >(I mean, won't some target align the structure more than its only field)?
>
> Hmm, maybe. I don't know.
>
> >Wouldn't it be safer to use alignof(_Tp2::_M_t) here?
>
> Yes.
>
> >Though, apparently that is a GNU extension, so you'd need to use __alignof__
> >instead.
>
> Yes, that's what I did in an earlier version of the patch, so I'll go
> back to that.
Just grepped around, and e.g. on powerpc64le-linux -std=c++11 -malign-power -O2
typedef double _Tp;
struct _Tp2 { _Tp _M_t; };
extern _Tp2 tp2e;
int a = alignof(_Tp2);
int b = __alignof__(_Tp2::_M_t);
int c = alignof(_Tp);
int d = __alignof__(tp2e._M_t);
int e = alignof(_Tp2::_M_t);
we have a = 8, b = 4, c = 8, d = 4, e = 4.
Note clang++ with -pedantic-errors errors out on alignof(_Tp2::_M_t) though.
Jakub
