On Tue, 7 Apr 2015, Jonathan Wakely wrote: > On 07/04/15 06:51 -0400, Hans-Peter Nilsson wrote: > > On Tue, 7 Apr 2015, Jonathan Wakely wrote: > > > On 05/04/15 21:07 -0400, Hans-Peter Nilsson wrote: > > > > We did specify that with the alignas. Is the alignof always > > > > exactly the same as an alignas, if one is specified? (And will > > > > that not change in a future amendment, standard and/or > > > > implementation?) Either way, is there a test-case to guard all > > > > this? > > > > > > The language guarantees that's what alignas() does, if the argument is > > > a valid alignment (which it must be if we derive it from some other > > > type's alignment). > > > > I'm more worried about alignof reporting a higher value for a > > specific object than alignas to be wrong. > > That shouldn't be possible because the C++ standard says it's an error > to use alignas with a less strict alignment than would be used if it > was omitted, i.e. an error to use alignas with a value less than the > result alignof would give. However, G++ doesn't reject it (PR65685). > > It still won't be possible here, because the alignas value we use is > not less than alignof(_Tp).
That's not what I meant. My worry is there being a case where alignof yields a *higher* value than the one that the alignas specified. > > Your question quoted just below seems to indicate a similar > > worry. > > I was thinking about cases like this: > > struct __attribute__((packed)) Bad { > char c; > std::atomic<long long> a; > }; > > But G++ ignores the packed attribute here, which is good (Clang > doesn't seem to ignore it, and mis-aligns the atomic). I was more thinking of something like: #include <atomic> #include <iostream> using std::cout; using std::endl; struct SoSo { double d; int x alignas(sizeof(int)); }; SoSo s __attribute__((__aligned__(16))); int main(void) { cout << "alignof(s): " << alignof(s) << endl; cout << "alignof(s.d): " << alignof(s.d) << endl; cout << "alignof(s.x): " << alignof(s.x) << endl; } in which I fear s.x would get an alignof the same as the s.d or s, now or after a while, i.e. higher than specified. (I get for cris-elf at revision 221891: alignof(s): 16 alignof(s.d): 1 alignof(s.x): 4 which is kind-of-expected except I thought s.d would get the s alignment so that just leaves it open whether that could possibly change.) brgds, H-P