[cfe-dev] c++11's alignas() ignored inside #pragma pack

[jahanian]

On Jul 22, 2014, at 3:58 PM, Richard Smith <richard at metafoo.co.uk> wrote:

> On Tue, Jul 22, 2014 at 1:03 PM, jahanian <fjahanian at apple.com> wrote:
> Implementation says that it is intentional:
> 
>   // The maximum field alignment overrides the aligned attribute.
>   if (!MaxFieldAlignment.isZero()) {
>     FieldAlign = std::min(FieldAlign, MaxFieldAlignment);
>     UnpackedFieldAlign = std::min(UnpackedFieldAlign, MaxFieldAlignment);
>   }
> - Fariborz
> 
> I too find this very surprising. Does this match GCC / MSVC behavior for #pragma pack? I don't think it matches __attribute__((packed)).

I don’t have access to GCC. But MSVC’s behavior takes into account current pack setting and enforces  alignas() of the field.


- Fariborz

>  
> On Jul 22, 2014, at 11:04 AM, jahanian <fjahanian at apple.com> wrote:
> 
>> This test shows that alignas() is ignored when used with #pragma.
>> Is this intentional behavior?
>> 
>> #pragma pack(push, 1)
>> struct S1 { char a; short b; double c; alignas(32) double d; char e; double f; };
>> #pragma pack(pop)
>> 
>> int main()
>> {
>>  return sizeof(struct S1);
>> }
>> 
>> - Thanks, Fariborz
>> 
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> 
> 
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