Alignment of unions (was: The great inapplicable attribute debate)

[Stewart Gordon]

Frits van Bommel wrote:
> Stewart Gordon wrote:
<snip>
>> But the offset of a union member is always zero.  So what would this do?
> 
> It should make sure the union is aligned appropriately in a containing 
> struct, meaning U.alignof >= M.alignof for all members M. Specifying 
> per-member alignment allows you to change that member's effect on the 
> union's alignment.

But alignment of a member within a structure and alignment of the whole 
structure relative to an outer one are two distinct concepts.  What 
sense is there in being able to use one to control the other?

>>> Most obviously, a union U consisting of a single member x should have
>>> U.alignof == x.alignof.
>> <snip>
>>
>> Yes, by propagating the union's alignment (relative to the containing 
>> struct) to the member.
> 
> But the union's alignment needs to be sufficient for all members, so it 
> depends on the maximum alignment of all members.

That's only because you want to be able to attach alignments to 
individual members of a union.  And I still don't know why.

If you want a union to have a certain alignment relative to a struct in 
which it's contained, in what cases is it not sufficient to put the 
align attribute on the union's instance in the struct?

Stewart.