Il 17/07/20 11:36, Lars Knoll ha scritto:
And it’s cleaner, because using the union trick, we do access several members of the union at the same time. It works on all compilers but I’m not 100% convinced it’s fully defined behavior according to C++, even if the members don’t have data.
To me, it's mostly the pointer arithmetic being sketchy (although probably legal).
I don't see a particular problem with the union trick -- given N empty classes
class Ex {};
And then the union
union U { E1 e1; E2 e2; ~~~ EN en; };
Then:
1) All of Ex and therefore U are standard layout classes [class.prop]
2) Therefore, the address of U is the same address as E1 [class.mem§26]
3) All non static data members of U have the same address [class.union]
It follows that U and all of its inner EN subobjects have the same
address, so you can apply the same offset to all of them reach the
QObject/gadget that contains U as subobject.
And why wouldn’t we do it, if the compilers support it?
Because it breaks ABI, and only relatively recent GCC versions have support for the attribute... or does GCC have a nonstandard one?
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