On Mon, Apr 16, 2018 at 10:31 AM, Markus Armbruster <arm...@redhat.com> wrote: > Markus Armbruster <arm...@redhat.com> writes: > >> Marc-André Lureau <marcandre.lur...@redhat.com> writes: >> >>> By moving the base fields to a QObjectBase_, QObject can be a type >>> which also has a 'base' field. This allows to write a generic >>> QOBJECT() macro that will work with any QObject type, including >>> QObject itself. The container_of() macro ensures that the object to >>> cast has a QObjectBase_ base field, giving some type safety >>> guarantees. However, for it to work properly, all QObject types must >>> have 'base' at offset 0 (which is ensured by static checking from >>> the previous patch) >> >> I'm afraid this condition is neither sufficient nor necessary. >> >> QOBJECT() maps a pointer to some subtype to the base type QObject. For >> this to work, the subtype must contain a QObject. >> >> Before the patch, this is trivially the case: the subtypes have a member >> QObject base, and QOBJECT() returns its address. >> >> Afterwards, the subtypes have a member QObjectBase_ base, and QOBJECT() >> returns the address of a notional QObject wrapped around this member. >> Works because QObject has no other members. >> >> The condition 'base is at offset 0' does not ensure this, and is >> therefore not sufficient. >> >> It's not necessary, either: putting base elsewhere would work just fine >> as long as we put *all* of QObject there. >> >> Please document the real condition "QObject must have no members but >> QObjectBase_ base, or else QOBJECT() breaks". Feel free to check their >> sizes are the same (I wouldn't bother). >> >> If requiring base to be at offset zero for all subtypes materially >> simplifies code, then go ahead and do that in a separate patch. My gut >> feeling is it doesn't, but I could be wrong. > > Uh, there's another reason: existing type casts from QObject * to > subtypes. I just spotted one in tests/check-qdict.c: > > dst = (QDict *)qdict_crumple(src, &error_abort); > > There may well be more. >
So we better have checks from patch 1, don't we?