Re: [Qemu-devel] [PATCH v4 2/4] qobject: use a QObjectBase_ struct

[Markus Armbruster <armbru@redhat.com>]

Markus Armbruster <armbru@redhat.com> writes:

> Marc-André Lureau <marcandre.lureau@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.

>> QObjectBase_ is not typedef and use a trailing underscore to make it
>> obvious it is not for normal use and to avoid potential abuse.
>
> Trailing underscore I like, lack of typedef I don't mind (but I'm firmly
> in the "eschew typedef for structs" camp).  It does violate the common
> QEMU coding style, though.
>
> A comment /* Not for use outside include/qapi/qmp/ */ next to
> QObjectBase_ wouldn't hurt.
>
>> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>