Re: [Qemu-devel] [PATCH v5 1/5] qobject: ensure base is at offset 0

[Marc-André Lureau]

Hi

On Wed, Apr 18, 2018 at 6:45 PM, Markus Armbruster <address@hidden> wrote:
> Eric Blake <address@hidden> writes:
>
>> On 04/17/2018 08:35 AM, Marc-André Lureau wrote:
>>> All QObject types have the base QObject as first field. This allows to
>>
>> s/as/as their/
>>
>>> simplify qobject_to() and will allow further simplification in
>>
>> s/allows to simplify/allows the simplification of/
>> s/in/in the/
>>
>>> following patch.
>>
>> Might also be worth mentioning that this explicitly guarantees that
>> existing casts work correctly (even though we'd prefer to get rid of
>> such casts in any location except the qobject.h macros); Markus pointed out:
>>
>>>> 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);
>
> As far as I'm concerned, that's the real reason.  The simplification
> plus the check to make it safe seems like a wash.
>
> The cast I spotted appears to be the only one, though:
>
>     $ git-grep '(Q[A-Z][a-z]* \*)'
>     hmp.c:    qmp_device_add((QDict *)qdict, NULL, &err);
>     include/qapi/qmp/qobject.h:        return (QObject *)obj;
>     qobject/qobject.c:static void (*qdestroy[QTYPE__MAX])(QObject *) = {
>     tests/check-qdict.c:    dst = (QDict *)qdict_crumple(src, &error_abort);
>
> The first two cast away const, the third isn't a type cast.  The fourth
> one should use qobject_to() instead, regardless of this patch.
>
> Do we want to force base to come first anyway?

to simplify qobject_to and allow static casting?

> Where does PATCH 2 exploit "base first"?

patch 2 no longer needs that afaict