Re: [Qemu-devel] [PATCH v6 1/5] qobject: ensure base is at offset 0

["Marc-André Lureau" <marcandre.lureau@gmail.com>]

On Thu, Apr 19, 2018 at 5:20 PM, Eric Blake <eblake@redhat.com> wrote:
> On 04/19/2018 10:01 AM, Marc-André Lureau wrote:
>> All QObject types have the base QObject as their first field. This
>> allows the simplification of qobject_to().
>>
>> 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)
>>
>> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
>> Reviewed-by: Eric Blake <eblake@redhat.com>
>
> My R-b stands that this is correct from a coding point of view.  But if
> I read Markus' review correctly, we could omit this patch, fix the one
> broken client in tests/check-qdict.c to use qobject_to() (why didn't you
> fix that in v6)?, and then just apply patches 2-5 without this patch,
> with no change in behavior and where we are no longer dependent on using
> offset 0 (even though all current instances do).  So, I'll leave that to
> maintainer discretion.
>

I don't think we have a good reason to allow offset different than 0.
The fact that we have code that rely on that behaviour already is a
sign that this could easily happen again, because it's the common
pattern in C for inheritance, and static casting is allowed, for
better or worse.



-- 
Marc-André Lureau