Re: [Qemu-devel] [PATCH 2/3] qobject: assume base of a qobject is at offset 0

[Eric Blake <eblake@redhat.com>]

On 03/21/2018 11:59 AM, Marc-André Lureau wrote:
>> Hmm, perhaps by making it "struct {} base"?
>>
>> Or even:
>>
>> struct QObjectCommon {
>>      QType type;
>>      size_t refcnt;
>> }
>>
>> struct QObject {
>>      QObjectCommon base;
>> }
>>
>> struct QString {
>>      QObjectCommon base;
>>      ...
>> }
> 
> I fail to see what that solves. You are moving the problem to
> QObjectCommon: we would have to replace all QObject * user to take
> QObjectCommon *, and then we wouldn't solve the problem for
> QObjectCommon.

Here, QObjectCommon is never used outside of qobject.h; all existing 
code continues to uwe QObject *.  But the conversion from QString* or 
QObject * to QObjectCommon * is trivial (&obj->base), and the conversion 
from QObjectCommon * to the public QObject * is trivial (container_of 
style cast).  So the QOBJECT() macro becomes comparable to a C++ 
reinterpret_cast<>() that converts any descendent of QObjectCommon into 
QObject, even if the original type was not QObject; and we never offer a 
public API to convert anything into or out of QObjectCommon (it only 
exists as a dummy type to make our other casts easier).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org