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 12:23 PM, Marc-André Lureau wrote:

>>>> struct QObjectCommon {
>>>>       QType type;
>>>>       size_t refcnt;
>>>> }
>>>>
>>>> struct QObject {
>>>>       QObjectCommon base;
>>>> }
>>>>
>>>> struct QString {
>>>>       QObjectCommon base;
>>>>       ...
>>>> }

> What is the QOJECT() macro you proposed with that?

Here's what you can do while still leaving all "subtypes" of QObject 
free to put base at a non-zero offset:

#define QOBJECT(o) ((QObject *)(&(o)->base))

QEMU_BUILD_BUG_MSG(offsetof(QObject, base),
     "base of QObject must be at offset 0");

#define qobject_to(type, obj) ({ \
     QObjectCommon *_tmp = qobject_check_type(obj, glue(QTYPE_CAST_TO_, 
type)); \
     _tmp ? container_of(_tmp, type, base) : (type *)NULL; })


static inline QObjectCommon *qobject_check_type(const QObject *obj, 
QType type)
{
     if (obj && qobject_type(obj) == type) {
         return QOBJECT(obj);
     } else {
         return NULL;
     }
}


Or, you keep your idea of requiring ALL subtypes to put base at offset 
0, so that qobject_to() also becomes simpler.


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