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

[Marc-André Lureau]

Hi

On Wed, Mar 21, 2018 at 3:01 PM, Paolo Bonzini <address@hidden> wrote:
> On 21/03/2018 14:40, Marc-André Lureau wrote:
>> +/* A typecast, checking for the type of arguments */
>> +/* QObject is at offset 0, for all QObject-derived types */
>> +#define QOBJECT(x) QEMU_GENERIC(x,              \
>> +    (QNull *, (QObject *) x),                   \
>> +    (const QNull *, (const QObject *) x),       \
>> +    (QNum *, (QObject *) x),                    \
>> +    (const QNum *, (const QObject *) x),        \
>> +    (QString *, (QObject *) x),                 \
>> +    (const QString *, (const QObject *) x),     \
>> +    (QDict *, (QObject *) x),                   \
>> +    (const QDict *, (const QObject *) x),       \
>> +    (QList *, (QObject *) x),                   \
>> +    (const QList *, (const QObject *) x),       \
>> +    (QBool *, (QObject *) x),                   \
>> +    (const QBool *, (const QObject *) x),       \
>> +    (QObject *, x),                             \
>> +    (const QObject *, x),                       \
>> +    qobject_unknown_type(x))
>
> Why not just
>
>         QEMU_GENERIC(x,
>                 (QObject *, x),
>                 (const QObject *, x),
>                 ({ \
>                         QEMU_BUILD_BUG_ON(offsetof(typeof(*x), base));
>                         &(x)->base;
>                 }))
>
> That is just an extension of what was being done before, and it is
> resilient against people putting a random "QObject base" in the middle
> of a struct.
>

That would work with an anonymous union though:

struct QObject {
    union {
        QType type;
        QType base;
    };
    size_t refcnt;
};

If it's acceptable, I think I'll take this approach.