Marc-André Lureau <[email protected]> writes: > 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)
In my opinion, type casts between QObject * and subtypes are plain wrong. I intend to apply my "[PATCH] qobject: Use qobject_to() instead of type cast" first, and drop the paragraph, if you don't mind. > Signed-off-by: Marc-André Lureau <[email protected]> > Reviewed-by: Eric Blake <[email protected]> > --- > include/qapi/qmp/qobject.h | 5 ++--- > qobject/qobject.c | 9 +++++++++ > 2 files changed, 11 insertions(+), 3 deletions(-) > > diff --git a/include/qapi/qmp/qobject.h b/include/qapi/qmp/qobject.h > index e022707578..5206ff9ee1 100644 > --- a/include/qapi/qmp/qobject.h > +++ b/include/qapi/qmp/qobject.h > @@ -61,9 +61,8 @@ struct QObject { > QEMU_BUILD_BUG_MSG(QTYPE__MAX != 7, > "The QTYPE_CAST_TO_* list needs to be extended"); > > -#define qobject_to(type, obj) ({ \ > - QObject *_tmp = qobject_check_type(obj, glue(QTYPE_CAST_TO_, type)); \ > - _tmp ? container_of(_tmp, type, base) : (type *)NULL; }) > +#define qobject_to(type, obj) \ > + ((type *)qobject_check_type(obj, glue(QTYPE_CAST_TO_, type))) > > /* Initialize an object to default values */ > static inline void qobject_init(QObject *obj, QType type) > diff --git a/qobject/qobject.c b/qobject/qobject.c > index 23600aa1c1..87649c5be5 100644 > --- a/qobject/qobject.c > +++ b/qobject/qobject.c > @@ -16,6 +16,15 @@ > #include "qapi/qmp/qlist.h" > #include "qapi/qmp/qstring.h" > > +QEMU_BUILD_BUG_MSG( > + offsetof(QNull, base) != 0 || > + offsetof(QNum, base) != 0 || > + offsetof(QString, base) != 0 || > + offsetof(QDict, base) != 0 || > + offsetof(QList, base) != 0 || > + offsetof(QBool, base) != 0, > + "base qobject must be at offset 0"); > + > static void (*qdestroy[QTYPE__MAX])(QObject *) = { > [QTYPE_NONE] = NULL, /* No such object exists */ > [QTYPE_QNULL] = NULL, /* qnull_ is indestructible */ As mentioned elsewhere in this thread, the coding errors this assertion can catch are vanishingly unlikely. I could flip a coin to decide whether to keep it. Instead I'm asking you :) With the commit message rephrased not to give anyone ideas about type casts, and with or without the assertion: Reviewed-by: Markus Armbruster <[email protected]>
