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qapi: Add qobject_to()

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This is a dynamic casting macro that, given a QObject type, returns an
object as that type or NULL if the object is of a different type (or
NULL itself).

The macro uses lower-case letters because:
1. There does not seem to be a hard rule on whether qemu macros have to
   be upper-cased,
2. The current situation in qapi/qmp is inconsistent (compare e.g.
   QINCREF() vs. qdict_put()),
3. qobject_to() will evaluate its @obj parameter only once, thus it is
   generally not important to the caller whether it is a macro or not,
4. I prefer it aesthetically.

The macro parameter order is chosen with typename first for
consistency with other QAPI macros like QAPI_CLONE(), as well as
for legibility (read it as "qobject to" type "applied to" obj).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20180224154033.29559-3-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
[eblake: swap parameter order to list type first, avoid clang ubsan
warning on QOBJECT(NULL) and container_of(NULL,type,base)]
Signed-off-by: Eric Blake <eblake@redhat.com>
This commit is contained in:
Max Reitz 2018-02-24 16:40:28 +01:00 committed by Eric Blake
parent 9139b56723
commit 1a56b1e2ab
1 changed files with 29 additions and 0 deletions
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29
include/qapi/qmp/qobject.h
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@ -50,6 +50,21 @@ struct QObject {
#define QDECREF(obj) \
qobject_decref(obj ? QOBJECT(obj) : NULL)
/* Required for qobject_to() */
#define QTYPE_CAST_TO_QNull QTYPE_QNULL
#define QTYPE_CAST_TO_QNum QTYPE_QNUM
#define QTYPE_CAST_TO_QString QTYPE_QSTRING
#define QTYPE_CAST_TO_QDict QTYPE_QDICT
#define QTYPE_CAST_TO_QList QTYPE_QLIST
#define QTYPE_CAST_TO_QBool QTYPE_QBOOL
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; })
/* Initialize an object to default values */
static inline void qobject_init(QObject *obj, QType type)
{
@ -102,4 +117,18 @@ static inline QType qobject_type(const QObject *obj)
return obj->type;
}
/**
* qobject_check_type(): Helper function for the qobject_to() macro.
* Return @obj, but only if @obj is not NULL and @type is equal to
* @obj's type. Return NULL otherwise.
*/
static inline QObject *qobject_check_type(const QObject *obj, QType type)
{
if (obj && qobject_type(obj) == type) {
return (QObject *)obj;
} else {
return NULL;
}
}
#endif /* QOBJECT_H */