qobject: use a QObjectBase_ struct

By moving the base fields to a QObjectBase_, QObject can be a type
which also has a 'base' field. This allows writing a generic QOBJECT()
macro that will work with any QObject type, including QObject
itself. The container_of() macro ensures that the object to cast has a
QObjectBase_ base field, giving some type safety guarantees. QObject
must have no members but QObjectBase_ base, or else QOBJECT() breaks.

QObjectBase_ is not a typedef and uses a trailing underscore to make
it obvious it is not for normal use and to avoid potential abuse.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180419150145.24795-3-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 20 insertions, 11 deletions

diff --git a/include/qapi/qmp/qobject.h b/include/qapi/qmp/qobject.h
index 5206ff9ee1..a713c0165b 100644
--- a/include/qapi/qmp/qobject.h
+++ b/include/qapi/qmp/qobject.h
@@ -34,13 +34,21 @@
 
 #include "qapi/qapi-builtin-types.h"
 
-struct QObject {
+/* Not for use outside include/qapi/qmp/ */
+struct QObjectBase_ {
     QType type;
     size_t refcnt;
 };
 
-/* Get the 'base' part of an object */
-#define QOBJECT(obj) (&(obj)->base)
+/* this struct must have no other members than base */
+struct QObject {
+    struct QObjectBase_ base;
+};
+
+#define QOBJECT(obj) ({                                         \
+    typeof(obj) _obj = (obj);                                   \
+    _obj ? container_of(&(_obj)->base, QObject, base) : NULL;   \
+})
 
 /* High-level interface for qobject_incref() */
 #define QINCREF(obj)      \
@@ -68,8 +76,8 @@ QEMU_BUILD_BUG_MSG(QTYPE__MAX != 7,
 static inline void qobject_init(QObject *obj, QType type)
 {
     assert(QTYPE_NONE < type && type < QTYPE__MAX);
-    obj->refcnt = 1;
-    obj->type = type;
+    obj->base.refcnt = 1;
+    obj->base.type = type;
 }
 
 /**
@@ -77,8 +85,9 @@ static inline void qobject_init(QObject *obj, QType type)
  */
 static inline void qobject_incref(QObject *obj)
 {
-    if (obj)
-        obj->refcnt++;
+    if (obj) {
+        obj->base.refcnt++;
+    }
 }
 
 /**
@@ -101,8 +110,8 @@ void qobject_destroy(QObject *obj);
  */
 static inline void qobject_decref(QObject *obj)
 {
-    assert(!obj || obj->refcnt);
-    if (obj && --obj->refcnt == 0) {
+    assert(!obj || obj->base.refcnt);
+    if (obj && --obj->base.refcnt == 0) {
         qobject_destroy(obj);
     }
 }
@@ -112,8 +121,8 @@ static inline void qobject_decref(QObject *obj)
  */
 static inline QType qobject_type(const QObject *obj)
 {
-    assert(QTYPE_NONE < obj->type && obj->type < QTYPE__MAX);
-    return obj->type;
+    assert(QTYPE_NONE < obj->base.type && obj->base.type < QTYPE__MAX);
+    return obj->base.type;
 }
 
 /**