Add support for using QWeakPointer with QObject, replacing QPointer.

The problem with QPointer is that it's a simple QObject*. So the only
way for QPointer to do what it's supposed to do is if the object it's
pointing to clears all QPointers when getting deleted. That means the
QObject must know each and every QPointer pointing to it. To make
matters worse, QPointers can be deleted while the object they're
pointing to also gets deleted. So deleting QObjects must do locking.

The solution to the QPointer problem is that both QObject and the
"QPointer" reference something outside the QObject. This way,
QObject doesn't have to lock anything to destroy itself: it's simply
setting a volatile integer to zero when it gets deleted. Since the
integer is outside the QObject, the integer is also refcounted. It's also
O(1), so there's no problem having as many "QPointer".

The two-atomic-ints structure is exactly what QSharedPointer and
QWeakPointer use internally. We just abuse this structure for QObject
needs, setting the strong reference count to -1 to indicate that it's
a QObject that cannot be managed by a QSharedPointer. But QWeakPointer
can still work and replace QPointer neatly.

Reviewed-by: Bradley T. Hughes
Reviewed-by: Jarek Kobus

1 files changed, 27 insertions, 0 deletions

diff --git a/src/corelib/tools/qsharedpointer.cpp b/src/corelib/tools/qsharedpointer.cpp
index ef4dfac..e2c5e95 100644
--- a/src/corelib/tools/qsharedpointer.cpp
+++ b/src/corelib/tools/qsharedpointer.cpp
@@ -864,6 +864,33 @@
 #include <qset.h>
 #include <qmutex.h>
 
+#if !defined(QT_NO_QOBJECT)
+#include "../kernel/qobject_p.h"
+
+QtSharedPointer::ExternalRefCountData *QtSharedPointer::ExternalRefCountData::getAndRef(const QObject *obj)
+{
+    Q_ASSERT(obj);
+    QObjectPrivate *d = QObjectPrivate::get(const_cast<QObject *>(obj));
+    ExternalRefCountData *that = d->sharedRefcount;
+    if (that) {
+        that->weakref.ref();
+        return that;
+    }
+
+    // we can create the refcount data because it doesn't exist
+    ExternalRefCountData *x = new ExternalRefCountData(Qt::Uninitialized);
+    x->strongref = -1;
+    x->weakref = 2;  // the QWeakPointer that called us plus the QObject itself
+    if (!d->sharedRefcount.testAndSetRelease(0, x)) {
+        delete x;
+        d->sharedRefcount->weakref.ref();
+    }
+    return d->sharedRefcount;
+}
+#endif
+
+
+
 #if !defined(QT_NO_MEMBER_TEMPLATES)
 
 //#  define QT_SHARED_POINTER_BACKTRACE_SUPPORT