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

4 files changed, 60 insertions, 5 deletions

diff --git a/src/corelib/kernel/qobject.cpp b/src/corelib/kernel/qobject.cpp
index 7e7dcaf..3417232 100644
--- a/src/corelib/kernel/qobject.cpp
+++ b/src/corelib/kernel/qobject.cpp
@@ -56,6 +56,7 @@
 #include <qvarlengtharray.h>
 #include <qset.h>
 #include <qsemaphore.h>
+#include <qsharedpointer.h>
 
 #include <private/qorderedmutexlocker_p.h>
 #include <private/qmutexpool_p.h>
@@ -768,6 +769,13 @@ QObject::~QObject()
         QObjectPrivate::clearGuards(this);
     }
 
+    if (d->sharedRefcount) {
+        // indicate to all QWeakPointers that this QObject has now been deleted
+        d->sharedRefcount->strongref = 0;
+        if (!d->sharedRefcount->weakref.deref())
+            delete d->sharedRefcount;
+    }
+
     emit destroyed(this);
     if (d->declarativeData)
         d->declarativeData->destroyed(this);
diff --git a/src/corelib/kernel/qobject_p.h b/src/corelib/kernel/qobject_p.h
index e58ee6c..056dee3 100644
--- a/src/corelib/kernel/qobject_p.h
+++ b/src/corelib/kernel/qobject_p.h
@@ -67,6 +67,7 @@ QT_BEGIN_NAMESPACE
 class QVariant;
 class QThreadData;
 class QObjectConnectionListVector;
+namespace QtSharedPointer { class ExternalRefCountData; }
 
 /* mirrored in QtTestLib, DON'T CHANGE without prior warning */
 struct QSignalSpyCallbackSet
@@ -187,6 +188,7 @@ public:
     // plus QPointer, which keeps a separate list
     QDeclarativeData *declarativeData;
     QGuard<QObject> *objectGuards;
+    QAtomicPointer<QtSharedPointer::ExternalRefCountData> sharedRefcount;
     int *deleteWatch;
 };
 
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
diff --git a/src/corelib/tools/qsharedpointer_impl.h b/src/corelib/tools/qsharedpointer_impl.h
index 3ee407c..1fed024 100644
--- a/src/corelib/tools/qsharedpointer_impl.h
+++ b/src/corelib/tools/qsharedpointer_impl.h
@@ -162,13 +162,22 @@ namespace QtSharedPointer {
 
     struct ExternalRefCountData
     {
-        QAtomicInt weakref;
-        QAtomicInt strongref;
+        QBasicAtomicInt weakref;
+        QBasicAtomicInt strongref;
 
-        inline ExternalRefCountData() : weakref(1), strongref(1) { }
-        virtual inline ~ExternalRefCountData() { Q_ASSERT(!weakref); Q_ASSERT(!strongref); }
+        inline ExternalRefCountData()
+        {
+            QBasicAtomicInt proto = Q_BASIC_ATOMIC_INITIALIZER(1);
+            weakref = strongref = proto;
+        }
+        inline ExternalRefCountData(Qt::Initialization) { }
+        virtual inline ~ExternalRefCountData() { Q_ASSERT(!weakref); Q_ASSERT(strongref <= 0); }
 
         virtual inline bool destroy() { return false; }
+
+#ifndef QT_NO_QOBJECT
+        Q_CORE_EXPORT static ExternalRefCountData *getAndRef(const QObject *);
+#endif
     };
     // sizeof(ExternalRefCount) = 12 (32-bit) / 16 (64-bit)
 
@@ -376,7 +385,7 @@ namespace QtSharedPointer {
                     tmp = o->strongref;  // failed, try again
                 }
 
-                if (tmp)
+                if (tmp > 0)
                     o->weakref.ref();
                 else
                     o = 0;
@@ -495,6 +504,15 @@ public:
 
     inline QWeakPointer() : d(0), value(0) { }
     inline ~QWeakPointer() { if (d && !d->weakref.deref()) delete d; }
+
+    // special constructor that is enabled only if X derives from QObject
+    template <class X>
+    inline QWeakPointer(X *ptr) : d(ptr ? d->getAndRef(ptr) : 0), value(ptr)
+    { }
+    template <class X>
+    inline QWeakPointer &operator=(X *ptr)
+    { return *this = QWeakPointer(ptr); }
+
     inline QWeakPointer(const QWeakPointer<T> &o) : d(o.d), value(o.value)
     { if (d) d->weakref.ref(); }
     inline QWeakPointer<T> &operator=(const QWeakPointer<T> &o)