Restore symmetry between QSharedPointer and QWeakPointer on QObjects.

With the previous commit, you could create a QWeakPointer from any
QObject-derived object. It's possible because QObject now has a
pointer to the QWeakPointer's d-pointer.

However, if you did:
    QSharedPointer<QObject> obj(new QObject);
    QWeakPointer<QObject> weak1(obj);
    QWeakPointer<QObject> weak2(obj.data());

Then weak1 would shared d-pointers with QSharedPointer, but weak2
wouldn't. Also, weak1.toStrongRef() would work, but
weak2.toStrongRef() wouldn't.

This change makes QObject know where the d-pointer created by
QSharedPointer is, so weak2 would get the same d-pointer.

As a nice side-effect, you can check if a given QObject is shared by
trying to promote its QWeakPointer to QSharedPointer.

Reviewed-by: Bradley T. Hughes

3 files changed, 31 insertions, 0 deletions

diff --git a/src/corelib/kernel/qobject.cpp b/src/corelib/kernel/qobject.cpp
index 3417232..e37b6d3 100644
--- a/src/corelib/kernel/qobject.cpp
+++ b/src/corelib/kernel/qobject.cpp
@@ -770,6 +770,11 @@ QObject::~QObject()
     }
 
     if (d->sharedRefcount) {
+        if (d->sharedRefcount->strongref > 0) {
+            qWarning("QObject: shared QObject was deleted directly. The program is malformed and may crash.");
+            // but continue deleting, it's too late to stop anyway
+        }
+
         // indicate to all QWeakPointers that this QObject has now been deleted
         d->sharedRefcount->strongref = 0;
         if (!d->sharedRefcount->weakref.deref())
diff --git a/src/corelib/tools/qsharedpointer.cpp b/src/corelib/tools/qsharedpointer.cpp
index e2c5e95..10f35c7 100644
--- a/src/corelib/tools/qsharedpointer.cpp
+++ b/src/corelib/tools/qsharedpointer.cpp
@@ -867,6 +867,29 @@
 #if !defined(QT_NO_QOBJECT)
 #include "../kernel/qobject_p.h"
 
+/*!
+    \internal
+    This function is called for a just-created QObject \a obj, to enable
+    the use of QSharedPointer and QWeakPointer.
+
+    When QSharedPointer is active in a QObject, the object must not be deleted
+    directly: the lifetime is managed by the QSharedPointer object. In that case,
+    the deleteLater() and parent-child relationship in QObject only decrease
+    the strong reference count, instead of deleting the object.
+*/
+void QtSharedPointer::ExternalRefCountData::setQObjectShared(const QObject *obj, bool)
+{
+    Q_ASSERT(obj);
+    QObjectPrivate *d = QObjectPrivate::get(const_cast<QObject *>(obj));
+
+    if (d->sharedRefcount)
+        qFatal("QSharedPointer: pointer %p already has reference counting", obj);
+    d->sharedRefcount = this;
+
+    // QObject decreases the refcount too, so increase it up
+    weakref.ref();
+}
+
 QtSharedPointer::ExternalRefCountData *QtSharedPointer::ExternalRefCountData::getAndRef(const QObject *obj)
 {
     Q_ASSERT(obj);
diff --git a/src/corelib/tools/qsharedpointer_impl.h b/src/corelib/tools/qsharedpointer_impl.h
index 421db34..8a34362 100644
--- a/src/corelib/tools/qsharedpointer_impl.h
+++ b/src/corelib/tools/qsharedpointer_impl.h
@@ -165,7 +165,9 @@ namespace QtSharedPointer {
 
 #ifndef QT_NO_QOBJECT
         Q_CORE_EXPORT static ExternalRefCountData *getAndRef(const QObject *);
+        Q_CORE_EXPORT void setQObjectShared(const QObject *, bool enable);
 #endif
+        inline void setQObjectShared(...) { }
     };
     // sizeof(ExternalRefCount) = 12 (32-bit) / 16 (64-bit)
 
@@ -328,6 +330,7 @@ namespace QtSharedPointer {
         inline void internalFinishConstruction(T *ptr)
         {
             Basic<T>::internalConstruct(ptr);
+            if (ptr) d->setQObjectShared(ptr, true);
 #ifdef QT_SHAREDPOINTER_TRACK_POINTERS
             if (ptr) internalSafetyCheckAdd2(d, ptr);
 #endif