QObject: Q_ASSERT the object type before calling a PMF

The old-syle signal-slot syntax had the advantage of not delivering
signals to slots in derived classes after that derived class's
destructor had finished running (because we called via the virtual
qt_metacall). The new syntax made no checks, so a conversion from the
old to the new syntax may introduce crashes or other data corruptions at
runtime if the destructor had completed.

This commit introduces a Q_ASSERT to print the class name that the
object is not any more. Since this is in inline code, this should get
enabled for users' debug modes and does not therefore depend on Qt being
built in debug mode.

It required some Private classes to be adapted to the new form, by
exposing the public q_func() in the public: part.

Pick-to: 6.2
Fixes: QTBUG-33908
Change-Id: Iccb47e5527544b6fbd75fffd16b874cdc08c1f3e
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

2 files changed, 36 insertions, 5 deletions

diff --git a/src/corelib/kernel/qobject_p.h b/src/corelib/kernel/qobject_p.h
index da171f5c7f..ac77d898da 100644
--- a/src/corelib/kernel/qobject_p.h
+++ b/src/corelib/kernel/qobject_p.h
@@ -98,9 +98,9 @@ public:
 
 class Q_CORE_EXPORT QObjectPrivate : public QObjectData
 {
+public:
     Q_DECLARE_PUBLIC(QObject)
 
-public:
     struct ExtraData
     {
         ExtraData(QObjectPrivate *ptr) : parent(ptr) { }
@@ -492,6 +492,12 @@ using FunctionStorage = typename std::conditional_t<
         FunctionStorageByValue<Func>
     >;
 
+template <typename ObjPrivate> inline void assertObjectType(QObjectPrivate *d)
+{
+    using Obj = std::remove_pointer_t<decltype(std::declval<ObjPrivate *>()->q_func())>;
+    assertObjectType<Obj>(d->q_ptr);
+}
+
 template<typename Func, typename Args, typename R>
 class QPrivateSlotObject : public QSlotObjectBase, private FunctionStorage<Func>
 {
diff --git a/src/corelib/kernel/qobjectdefs_impl.h b/src/corelib/kernel/qobjectdefs_impl.h
index a3068e1712..da9d0f6390 100644
--- a/src/corelib/kernel/qobjectdefs_impl.h
+++ b/src/corelib/kernel/qobjectdefs_impl.h
@@ -50,6 +50,7 @@
 
 QT_BEGIN_NAMESPACE
 class QObject;
+class QObjectPrivate;
 
 namespace QtPrivate {
     template <typename T> struct RemoveRef { typedef T Type; };
@@ -139,6 +140,22 @@ namespace QtPrivate {
 
     template<typename Func> struct FunctionPointer { enum {ArgumentCount = -1, IsPointerToMemberFunction = false}; };
 
+    template<typename ObjPrivate> inline void assertObjectType(QObjectPrivate *d);
+    template<typename Obj> inline void assertObjectType(QObject *o)
+    {
+        // ensure all three compile
+        [[maybe_unused]] auto staticcast = [](QObject *obj) { return static_cast<Obj *>(obj); };
+        [[maybe_unused]] auto qobjcast = [](QObject *obj) { return Obj::staticMetaObject.cast(obj); };
+#ifdef __cpp_rtti
+        [[maybe_unused]] auto dyncast = [](QObject *obj) { return dynamic_cast<Obj *>(obj); };
+        auto cast = dyncast;
+#else
+        auto cast = qobjcast;
+#endif
+        Q_ASSERT_X(cast(o), Obj::staticMetaObject.className(),
+                   "Called object is not of the correct type (class destructor may have already run)");
+    }
+
     template <typename, typename, typename, typename> struct FunctorCall;
     template <int... II, typename... SignalArgs, typename R, typename Function>
     struct FunctorCall<IndexesList<II...>, List<SignalArgs...>, R, Function> {
@@ -148,25 +165,33 @@ namespace QtPrivate {
     };
     template <int... II, typename... SignalArgs, typename R, typename... SlotArgs, typename SlotRet, class Obj>
     struct FunctorCall<IndexesList<II...>, List<SignalArgs...>, R, SlotRet (Obj::*)(SlotArgs...)> {
-        static void call(SlotRet (Obj::*f)(SlotArgs...), Obj *o, void **arg) {
+        static void call(SlotRet (Obj::*f)(SlotArgs...), Obj *o, void **arg)
+        {
+            assertObjectType<Obj>(o);
             (o->*f)((*reinterpret_cast<typename RemoveRef<SignalArgs>::Type *>(arg[II+1]))...), ApplyReturnValue<R>(arg[0]);
         }
     };
     template <int... II, typename... SignalArgs, typename R, typename... SlotArgs, typename SlotRet, class Obj>
     struct FunctorCall<IndexesList<II...>, List<SignalArgs...>, R, SlotRet (Obj::*)(SlotArgs...) const> {
-        static void call(SlotRet (Obj::*f)(SlotArgs...) const, Obj *o, void **arg) {
+        static void call(SlotRet (Obj::*f)(SlotArgs...) const, Obj *o, void **arg)
+        {
+            assertObjectType<Obj>(o);
             (o->*f)((*reinterpret_cast<typename RemoveRef<SignalArgs>::Type *>(arg[II+1]))...), ApplyReturnValue<R>(arg[0]);
         }
     };
     template <int... II, typename... SignalArgs, typename R, typename... SlotArgs, typename SlotRet, class Obj>
     struct FunctorCall<IndexesList<II...>, List<SignalArgs...>, R, SlotRet (Obj::*)(SlotArgs...) noexcept> {
-        static void call(SlotRet (Obj::*f)(SlotArgs...) noexcept, Obj *o, void **arg) {
+        static void call(SlotRet (Obj::*f)(SlotArgs...) noexcept, Obj *o, void **arg)
+        {
+            assertObjectType<Obj>(o);
             (o->*f)((*reinterpret_cast<typename RemoveRef<SignalArgs>::Type *>(arg[II+1]))...), ApplyReturnValue<R>(arg[0]);
         }
     };
     template <int... II, typename... SignalArgs, typename R, typename... SlotArgs, typename SlotRet, class Obj>
     struct FunctorCall<IndexesList<II...>, List<SignalArgs...>, R, SlotRet (Obj::*)(SlotArgs...) const noexcept> {
-        static void call(SlotRet (Obj::*f)(SlotArgs...) const noexcept, Obj *o, void **arg) {
+        static void call(SlotRet (Obj::*f)(SlotArgs...) const noexcept, Obj *o, void **arg)
+        {
+            assertObjectType<Obj>(o);
             (o->*f)((*reinterpret_cast<typename RemoveRef<SignalArgs>::Type *>(arg[II+1]))...), ApplyReturnValue<R>(arg[0]);
         }
     };