qobject_cast: optimize when the target is marked final

If it is, then we can perform a direct meta object pointer comparison.
Comparing meta objects is, after all, what QMetaObject::inherits() and
QMetaObject::cast()) are doing:

    const QMetaObject *m = this;
    do {
        if (metaObject == m)
            return true;
    } while ((m = m->d.superdata));

But if we know the class is final, we know an object can either be
exactly of the type we want or not be so at all.

Change-Id: If6333547d215c1d84d1cfffdfd82c04e75251608
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/kernel/qobject.h

@@ -420,28 +420,17 @@ bool QObject::setProperty(const char *name, QVariant &&value)
 template <class T>
 inline T qobject_cast(QObject *object)
 {
-    static_assert(std::is_pointer_v<T>,
-                  "qobject_cast requires to cast towards a pointer type");
-    typedef typename std::remove_cv<typename std::remove_pointer<T>::type>::type ObjType;
-    static_assert(QtPrivate::HasQ_OBJECT_Macro<ObjType>::Value,
-                    "qobject_cast requires the type to have a Q_OBJECT macro");
-    return static_cast<T>(ObjType::staticMetaObject.cast(object));
+    return QtPrivate::qobject_cast_helper<T>(object);
 }
 
 template <class T>
 inline T qobject_cast(const QObject *object)
 {
-    static_assert(std::is_pointer_v<T>,
-                  "qobject_cast requires to cast towards a pointer type");
     static_assert(std::is_const_v<std::remove_pointer_t<T>>,
                   "qobject_cast cannot cast away constness (use const_cast)");
-    typedef typename std::remove_cv<typename std::remove_pointer<T>::type>::type ObjType;
-    static_assert(QtPrivate::HasQ_OBJECT_Macro<ObjType>::Value,
-                      "qobject_cast requires the type to have a Q_OBJECT macro");
-    return static_cast<T>(ObjType::staticMetaObject.cast(object));
+    return QtPrivate::qobject_cast_helper<T>(object);
 }
 
-
 template <class T> constexpr const char * qobject_interface_iid() = delete;
 template <class T> inline T *
 qobject_iid_cast(QObject *object, const char *IId = qobject_interface_iid<T *>())

src/corelib/kernel/qobjectdefs.h

@@ -737,6 +737,24 @@ namespace QtPrivate {
         static int test(int (Object::*)(QMetaObject::Call, int, void **));
         enum { Value =  sizeof(test(&Object::qt_metacall)) == sizeof(int) };
     };
+
+    template <class TgtType, class SrcType>
+    inline TgtType qobject_cast_helper(SrcType *object)
+    {
+        using ObjType = std::remove_cv_t<std::remove_pointer_t<TgtType>> ;
+        static_assert(std::is_pointer_v<TgtType>,
+                "qobject_cast requires to cast towards a pointer type");
+        static_assert(HasQ_OBJECT_Macro<ObjType>::Value,
+                "qobject_cast requires the type to have a Q_OBJECT macro");
+
+        if constexpr (std::is_final_v<ObjType>) {
+            if (object && object->metaObject() == &ObjType::staticMetaObject)
+                return static_cast<TgtType>(object);
+            return nullptr;
+        } else {
+            return static_cast<TgtType>(ObjType::staticMetaObject.cast(object));
+        }
+    }
 }
 
 QT_END_NAMESPACE

tests/auto/corelib/kernel/qobject/tst_qobject.cpp

@@ -92,6 +92,7 @@ private slots:
     void dumpObjectTree();
     void connectToSender();
     void qobjectConstCast();
+    void qobjectCastFinal();
     void uniqConnection();
     void uniqConnectionPtr();
     void interfaceIid();
@@ -2429,6 +2430,13 @@ public:
     int rtti() const override { return 43; }
 };
 
+class FinalObject final: public FooObject
+{
+    Q_OBJECT
+public:
+    int rtti() const override { return 44; }
+};
+
 void tst_QObject::declareInterface()
 {
     FooObject obj;
@@ -3691,9 +3699,29 @@ void tst_QObject::qobjectConstCast()
     const QObject *cptr = &obj;
 
     QVERIFY(qobject_cast<FooObject *>(ptr));
+    QVERIFY(qobject_cast<const FooObject *>(ptr));
     QVERIFY(qobject_cast<const FooObject *>(cptr));
 }
 
+void tst_QObject::qobjectCastFinal()
+{
+    FooObject foo;
+    QObject *ptr = &foo;
+    const QObject *cptr = &foo;
+
+    QCOMPARE(qobject_cast<FinalObject *>(ptr), nullptr);
+    QCOMPARE(qobject_cast<const FinalObject *>(ptr), nullptr);
+    QCOMPARE(qobject_cast<const FinalObject *>(cptr), nullptr);
+
+    FinalObject final;
+    ptr = &final;
+    cptr = &final;
+
+    QCOMPARE(qobject_cast<FinalObject *>(ptr), &final);
+    QCOMPARE(qobject_cast<const FinalObject *>(ptr), &final);
+    QCOMPARE(qobject_cast<const FinalObject *>(cptr), &final);
+}
+
 void tst_QObject::uniqConnection()
 {
     SenderObject s;