QQmlRefCount: de-virtualize dtor

Now that QQmlRefCounted ensures no-one uses ~QQmlRefCount() directly
anymore, we can make it non-virtual, shrinking the
sizeof(QQmlRefCount) from 8/16 to 4/4 bytes (32/64 bit platforms).

This requires moving the release() function from QQmlRefCount down
into QQmlRefCounted<T>, and static_cast'ing *this to T before calling
delete.

We need to be careful, of course, that no derived class relied on the
implied virtualness of ~QQmlRefCount() making their dtors virtual, so
require that all classes that use QQmlRefCounted are either final or
have a virtual destructor.

Update the toolsupport test and the TypeInformationVersion, as
sizeof(QQmlRefCount) was one the items checked.

Fixes: QTBUG-114817
Change-Id: I69afd36ec5b63313842c1438e0244503603ed96f
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 17 insertions, 5 deletions

diff --git a/src/qml/qml/ftw/qqmlrefcount_p.h b/src/qml/qml/ftw/qqmlrefcount_p.h
index 2914e6279c..8cd1cbdaf1 100644
--- a/src/qml/qml/ftw/qqmlrefcount_p.h
+++ b/src/qml/qml/ftw/qqmlrefcount_p.h
@@ -30,11 +30,10 @@ class Q_QML_PRIVATE_EXPORT QQmlRefCount
 public:
     inline QQmlRefCount();
     inline void addref() const;
-    inline void release() const;
     inline int count() const;
 
 private:
-    inline virtual ~QQmlRefCount();
+    inline ~QQmlRefCount();
     template <typename T> friend class QQmlRefCounted;
 
 private:
@@ -44,8 +43,10 @@ private:
 template <typename T>
 class QQmlRefCounted : public QQmlRefCount
 {
+public:
+    inline void release() const;
 protected:
-    ~QQmlRefCounted() = default;
+    inline ~QQmlRefCounted();
 };
 
 template<class T>
@@ -130,11 +131,22 @@ void QQmlRefCount::addref() const
     refCount.ref();
 }
 
-void QQmlRefCount::release() const
+template <typename T>
+void QQmlRefCounted<T>::release() const
 {
+    static_assert(std::is_base_of_v<QQmlRefCounted, T>,
+                  "QQmlRefCounted<T> must be a base of T (CRTP)");
     Q_ASSERT(refCount.loadRelaxed() > 0);
     if (!refCount.deref())
-        delete this;
+        delete static_cast<const T *>(this);
+}
+
+template <typename T>
+QQmlRefCounted<T>::~QQmlRefCounted()
+{
+    static_assert(std::is_final_v<T> || std::has_virtual_destructor_v<T>,
+                  "T must either be marked final or have a virtual dtor, "
+                  "lest release() runs into UB.");
 }
 
 int QQmlRefCount::count() const