refs #684, #766, #776 Support %OwnerName% in cache keys by allowing CCached::m_element to be a dummy

and by using a trick with explicit destructor call and placement new to reconstruct CCached when owner's name changes.

src/blackmisc/valuecacheprivate.h

@@ -43,6 +43,16 @@ namespace BlackMisc
         };
         //! \endcond
 
+        /*!
+         * \private Destroy an object and reconstruct it with the given constructor arguments.
+         */
+        template <typename T, typename... Args>
+        void reconstruct(T *object, Args &&... args)
+        {
+            object->~T();
+            new (object) T(std::forward<Args>(args)...);
+        }
+
         /*!
          * \private QObject subclass used by CCached<T> class template for signal/slot communication with CValueCache.
          * An instance of this class is shared between all CCached<T> referring to the same CValueCache and owned by the same QObject,
@@ -76,6 +86,9 @@ namespace BlackMisc
             //! Set the functor to call to notify that the value corresponding to the element's key was modified.
             void setNotifySlot(Element &element, NotifySlot slot);
 
+            //! True if the currently paged value corresponds to a valid key.
+            bool isInitialized(const Element &element) const;
+
             //! True if the currently paged value is a valid instance of the given type.
             //! \threadsafe
             bool isValid(const Element &element, int typeId) const;