[Qt-interest] Q_DECLARE_METATYPE and abstract classes

[Schimkowitsch Robert]

> How do I declare a metatype of an abstract base class? I have seen
> nothing in the Q_DECLARE_METATYPE-docs that would forbid it.
> 
> I need the metatype to create a Q_PROPERTY of (a pointer to) that
> abstract type. The compiler errors are clear enough: It wants to
> create a new object of that type, which doesn't work with an abstract
> class.
> 
> How can I work around this? Of course, I could make a dummy
> implementation in the abstract base class, but would the polymorphism
> mechanisms even work correctly when passing my objects to and from
> QVariants?
> 
> 
> 
> The longer story: What do I actually want to achieve?
> 
> I have found a very elegant solution to the problem: How to create
> (graphics) widgets that can handle events differently in different
> modes or contexts, without needing to know about the modes or
> contexts.
> 
> The idea is that the graphics widget always draws the same way, but
> will respond differently on various events, e.g. mouse clicks.
> 
> There are two use cases I am trying to address:
> 1) The same graphics widget is used on a touch device and on a PC with
> mouse. I need to respond to the events differently to achieve the same
> results.
> 2) The same graphics widget can be in different "modes", and act
> differently in those modes. An example would be a painting surface
> that can be in "pan", "zoom in" or "zoom out" mode. A click would
> cause a different course of action depending on mode.
> 
> To make this all easily extensible, I have created an event handler
> base class, and a concrete event handler class for each specific mode
> or context. The event handler knows its "buddy" graphics widget via
> pointer, so it can act on the widget if need be.
> 
> To switch the mode, I simply set a different concrete event handler on
> the graphics widget. The widget forwards all calls to "event(QEvent*)"
> to the event handler, if one is present.
> 
> And now the final trick: I intend to do the "mode switches" via State
> machine.
> But for that, I need to be able to assign the concrete event handler
> pointer via Q_PROPERTY (or is there another way)?
> 
> 
> This is my abstract event class, which must be implemented by all
> concrete event handlers:
> 
> template<class buddyT>
> class CGlib_AbstractGraphicsEventHandler
> {
> public:
>     CGlib_AbstractGraphicsEventHandler()
>         : m_Buddy(0)
>     {
>     }
>     virtual ~CGlib_AbstractGraphicsEventHandler()
>     {
>     }
> 
>     virtual const buddyT* buddy()
>     {
>         return m_Buddy;
>     }
> 
>     virtual void setBuddy(buddyT* newBuddy)
>     {
>         m_Buddy = newBuddy;
>     }
> 
>     virtual bool performEvent(QEvent* event) = 0;
> private:
>     buddyT* m_Buddy;
> 
> 
> Ideas?
> 
> 
> Kind regards
> 
> Robert Schimkowitsch


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