Handling QMetaType registration in Qt5 with dynamic plug-ins

Question

My company is considering the jump from Qt 4.8.4 to Qt 5.4, but I came across a change that could be a showstopper for us: QMetaType::unregisterType() is removed (http://doc.qt.io/qt-5/sourcebreaks.html).

Our GUI requires plug-ins to be loaded at runtime, with the same plug-in potentially loaded and unloaded more than once during a session of the GUI. In Qt 4, we ran into an issue where when the plug-in was loaded the second time, any signal/slot that used one of the custom types registered by the plug-in would cause access violations because the meta type had been registered by the first instance of the plug-in (which was now unloaded, so the memory space was invalid). We worked around this issue by defining our own macros to register and unregister meta types safely as the plug-in was loaded and unloaded.

With QMetaType::unregisterType() no longer present, I fear that this issue will come back with no real way to solve the problem. Upgrading to Qt 5.4 would be a significant investment to even get to the point that I could test this issue, so I'm hoping I can get some indication from the experts here.

Is there any way to unregister a meta type in Qt 5? If not, does Qt 5 now have some sort of system that can detect when the DLL is being unloaded and unregister the meta types itself (highly unlikely I'd assume)? Alternatively, if we switch to the new Qt 5 signal/slot syntax, does that absolve us of the need for meta types entirely? If so, does the new syntax still allow for queued connections? Please forgive my ignorance on the subject, but I don't see it explicitly listed as supported or not.

Answer 1

Please forgive my ignorance on the subject, but I don't see it explicitly listed as supported or not.

This is currently unsupported, which means that do not unload plugins with Qt 5 as of writing this. Usually, you do not load and unload plugins anyway, as it is done during the launch up in general. The corresponding change in the repository also claims:

The function hasn't been working properly. It was not well tested, for example it is undefined how QVariant should behave if it contains an instance of an unregistered type.

Concept of unregistering types was inspired by plug-in system, but in most supported platforms we do not unload plug-ins.

Idea of type unregistering may block optimizations in meta object system, because it would be not possible to cache a type id. QMetaType::type() could return different ids for the same name.

Thereby, even though you thought it was working, it was unreliable which means you could have observed difficult to find bugs in its operation, resulting unreliable software on your part. I am sure you do not want to release such a software, especially if it is not recommended by the Qt Project to be used.