On 23 February 2017 at 05:42, Thiago Macieira <thiago.macie...@intel.com> wrote: > On quarta-feira, 22 de fevereiro de 2017 02:40:37 PST Ch'Gans wrote: >> > If you're going to call the QVariant constructor with a pointer, you MUST >> > pass a pointer to the correct, concrete object. Forget that it was a >> > template: just >> Well that's my problem, i cannot instanciate and assign the instance >> to a variable of the concrete type since i don't have access to the >> class declaration, not even to a forward declaration. > > That seals your decision. If you don't have a pointer to an object of type > MyObjectList, then your QVariant can only contain be of one value: a null > QVariant.
Ok, makes sense. No black magic here. [...] > >> >> Does anyone know a "proper" solution for this? Or is my approach >> >> correct and safe? >> > >> > Your approach is to redesign and think the problem from another angle. >> >> Well, what is the point of Qt meta-type/object then? Is it an abuse to >> do introspection and dynamic creation? Doesn't Qt (to some extent) >> brings to C++ what Java and C# provides natively? > > The point of the metatype system is to like std::any. It is a simple way of > registering functions to register in a central database a set of functions to > manipulate types. > > The metaobject system includes reflection. The metatype system does not. > > Neither abuse anthing. They're fully within the language. So yes, Qt tries to > bring some niceties of other languages into C++, but its hands are tied in > some ends, where there's simply no solution. Despie what many say, Qt is still > C++ and does not change any of the language rules. > >> 'MyObject*' is registered to the QMetaType system, 'MyObject' class is >> 'registered' to the QMetaObject system. Qt can do "magic" stuff (C++ >> <-> QML & JS), but i cannot do the same myself? > > Sure you can. > > But there's a difference between magic and undefined behaviour. According to > Clarke's Third Law, magic is undistinguishable from sufficiently advanced > technology -- or, in our case, sufficiently advanced code/implementation. But > it > still remains within the boundaries of the language. The moment you step > outside, everything falls apart. > > That's what you asked to do in your code. > > Now, you *can* implement what you want. You can create your own list type > erasure database that permits manipulation (unlike QSequentialIterable). That > way, you can ask it to create an empty list for you, add items, etc. > > But that's not the metatype system. > >> This might work, but i'm trying to find a solution that wouldn't >> require to write lot of boiler-plate code, b/c with the above solution >> one will have to write boiler-plate code (create object, create list >> of objects, create map of objects, create set of objects, create hash >> of objects...) for every single object type... > > And what is the metatype system besides boilerplate? It is smart about using > templates and a single macro, but it's nothing different. > >> Tanks for all your inputs, but I'm still puzzled, I can't believe this >> is not achievable in a generic way... > > It is achievable. I'm saying that the Qt metatype system doesn't do what you > want and that the way you tried was UB. OK, I take it as "Undefined Behaviour", since you mentioned this above. As my code works, i'll keep it for now (not mission-critical), but i take note that QList<A*> has nothing to do with QList<B*> from a type (and QVariant) point of view, whatever the relationship is between A and B. In the long run, i will use a factory approach, and fully delegate Object* management to the factory. Thanks a lot for sharing on the technical "details". Chris > > -- > Thiago Macieira - thiago.macieira (AT) intel.com > Software Architect - Intel Open Source Technology Center > > _______________________________________________ > Interest mailing list > Interest@qt-project.org > http://lists.qt-project.org/mailman/listinfo/interest _______________________________________________ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
