Qt: any way to automatically generate QSharedData-based structures?

Question

Hello.

Qt has a build-in supprt for creating objects with integrated reference counting via QSharedData and QSharedDataPointer. All wordks great, but for each such object i need to write a lot of code: QSharedData-based implementation class with constructor and copy constructor, object class itsef with accessor methods for each filed. For a simple structures with 5-10 fields this requires really lot of near same code. Is it some ways to automate such classes generation? Maybe it's some generators exists that take a short description and automatically generates implementation class and object class with all accessors?

Answer 1

I don't think generators would exist for these things, but I suggest two things:

• (ab)use existing shared containers (QVector, QList...)
• read the documentation (with examples) on QSharedData, QSharedDataPointer, and QExplicitelySharedDataPointer

The two subclasses have simple examples that show how to implement the shared-ness it seems. I can't help you further though, because I've never had the need to create my own.

On second thought, why not make all data fields public, and use the QSharedData derivative as a struct-like class with reference counting? Maybe not nice on encapsulation, but if you're careful, nothing wrong should happen.

Answer 2

You usually don't have to implement copy ctor or operator= when using QSharedData/Pointer. The default impls copy/assign the QSharedData-derived member, which usually does the Right Thing (TM). For the public class, you need to implement the ctor creating the private object, and if the private class is not declared in the header but in the implementation (which is better), a dtor (doing nothing, the only point is that is not inlined and defined in the .cpp, after the private declaration). For the private class, no method/ctor/dtor implementations are necessary. For simple value-based classes, writing setters is of course tedious, but the same is true if you use plain private member variables. The overhead in LOC doesn't grow with the number of members.

And no, there is no standard generator solution for that I know of, although writing a script or emacs macro etc. doing it is not that hard. Probably would make sense to add such things to a publicly available toolbox, or QtCreator...