Hi Cristián, thank you very much for the detailed answer. Indeed it was very enlightening, even if I still have troubles to imagine the consequences of class attribute. Probably this needs some more experimenting.
I'm going to try to come up with a doc enhancement as requested, but it may take some time. Best Regards Christoph On 27.02.20 10:59, Cristián Maureira-Fredes wrote: > Hello, > > I will answer inline: > > > On 2/25/20 12:05 AM, ic...@gmx.net wrote: >> Hi, >> (snip) >> One of the biggest unknowns here is the default ownership assumptions of >> shiboken2. It is mentioned that there are some heuristics, but >> unfortunately the documentation seems to be spread around the >> aforementioned places, and so I'm not sure if I understood everything >> correctly. I would like to give some examples in C++ code (assuming "A" >> is a wrapped C++ class which is wrapped as <object-type>) and the >> question is whether or not my assumptions are correct? >> >> 1. A* createA(); // default ownership of return value: target language, >> unless --enable-return-value-heuristic is given > yes > >> 2. void doSomething(A* arg); // default ownership of argument: target >> language > yes > >> 3. void doSomething(A& arg); // default ownership of argument: target >> language > yes > >> 4. const A& getReference(); // what happens here? a copy of the result >> is transferred to target ownership? > yes, > for example look at the generated code for the function "font()" > from QWidget: > > > // font()const > > const QFont & cppResult = const_cast<const ::QWidget *>( > cppSelf)->font(); > > pyResult = Shiboken::Conversions::copyToPython( > reinterpret_cast<SbkObjectType *>( > SbkPySide2_QtGuiTypes[SBK_QFONT_IDX]), &cppResult); > > the value is just copied to target (python). > > >> It seems that the default is that the target language owns all objects >> passed around and transfer to c++ has to be made explicit in >> typesystem.xml? > yes > >> In the <define-ownership> tag documentation, for me it is completely >> unclear what the class attribute means? I think the default is "target", >> but what happens when you specify "native"? > The class attribute will specify if you want to modify > the native or target functions. > - The native being the one called e.g.: > QMimeData * QTreeWidgetWrapper::mimeData(...) > in qtreewidget_wrapper.cpp > - or the target function: > static PyObject *Sbk_QTreeWidgetFunc_mimeData(...) > in qtreewidget_wrapper.cpp > > > Since the default is target, > using class="native" will allow the python object > to keep the ownership of it: > - the variable called pyResult (in the native function) > > But when you specify: > <define-ownership class="native" owner="c++" ...> > you are telling shiboken, don't allow the python object > to keep ownership of the object, so then in the generated code > you get something like: > > Shiboken::Object::releaseOwnership(pyResult) > > Another case is: > > <define-ownership class="target" owner="default", > > in that case nothing is done in the target function, > the variable will just exists in that scope, > However when using: > > <define-ownership class="target" owner="target", > > you will explicitly get the ownership of the object, > and the following line will be added to the target function: > > Shiboken::Object::getOwnership(pyResult); > > and you have yet another case of: > > <define-ownership class="target" owner="c++", > > that will release the ownership (from target) > so it will leave it to C++: > > Shiboken::Object::releaseOwnership(pyArg) > > as you can see, this is the same case as the first with the > class="native" owner="c++", but this happens in the > target function and not in the native. > > >> In the object ownership documentation, it is mentioned that there is the >> invalidate-after-use attribute, but this seems to be missing in the type >> system reference. The given use case is completely unclear to me and it >> would be good to have an example when this might be useful. > Maybe the test case for this situation can clarify the situation better: > https://code.qt.io/cgit/pyside/pyside-setup.git/tree/sources/shiboken2/ApiExtractor/tests/testmodifyfunction.cpp#n102 > >> All in all, the object ownership documentation could be improved greatly >> by giving some examples for the mentioned cases, not only in the xml >> domain, but also in C++ domain. > Sure, I'm totally with you on this, > but it's always a matter of people available to do so, > and priorities :( > > I really hope you can help us improving the documentation too, > even if you find a small mistake you want to submit, > please do, I will happily review it :) > > Cheers > > > _______________________________________________ PySide mailing list PySide@qt-project.org https://lists.qt-project.org/listinfo/pyside
