[C++-sig] Pyste feature: class member deletion order solution

[Niall Douglas]

Just encountered an interesting problem in my now fully working 
python test program - python does not delete class instance data 
members in the reverse order of which they were created. For example:

class MainWindow(FXMainWindow):
    def __init__(self, app, title):
        FXMainWindow.__init__(self, app, title)
        self._vframe=FXVerticalFrame(self, LAYOUT_FILL)
        self._text=FXText(self._vframe, None, 0, LAYOUT_FILL)
        self._text.setEditable(False)
        self._text.setText("Hello World!")

        self._okb=FXButton(self._vframe, "&Close", None, app, 
FXApp.ID_QUIT)
        self.resize(320, 256)

The problem is when python is deleting everything it goes to delete 
MainWindow - according to the python spec, if an instance data member 
starts with "_" then it gets "del" called upon it before all else 
(section 3.3.1 reference manual). Unfortunately, in the case above it 
deletes self._vframe before self._text.

This is a problem because under C++ _text is a child of _vframe and 
deleting _vframe causes auto-delete of _text - and so when python 
comes to delete _text, the memory is vacant and nasty fatal exit 
happens.

I was thinking that the obvious solution is for representations of 
C++ objects to be able to hold a boost::python::handle<> of their 
parent, thus ensuring that the parent stays alive as long as its 
children. For that, pyste would merely need to add a handle<> to the 
class wrapper with something like:

struct FX_FXText_Wrapper: FX::FXText
{
    handle<> parentHandle;
    PyObject* self;
    FX_FXText_Wrapper(PyObject* self_, FX::FXComposite* p0):
        FX::FXText(p0), parentHandle(object(p0 ? *p0 : 0).ptr()), 
self(self_)
    { }

The only thing is, I've not determined if object() matches the 
FXComposite pointer to what it really is, a FXVerticalFrame.

Thus Q: Does boost::python::object(type &) search a list of pointers 
to all known instantiated things and thus correctly increment the 
reference of the right thing? Or does it depend on type, which being 
that of a base class of FXVerticalFrame means it might get confused?

Bruno, how easy is this to implement in pyste?

Cheers,
Niall




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