[C++-sig] Custodian & ward

["Jarno Seppänen"]

Hi,

Niall asked:
> ... and each FXWindow instance maintains a list of its children, and
> deletes its children when it gets deleted, then how do I apply 
> custodian & ward to ensure that when the parent instance is deleted, 
> python knows not to delete the children?

I've experienced the same problems when writing wrappers for LLVM.  My
conclusion is that BPL doesn't obey with_custodian_and_ward and/or
return_value_policy in case of constructors.  This is what I think should
work, but eventually this crashes in a double delete:

struct P;
struct C {
    explicit C(P& parent); // transfers ownership of this to parent
};
BOOST_PYTHON_MODULE(init_policy) {
    class_<C, boost::noncopyable>
        ("C", init<P&>()
         [with_custodian_and_ward<2, 1,
              return_value_policy<reference_existing_object> >()]);
}

I've attached a compiling example of the crash, and how to work around by
using a factory function.

BR, Jarno

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