[Swig-user] Python director + shared_ptr + ownership

[Sander v. D. <sgv...@gm...>]

Hello all,

I am making a Python interface for a project, using directors, and I am
confused about the proper way to make sure ownership is correct. The
situation:

I have a C++ class Option, with the following in the header, which is
%include-d in the SWIG interface file:

class Option
{
public:
  Option(std::string const& id);
  std::string getID() const;
  virtual bool isAvailable() { return true; }
};

Furthermore, I have a container for these Options, OptionTree, with the
following interface in the SWIG interface file:

class OptionTree
{
public:
  void addOption(std::shared_ptr<Option> option);
  std::shared_ptr<Option> getOption(std::string const& id) const;
};

As you see, I use std::shared_ptr, I have a %shared_ptr(Option) at the
start of the interface file.

Finally, I extend the class and use it in Python as follows:

class DummyOption(Option):
    def __init__(self, id):
        Option.__init__(self, id)

    def isAvailable(self):
          print(self.getID()) # 1
          return True

def addDummy(tree):
    dummy = DummyOption("dummy")
    tree.addOption(dummy)

OptionTree tree
addDummy(tree)

Then a C++ method is called from Python, which ends up calling isAvailable
on the option. This causes a segfault in the getID call at the line marked
# 1, because after addDummy returns, the Python object is garbage
collected, which also deletes the C++ director object (right?). To overcome
this, I followed the docs and did tree.addOPtion(dummy.__disown__())
instead, but this causes a segfault in swig_pyobj_disown, when it tries to
do a dynamic cast. After digging around with gdb and some printing, my
guess is that there it tries to cast the shared_ptr to an Option, which
blows up.

I now got it working without crashing by setting dummy.thisown = 0, however
I don't think this must be the right way to do it right? With this the
Python object still gets garbage collected, which I see happen when
implementing its __del__ method. However, subsequent calls into Python,
such as isAvailable, still work, even when I access some member variables
in there that I set on the DummyOption in __init__, which confuses me..

So my questions: what is the best way to do it, or can I stick with thisown
= 0? Does SWIG get confused by the shared_ptr?  Do I need some typemap? Is
my thinking completely wrong?

I use SWIG 2.0.10 and Python 3.3.2

Many thanks in advance for any insight you may have!

Sander