[Swig-user] swig python - having c/c++ increment python reference counts to avoid garbage collection

[Daniel M. <ang...@gm...>]

I have started wrapping some C++ objects for use in python using swig and I
am having problems with object ownership.

Here is a rather contrived simplest case example of what is happening to me.

Given this C++ code:
/////////////////////////////
class Hold
{
public:
   Hold(const char * label)
   {
      m_label = label;
   }

   void tell()
   {
      std::cout << "This is in held: " << m_label << std::endl;
   }

protected:
   const char * m_label;
};


class Container
{
public:
   Container(Hold * obj)
   {
      m_obj = obj;
   }

   void doTell()
   {
      if (m_obj)
         m_obj->tell();
   }

protected:
   Hold * m_obj;
};
/////////////////////////////

And if you then use this example module in python like so:

l = list()

def start():

    h = Hold('h1')
    c = Container(h)

    print 'tell1'
    c.doTell()

    l.append(c)

start()

print 'tell2'
c = l.pop(0)
c.doTell()


tell1 is ok
tell2 is garbage

This is I assume because python doesn't know "Container" is holding a
reference to "Hold" so as soon as it leaves start it garbage collects it.

How do I tell python that Container is going to hold a reference for this
parameter?
While searching for an answer I found section "31.4.2 Memory management" in
the python swig documentation sort of talk about this with "thisown", it is
talking about returning new objects, I am trying to hold a reference to an
existing one

%ref %unref seem to be about reference counting the C++ objects themselves
if it was being held in a smart pointer not telling python at all

so I assume the answer lies in a typemap.  I can use Py_INCREF to increment
the reference count in a special typemap as it comes in, but then I have no
way to decrement the reference count on containers destruction.

Maybe I am just missing something obvious, any help would be appreciated.