Re: [Swig-user] [SPAM] Reference counting redux

[David P. <dpi...@me...>]

> I took another swipe at adding reference counting to our C++/Python
> interface and hit another wall. I have reference counted pointers in
C+
> + and use the %ref and %unref directives to try to add reference
> counting to python. The problem is that, storing pointers to objects
> which are returned from C++ code do not cause the reference count to
> be incremented. Example code:
> 
> # this increments
> m= Lib.Container()
> p= Lib.Object()
> index= m.store_object(p)
> p=None
> #the reference count is not incremented here
> p2=m.get_object(index)
> # nor is it decremented here
> p2= None
> 
> This is with swig 1.33. There is something in the change notes for
> 1.34 about adding support for shared_ptr, but I couldn't find
> documentation online. Are there other changes to improve reference
> counting?

I'm a bit surprised, as I thought python ref-counting worked properly ;)

I have the same problem in C#. My solution, like Josh's, is to use "out"
typemaps in place of %feature("ref"):

%define %counted_obj(TYPE)
// The 'ref' and 'unref' features enable reference counting on 'new' and

// 'delete' only (in C#, delete is called during garbage collection.) 
// These are nice features because they apply to a whole type hierarchy.
// Unfortunately, they are not enough. The refcount must be incremented 
// whenever a function returns TYPE*, so we use the 'out' typemap to
// accomplish that. Since the 'out' typemap applies to the 'new'
operator,
// we cannot use 'ref' to add a reference at the same time, for it would

// cause a second increment. Unfortunately, typemaps do not apply to 
// entire hierarchies, so %counted_obj must be used on EVERY class in a
// hierarchy, not just the base class.
	%feature("ref")   TYPE "// $this->AddRef();"
	%feature("unref") TYPE "$this->ReleaseRef();"
	%typemap(out, null="NULL") TYPE* %{
		if ($1 != NULL) $1->AddRef();
		$result = $1;
	%}
	
	// in the target language you must make typemaps to ensure the
	// "owner" flag is set.
	...
	...
%enddef