Re: C++ to Python and returning members of a C++ object

Terry,

You should open a pull request on Github with your proposed change.
William or another maintainer will have to accept it to get in.

I'm not a maintainer, but I would recommend the following:

Replace my implementation of the container owner function by your own, but
keep the template specialization so the container code works as before.
In your insertion points, use swig::pointer_category as the template
argument so the ref-counting specialization is called.

Yes, I replaced 'args' with 'self' where you showed.  'self' is the wrapper
for the parent instance, 'args' is a tuple of function arguments to be
unpacked.

When you have in Python:

m = Mixed()
o = m.obj

And Mixed.obj is some wrapped type (e.g. not a string or other value that
gets copied), look at the generated code:

 6903 SWIGINTERN PyObject *_wrap_Mixed_obj_get(PyObject *self, PyObject
*args) {

 6904   PyObject *resultobj = 0;

 6905   Mixed *arg1 = (Mixed *) 0 ;

 6906   void *argp1 = 0 ;

 6907   int res1 = 0 ;

 6908   PyObject *swig_obj[1] ;

 6909   MyObject *result = 0 ;

 6910

 6911   if (!SWIG_Python_UnpackTuple(args, "Mixed_obj_get", 0, 0, 0))
SWIG_fail;

 6912   res1 = SWIG_ConvertPtr(self, &argp1,SWIGTYPE_p_Mixed, 0 |  0 );

 6913   if (!SWIG_IsOK(res1)) {

 6914     SWIG_exception_fail(SWIG_ArgError(res1), "in method '"
"Mixed_obj_get" "', argument " "1"" of type '      " "Mixed *""'");

 6915   }

 6916   arg1 = reinterpret_cast< Mixed * >(argp1);

 6917   result = (MyObject *)& ((arg1)->obj);

 6918   resultobj = SWIG_NewPointerObj(SWIG_as_voidptr(result),
SWIGTYPE_p_MyObject, 0 |  0 );

 6919   SwigContainerReference(args, resultobj); // Increment reference
count of the container object and add       a pointer to the container to
this in the returned object

Without that change I was getting crashes.

-Jake



On Tue, Sep 22, 2020 at 10:28 AM Terry Barnaby <[email protected]> wrote:

> Hi Jake,
>
> I have been away from this for a while with other work.
>
> You stated:
> On 19/08/2020 09:36, Terry Barnaby wrote:
> > It segfaults on pointer-category objects because you should be passing
> > "self" as the first argument not "args", I corrected that for the
> > benchmarks above.
>
> Not sure what you mean here, the only place I use "args " is in:
>
>     Printf(f->code, "SwigContainerReference(args, resultobj); //
> Increment reference count of the container object and add a pointer to
> the container to this in the returned object\n");
>
> And this seems correct, certainly I can't use "self" in place of "args"
> here as the code won't compile. May I ask what change you made here ?
>
> I'm not sure where the SWIG group want to go on this. For the projects
> I'm involved in I need this fix to eliminate the memory faults when
> returning C++ members in Python so I will be creating my own local
> version of SWIG, with my patch applied, for now. But I am keen to try
> and get something like this included in the main tree if possible.
>
> Terry
>
>

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