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

I have had a quick look at the SWIG code.

It seems that it would be possible, and desirable to do a Py_INCREF() on 
the PyObject* object that is wrapping the C++ object which you are 
returning a pointer to the member from. This would hold the whole 'C++' 
object in memory so that returning a pointer to one of this objects 
members would be ok. The PyObject* returned for the member would have to 
have a pointer to its parents PyObject* and do a Py_DECREF() on the 
parent object when it is finally deleted so that the original C++ object 
gets cleaned up eventually.

I don't know the SWIG code to know if this is really possible and if 
people think this is a reasonable SWIG feature that could be added ?
At the moment a user of your SWIG generated Python API gets a pointer to 
a destroyed object which is not good.

Terry
On 24/06/2020 10:04, Terry Barnaby wrote:
> I have a class library that has List objects as members of the class. 
> If I use this class in a Python function and try to return one of 
> these more complex members all hell breaks loose. It appears the SWIG 
> generated wrapper returns a pointer to the objects member rather than 
> making a copy of it when returning the member from the function.
>
> So I have something like:
>
> class TypeB {
>
> public:
>
>     List<TypeA> list1;
>
> };
>
> If in Python I have a function like the following, the returned item 
> is effectively (SWIG wrapped) a pointer the b objects member and b 
> disappears when the function returns.
>
> def func1():
>
>     b = TypeB();
>
>     b.list1.append(TypeA());
>
>     return b.list1;
>
> Now I can use "return ListTypeA(b.list1)", where ListTypeA has been 
> declared using a %template declaration such that the copy constructor 
> is called to explicitly make a copy of the list, but is there a better 
> way of handing this ?
>
> I know this is difficult to handle, but perhaps something like a 
> %template(return) to add code when returning a type in a Python 
> function somehow or maybe the overall objects reference counter can be 
> incremented ?
>
>
>
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