Object of self type in class - TypeError: No to_python (by-value) converter found for C++ type

Question

I am trying to expose a class (Child) with 2 members (val and parent) using boost.python. Both are public and parent is of type Child. The compilation gives no errors but my python wrapper brings the following when I try to access parent:

TypeError: No to_python (by-value) converter found for C++ type: class

I am pretty new to C++, python and therefore boost.python. I am probably missing something pretty obvious but I can't quite see what's wrong here.

I am using Visual Studio Community 2017 and python2.7 (both 64bit) on Windows 7.

Here are the different pieces of code:

Child.h

class Child {
public:
    Child();
    double val;
    Child* parent;
};

Child.cpp

#include "Child.h"

Child::Child()
{
    val = 0.0;
}

Wrapper.cpp

#include <boost/python.hpp>
#include "Child.h"

BOOST_PYTHON_MODULE(WrapPyCpp)
{
    boost::python::class_<Child>("Child")
        .def_readwrite("val", &Child::val)
        .def_readwrite("parent", &Child::parent)
        ;
}

Wrapper.py

import WrapPyCpp

class_test = WrapPyCpp.Child()
class_test.val = 1.0

class_test_parent = WrapPyCpp.Child()
class_test.parent = class_test_parent

print class_test_parent.val
print dir(class_test)
print class_test.val
print class_test.parent.val

And when I launch Wrapper.py, the last line throws an exception:

C:\Users\Visual Studio 2017\Projects\MinExBoostPython2\x64\Debug>Wrapper.py
0.0
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__instance_size__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'parent', 'val']
1.0
Traceback (most recent call last):
  File "C:\Users\Visual Studio 2017\Projects\MinExBoostPython2\x64\Debug\
Wrapper.py", line 12, in <module>
    print class_test.parent.val
TypeError: No to_python (by-value) converter found for C++ type: class Child * __ptr64

C:\Users\Visual Studio 2017\Projects\MinExBoostPython2\x64\Debug>

I expect to access parent and its member val. So far as I understand the error, I imagine there is a problem with how I expose parent which is declared as a pointer. Am I correct?

I used .def_readwrite("parent", &Child::parent) because it is a public member of Child but I am not sure that it is the right way to access it.

I tried to find informations in the boost.python documentation and took a look at

TypeError: No to_python (by-value) converter found for C++ type

and

Boost.Python call by reference : TypeError: No to_python (by-value) converter found for C++ type:

but if the solution for my problem is in there, I did not get it.

Anyway, I would be very grateful if anyone could help me to correct my mistake and explain me why I am wrong!

Thanks!

Answer 1

Edit

I figured out that, replacing the line

boost::python::class_<Child>("Child")

by

boost::python::class_<Child, Child*>("Child")

in Wrapper.cpp solved my problem. The result is then as expected:

C:\Users\Visual Studio 2017\Projects\MinExBoostPython2\x64\Debug>Wrapper.py
0.0
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__instance_size__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'parent', 'val']
1.0
0.0

C:\Users\Visual Studio 2017\Projects\MinExBoostPython2\x64\Debug>

Hope it helps someone!