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Showing content from https://github.com/pybind/pybind11/issues/1333 below:

[BUG] Problem when creating derived Python objects from C++ (inheritance slicing) · Issue #1333 · pybind/pybind11 · GitHub

EDIT(eric): Adding a tracking list.

• Enumerate possible solutions and drawbacks, e.g.
  • Should investigate usage of py::wraper<> trampoline shim with GC garbage collection (to prevent inheritance slicing) #2757 Using py::wrapper<> and GC ressurrection
  • Others...
• Identify best path forward, implement.

Hello,
Consider deriving a C++ class in Python and overloading virtual functions, including a 'Clone' function.
When passing a derived Python object to C++ without holding a reference (like calling Clone()), the information about the Python type is lost. In this case the wrong base-class function is called. More complex cases may result in a SEGFAULT.

Is there any way to work around/fix this without causing memory leaks?

Thanks!

// test.cpp
#include <pybind11/pybind11.h>
#include <Python.h>

#include <iostream>
#include <memory>

using std::cout;
using std::endl;
using std::unique_ptr;
using std::shared_ptr;
using std::make_shared;

namespace py = pybind11;

class Base {
public:
    virtual void Print() { 
        cout << "Base!" << endl;
    }

    virtual shared_ptr<Base> Clone() {
        return make_shared<Base>();
    }
};

class PyBase : public Base {
public:
    using Base::Base; // Inherit constructors
    void Print() override { PYBIND11_OVERLOAD(void, Base, Print, ); }
    shared_ptr<Base> Clone() override { PYBIND11_OVERLOAD(shared_ptr<Base>, Base, Clone, ); }
};

PYBIND11_MODULE(libtest, m) {
    py::class_<Base, py::wrapper<PyBase>, shared_ptr<Base>>(m, "Base")
      .def(py::init<>())
      ;

    m.def("Test", [] ( shared_ptr<Base> b ) {
          shared_ptr<Base> d = b->Clone();
          // Python object of d is already dead!
          d->Print();  // prints "Base!"
    });
}

from libtest import *

class Derived(Base):
    def __init__(self):
        super().__init__()

    def Print(self):
        print("Derived!")

    def Clone(self):
        return Derived()

Test(Derived())

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