Return c++ object (not a pointer preferably) in cython

Question

I have two classes (let's assume the most simple ones, implementation is not important). My defs.pxd file (with cython defs) looks like this:

cdef extern from "A.hpp":
  cdef cppclass A:
    A() except +

cdef extern from "B.hpp":
  cdef cppclass B:
    B() except +
    A func ()

My pyx file (with python defs) looks like this:

from cython.operator cimport dereference as deref
from libcpp.memory cimport shared_ptr

cimport defs

cdef class A:
    cdef shared_ptr[cquacker_defs.A] _this

    @staticmethod
    cdef inline A _from_this(shared_ptr[cquacker_defs.A] _this):
        cdef A result = A.__new__(A)
        result._this = _this
        return result

    def __init__(self):
        self._this.reset(new cquacker_defs.A())

cdef class B:
    cdef shared_ptr[cquacker_defs.B] _this

    @staticmethod
    cdef inline B _from_this(shared_ptr[cquacker_defs.B] _this):
        cdef B result = B.__new__(B)
        result._this = _this
        return result

    def __init__(self):
        self._this.reset(new cquacker_defs.B())

    def func(self):
      return deref(self._this).func()

The thing is that I cannot return non-python object from Python. Actually, I don't want to change my c++ code to return pointer instead of new object (because there are many functions like that). Now it gives me the error:

Cannot convert 'B' to Python object

How can I return one python object holding internal c++ object from another one's method in python? If I can only do that after some c++ changes, I want the most elegant solution, if possible.

Answer 1

Your issue is that your wrapper class requires a pointer (to a new allocated object) but your function returns an C++ object on the stack. To solve this you have to copy or move the object from the stack.

First ensure that your C++ class A has a working copy or move constructor. A move constructor is better if you're c++ class contains large members. Wrap this is Cython like so:

cdef extern from "A.hpp":
  cdef cppclass A:
    A() except +
    A(const A&) except +
    # or A(A&&) except +

(Don't tell Cython about both a copy constructor and a move constructor - it gets confused! C++ finds the right one when it's compiled anyway).

Then, in func use your copy/move constructor with new to pass to your python wrapper:

def func(self):
      return A._from_this(new cquacker_defs.A(self._this.func()))