Convert C++ class pointer to Python object

[Rafał Lalik]

I have a class (A), which returns pointer to another class (B). I would like to port it into cython and be able to return python object B from python object A, but have a problem with defining properly function which return python object. Here is my code (it's simplified version of the problem) (all the code is also attached).

// a.h

#ifndef CLASS_A
#define CLASS_A
#include "b.h"

class A {
public:
  A() {}
  B* getB();
};
#endif

// a.cpp

#include "a.h"

B * A::getB() { return new B; }

// b.h

#ifndef CLASS_B
#define CLASS_B

class B {
public:
  B() {}
  void nop() const;
};
#endif

// b.cpp

#include "b.h"
#include <cstdio>

void B::nop() const
{
  std::printf("My address is %p\n", this);
}

and following cython files

// class_a.pxd

from class_b cimport _B

cdef extern from "a.h":
  cdef cppclass _A "A":
    _A() except +
    _B * getB()

// class_a.pyx

from class_b cimport _B

cdef class A:
  cdef _A c_obj
    def __cinit__(self):
      self.c_obj = _A()

  def getB(self):
    _b = self.c_obj.getB()
    b = B(*_b)
    return b

// class_b.pxd

cdef extern from "b.h":
  cdef cppclass _B "B":
    _B() except +

  void nop()

// class_b.pyx

cdef class B:
  cdef _B c_obj
    def __cinit__(self, _B _Object=None):
      if _B is None:
        self.c_obj = _B()
      else:
        self.c_obj = _Object

  def nop(self):
    self.c_obj.nop()

// setup.py

from distutils.core import setup, Extension
from Cython.Build import cythonize

modules = [
  Extension(
    "class_a",
    sources = [ "class_a.pyx" ],
    language = "c++",
    library_dirs = [ "." ],
    libraries = [ "class" ],
  ),
  Extension(
    "class_b",
    sources = [ "class_b.pyx" ],
    language = "c++",
    library_dirs = [ "." ],
    libraries = [ "class" ],
  ),
]

setup(
  name="pyClass",
  package_dir={'' : ''},
  ext_modules=cythonize(modules, build_dir="./")
)

And here is the build order
// build.sh

g++ -fPIC -o class_a.o a.cpp -c
g++ -fPIC -o class_b.o b.cpp -c
g++ -shared -Wl,-soname,libclass.so -o libclass.so class_a.o class_b.o

python setup.py build_ext install

The error I have is:

Compiling class_a.pyx because it changed.
Compiling class_b.pyx because it changed.
[1/2] Cythonizing class_a.pyx

Error compiling Cython file:
------------------------------------------------------------
...
   def __cinit__(self):
       self.c_obj = _A()

   def getB(self):
       _b = self.c_obj.getB()
       b = B(*_b)
          ^
------------------------------------------------------------

class_a.pyx:10:12: undeclared name not builtin: B

Error compiling Cython file:
------------------------------------------------------------
...
   def __cinit__(self):
       self.c_obj = _A()

   def getB(self):
       _b = self.c_obj.getB()
       b = B(*_b)
             ^
------------------------------------------------------------

class_a.pyx:10:15: Cannot convert '_B *' to Python object

[Robert Bradshaw]

On Wed, Oct 25, 2017 at 4:52 AM, Rafał Lalik <rafal...@gmail.com> wrote:
> The error I have is:
>
>> Compiling class_a.pyx because it changed.
>> Compiling class_b.pyx because it changed.
>> [1/2] Cythonizing class_a.pyx
>>
>> Error compiling Cython file:
>> ------------------------------------------------------------
>> ...
>> def __cinit__(self):
>> self.c_obj = _A()
>>
>> def getB(self):
>> _b = self.c_obj.getB()
>> b = B(*_b)
>> ^
>> ------------------------------------------------------------
>>
>> class_a.pyx:10:12: undeclared name not builtin: B

This is because you import the class as _B and are trying to invoke it as B.

>> Error compiling Cython file:
>> ------------------------------------------------------------
>> ...
>> def __cinit__(self):
>> self.c_obj = _A()
>>
>> def getB(self):
>> _b = self.c_obj.getB()
>> b = B(*_b)
>> ^
>> ------------------------------------------------------------
>>
>> class_a.pyx:10:15: Cannot convert '_B *' to Python object

__init__ (and, consequently) __cinit__ methods cannot take non-Python
arguments. The typical solution to this is to add a static create
method that can take such arguments. E.g.

cdef class B(object):
cdef B* c_obj
@staticmethod
cdef B create(B* ptr):
cdef B result = B()
result.c_obj = ptr
return result

[Rafał Lalik]

>>    def getB(self):
>>        _b = self.c_obj.getB()
>>        b = B(*_b)
>>           ^
>> ------------------------------------------------------------
>>
>> class_a.pyx:10:12: undeclared name not builtin: B

This is because you import the class as _B and are trying to invoke it as B.

Yes, correct. But I didn't know how to handle this with pxd files. But later I tried some things and made it working. Would be great if cython dicumentation contain better example on C++ class and pxd files. 

 

__init__ (and, consequently) __cinit__ methods cannot take non-Python
arguments. The typical solution to this is to add a static create
method that can take such arguments. E.g.

cdef class B(object):
    cdef B* c_obj
    @staticmethod
    cdef B create(B* ptr):
        cdef B result = B()
        result.c_obj = ptr
        return result

Thanks for this code, I put it into my code and now it works.

I had to make also a several more changes, so I will not post all the files again, I just attach the whole code as an archive. Maybe someone who has the same problem will find it useful.