Calling C++ methods from python

Question

I stuck with some problem in calling C++ methods in python I got this following link in stack overflow

But I don't to use Boost package and swig. Is there any approach in python can used to call these C++ methods. I am compiling C++ as a shared object in Linux and using in Python 2.7

#include <iostream>
using namespace std;

class Polygon {
  protected:
    int width, height;
  public:
    void set_values (int a, int b)
      { width=a; height=b; }
};

class Rectangle: public Polygon {
  public:
    int area()
      { return width*height; }
};

class Triangle: public Polygon {
  public:
    int area()
      { return width*height/2; }
};

int main () {
  Rectangle rect;
  Triangle trgl;
  Polygon * ppoly1 = &rect;
  Polygon * ppoly2 = &trgl;
  ppoly1->set_values (4,5);
  ppoly2->set_values (4,5);
  cout << rect.area() << '\n';
  cout << trgl.area() << '\n';
  return 0;
}

Could please guide to achieve this. Any code snippet or example will be highly appreciated.

Thanks in advance --

Answer 1

This answer is for Python2.x only!

When you want to write a C++-extension, you first need to download the python-dev package to get all the needed libs and header files (only needed on Linux)

The most important part in the following source is PolygonObject and PyTypeObject .

PolygonObject

An object of this class represents your Polygon instance in Python. As you can see it contains a pointer obj which is your original object.

PyTypeObject

https://docs.python.org/2/c-api/type.html#c.PyTypeObject

The C structure of the objects used to describe built-in types.

Here is how you can use your Polygon object:

import libfoo
po = libfoo.Polygon()
po.set_values(1, 2)

Here is the module implementation ( libfoo.cpp ). This example does not contain inheritance but the keyword you have to look for is tp_base . Also the original Python source has a lot of examples which can help here a lot.

#include <Python.h>

// this is your original class
class Polygon {
protected:
  int width, height;
public:
  void set_values (int a, int b)
  { width=a; height=b; }
};

typedef struct {
  PyObject_HEAD
  Polygon* obj;
} PolygonObject;

static void
Polygon_dealloc(PolygonObject* self)
{
  delete self->obj;
  self->ob_type->tp_free((PyObject*)self);
}

static PyObject *
Polygon_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
  PolygonObject *self;

  self = (PolygonObject*)type->tp_alloc(type, 0);
  if (self != NULL) {
    self->obj = new Polygon;
    // do your cleanup here
  }

  return (PyObject *)self;
}

static PyObject* Polygon_set_values(PolygonObject* self, PyObject *args, PyObject *kwds)
{
  int a, b;
  const char* kwlist[] = {"a", "b", NULL};

  if (! PyArg_ParseTupleAndKeywords(args, kwds, "ii", (char**)kwlist, &a, &b))
    return NULL;

  self->obj->set_values(a, b);
  Py_INCREF(Py_None);
  return Py_None;
}

static PyMethodDef Polygon_methods[] = {
  {"set_values", (PyCFunction)Polygon_set_values, METH_VARARGS, "set the value"},
  {NULL}  /* Sentinel */
};

static PyTypeObject PolygonType = {
  PyObject_HEAD_INIT(NULL)
  0,                         /*ob_size*/
  "mod.Polygon",             /*tp_name*/
  sizeof(PolygonObject),           /*tp_basicsize*/
  0,                         /*tp_itemsize*/
  (destructor)Polygon_dealloc, /*tp_dealloc*/
  0,                         /*tp_print*/
  0,                         /*tp_getattr*/
  0,                         /*tp_setattr*/
  0,                         /*tp_compare*/
  0,                         /*tp_repr*/
  0,                         /*tp_as_number*/
  0,                         /*tp_as_sequence*/
  0,                         /*tp_as_mapping*/
  0,                         /*tp_hash */
  0,                         /*tp_call*/
  0,                         /*tp_str*/
  0,                         /*tp_getattro*/
  0,                         /*tp_setattro*/
  0,                         /*tp_as_buffer*/
  Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, /*tp_flags*/
  "Polygon class",           /* tp_doc */
  0,                         /* tp_traverse */
  0,                         /* tp_clear */
  0,                         /* tp_richcompare */
  0,                         /* tp_weaklistoffset */
  0,                         /* tp_iter */
  0,                         /* tp_iternext */
  Polygon_methods,           /* tp_methods */
  0,                         /* tp_members */
  0,                         /* tp_getset */
  0,                         /* tp_base */
  0,                         /* tp_dict */
  0,                         /* tp_descr_get */
  0,                         /* tp_descr_set */
  0,                         /* tp_dictoffset */
  0,                         /* tp_init */
  0,                         /* tp_alloc */
  Polygon_new,               /* tp_new */
};

static PyMethodDef module_methods[] = {
  {NULL}  /* Sentinel */
};

PyMODINIT_FUNC
initlibfoo()
{
  PyObject* m;

  if (PyType_Ready(&PolygonType) < 0)
    return;

  m = Py_InitModule3("libfoo", module_methods, "Example module that creates an extension type.");
  if (m == NULL)
    return;

  Py_INCREF(&PolygonType);
  PyModule_AddObject(m, "Polygon", (PyObject *)&PolygonType);
}

clang++ -shared -I/usr/include/python2.7/ -fPIC libfoo.cpp -o libfoo.so -lpython

Here are two additional links which give you more information and a deeper technical background how to extend Python.

https://docs.python.org/2/extending/newtypes.html https://docs.python.org/2/extending/extending.html