Question

How to use Python object in C++?

Answer 1

Heya!

Here is an example in which a simple Python object is wrapped and embedded. We are using  .c for this, c++ has similar steps:

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class PyClass(object):

   def __init__(self):

       self.data = []

   def add(self, val):

       self.data.append(val)

   def __str__(self):

       return "Data: " + str(self.data)

cdef public object createPyClass():

   return PyClass()

cdef public void addData(object p, int val):

   p.add(val)

cdef public char* printCls(object p):

   return bytes(str(p), encoding = 'utf-8')

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We compile with cython pycls.pyx (use --cplus for c++) to generate a .c and .h file containing the source and the function declarations respectively. We now create a main.c file that starts up Python and we are ready to call these functions:

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#include "Python.h"   // Python.h always gets included first.

#include "pycls.h"    // Include your header file.

int main(int argc, char *argv[])

{

   Py_Initialize();   // initialize Python

   PyInit_pycls();    // initialize module (initpycls(); in Py2)

   PyObject *obj = createPyClass();

   for(int i=0; i<10; i++){

       addData(obj, i);

   }

   printf("%s\n", printCls(obj));

   Py_Finalize();

   return 0;

}

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Compiling this with the proper flags (which you can obtain from python3.5-config of python-config [Py2]):

gcc pycls.c main.c -L$(python3.5-config --cflags) -I$(python3.5-config --ldflags) -std=c99

will create our executable which interacts with our object:

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./a.out

Data: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

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All this was done by using Cython along with the public keyword that generates the .h header file. We could alternatively just compile a python module with Cython and create the header/handle the additional boilerplate ourself.

Hope it works!!