Python – Create objects in C++ and pass pointers to python

Create objects in C++ and pass pointers to python… here is a solution to the problem.

Create objects in C++ and pass pointers to python

I’m trying to share C++ objects with Python using ctypes by creating the object in C++ and then passing pointers to Python via a C wrapper. I hope to be able to manipulate the object later using other functions in the Python class (do_something in the following code).

I tried the code below but got a segfault. I’m not familiar with C vs. C++ and C vs. Python interfaces, so I’m not sure if I’m making some fundamental mistake when passing pointers, or if after me, memory is being cleared/offloaded by Python garbage collection to create objects?

This question is discussed Similar question for boost, but the answer is not very useful for ctypes.

object.h

class object {  
public:

 constructor 
  object() { 
    pointer = nullptr;
  }

 destructor
  virtual ~object() {
    delete pointer;
    pointer = nullptr;
  }

 get member functions of object_pointer
   from C++
  double do_something();

protected:

 pointer to the object
  object_pointer *pointer;

};

extern "C" {
  object* object_new();
  void object_delete(object *Ob);
  double object_do_something(object *Ob);
}

Object .cpp

#include "object.h"
double object::do_something() { return pointer->do_something(); }

extern "C" {
    object *object_new() { return new object(); }
    void object_delete(object *Ob) { delete Ob; }
    double object_do_something(object *Ob) { return Ob->do_something(); }
}

Object .py

from ctypes import *

lib = cdll.LoadLibrary('./lib_object.so')
lib.object_new.argtypes = ()
lib.object_new.restype = c_void_p
lib.special_delete.argtypes = c_void_p,
lib.special_delete.restype = None
lib.object_pointer.argtypes = c_void_p
lib.object_pointer.restype = c_void_p

class Object:
    def __init__(self):
        self.obj = lib.object_new()
        print self.obj
    def __del__(self):
        lib.object_delete(self.obj)
    def do_something(self):
        lib.object_do_something(self.obj)

s = Object()
>> 94549743086144
s.do_something()
>> Segfault

Any help would be appreciated!

Solution

Notes:

  • All files (.cpp, .py) in the issue are not compiled. They contain syntax errors as well as semantic errors
  • I don’t know what the pointer does (it produces a syntax error). I can only assume that a singleton implementation was attempted
  • With the above in mind, instead of pointing out the errors in the existing file (there are a lot of them), I created a whole new basic example
  • While ctypes

  • aren’t the only areas for improvement, I did point it out: [Python 2]: ctypes – A foreign function library for Python

object.h:

class Object {
    public:

Object() {
        m_double = 2.718282;
    }

virtual ~Object() {}

double do_something();

private:

double m_double;
};

extern "C" {
    Object* object_new();
    void object_delete(Object *ob);
    double object_do_something(Object *ob);
}

object.cpp:

<pre class=”lang-c prettyprint-override”>#include <iostream>
#include "object.h"

using std::cout;

double Object::do_something() {
std::cout << "Doing something in C++\n";
return m_double;
}

extern "C" {
Object *object_new() { return new Object(); }
void object_delete(Object *pObj) { delete pObj; }
double object_do_something(Object *pObj) { return pObj->do_something(); }
}

object.py:

#!/usr/bin/env python2

import sys
import ctypes

lib = ctypes.cdll.LoadLibrary('./lib_object.so')

lib.object_new.argtypes = []
lib.object_new.restype = ctypes.c_void_p
lib.object_delete.argtypes = [ctypes.c_void_p]
lib.object_do_something.argtypes = [ctypes.c_void_p]
lib.object_do_something.restype = ctypes.c_double

class Object:
    def __init__(self):
        self.obj = lib.object_new()
        print("`Object` instance (as a `void *`): 0x{:016X}".format(self.obj))

def __del__(self):
        lib.object_delete(self.obj)

def do_something(self):
        return lib.object_do_something(self.obj)

def main():
    obj = Object()
    ret = obj.do_something()
    print(ret)

if __name__ == "__main__":
    print("Python {:s} on {:s}\n".format(sys.version, sys.platform))
    main()

Output:

[cfati@cfati-5510-0:/cygdrive/e/Work/Dev/StackOverflow/q054594122]> ls
object.cpp  object.h  object.py
[cfati@cfati-5510-0:/cygdrive/e/Work/Dev/StackOverflow/q054594122]> gcc -shared -fPIC -o lib_object.so object.cpp -lstdc++
[cfati@cfati-5510-0:/cygdrive/e/Work/Dev/StackOverflow/q054594122]> python2 object.py
Python 2.7.14 (default, Oct 31 2017, 21:12:13)
[GCC 6.4.0] on cygwin

`Object` instance (as a `void *`): 0x0000000600078870
Doing something in C++
2.718282

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