Create object in C++ and pass pointer out to python

Question

I am attempting to use ctypes to share a C++ object with Python, by creating the object in C++, and passing the pointer out to Python via a C wrapper. I want to be able to act on this object later using the other functions in the Python class, do_something in the code below.

I've tried the following code, but I get a segfault. I'm new to interfacing C with C++ and C with Python, so I'm not sure if I'm doing something fundamentally wrong when passing the pointer, or if the memory is being cleared/shifted by Python garbage-collection, after I create the object?

This question discusses a similar problem for boost, but the answer isn't 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 greatly appreciated!

What command did you use to compile object.cpp into lib_object.so? — pschill
– pschill, Commented Feb 8, 2019 at 14:24
Do you ever change pointer? Otherwise pointer->do_something() dereferences a nullpointer. Also, why is it S->do_something() and not Ob->do_something? — Artyer
– Artyer, Commented Feb 8, 2019 at 15:06
The pointer does get changed: an object is reconstructed from a character buffer, and then that object is pointed to by pointer — Orca
– Orca, Commented Feb 8, 2019 at 15:53
argtypes should be a list (or tuple): lib.object_pointer.argtypes = [c_void_p]. — CristiFati
– CristiFati, Commented Feb 8, 2019 at 22:04

CristiFati · Accepted Answer · 2023-10-07 13:50:23Z

Notes:

None of the files (.cpp, .py) from the question compiled. They contain syntax errors and also semantic errors
I don't know what the pointer role was intended to be (it generated syntax errors). I can only assume that a Singleton implementation was attempted
Considering the above, instead of pointing the errors (there are many of them) in the existing files, I'm creating a brand new basic example
Although CTypes is not the only area to improve, I'm also pointing it out: [Python.Docs]: ctypes - A foreign function library for Python

object.hpp:

#pragma once

#if defined(_WIN32)
#  if defined(OBJECT_EXPORTS)
#    define OBJECT_EXPORT_API __declspec(dllexport)
#  else
#    define OBJECT_EXPORT_API __declspec(dllimport)
#  endif
#else
#  define OBJECT_EXPORT_API
#endif


class Object {
public:
    Object();
    virtual ~Object();
    virtual double doSomething();

private:
    double m_double;
};


extern "C" {
    OBJECT_EXPORT_API void* objectNew();
    OBJECT_EXPORT_API void objectDel(void *pObj);
    OBJECT_EXPORT_API double objectDoSomething(void *pObj);
}

object.cpp:

#define OBJECT_EXPORTS
#include "object.hpp"
#include <iostream>

#define DBG_MSG0() std::cout << "CPP - [" << __FILE__ << "] " << __LINE__ << " (" << __FUNCTION__ << ")\n"


Object::Object():
    m_double(2.718282)
{
    DBG_MSG0();
}

Object::~Object()
{
    DBG_MSG0();
}

double Object::doSomething()
{
    DBG_MSG0();
    return m_double;
}


extern "C" {

void* objectNew()
{
    DBG_MSG0();
    return new Object();
}
void objectDelete(void *pObj)
{
    DBG_MSG0();
    delete reinterpret_cast<Object*>(pObj);
}

double objectDoSomething(void *pObj)
{
    DBG_MSG0();
    if (pObj) {
        return (reinterpret_cast<Object*>(pObj))->doSomething();
    }
    return 0.0;
}

}

code00py:

#!/usr/bin/env python

import ctypes as cts
import sys


DLL_NAME = "./libobject.{:s}".format("dll" if sys.platform[:3].lower() == "win" else "so")

dll = cts.CDLL(DLL_NAME)

objectNew = dll.objectNew
objectNew.argtypes = ()
objectNew.restype = cts.c_void_p
objectDelete = dll.objectDelete
objectDelete.argtypes = (cts.c_void_p,)
objectDoSomething = dll.objectDoSomething
objectDoSomething.argtypes = (cts.c_void_p,)
objectDoSomething.restype = cts.c_double


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

    def __del__(self):
        print("Deleting instance")
        objectDelete(self.obj)
        self.obj = None

    def do_something(self):
        print("Doing something")
        return objectDoSomething(self.obj)


def main(*argv):
    obj = ObjectWrapper()
    ret = obj.do_something()
    print("do_something() returned: {:.6f}".format(ret))


if __name__ == "__main__":
    print("Python {:s} {:03d}bit on {:s}\n".format(" ".join(elem.strip() for elem in sys.version.split("\n")),
                                                   64 if sys.maxsize > 0x100000000 else 32, sys.platform))
    rc = main(*sys.argv[1:])
    print("\nDone.\n")
    sys.exit(rc)

Output:

[cfati@cfati-5510-0:/mnt/e/Work/Dev/StackExchange/StackOverflow/q054594122]> . ~/sopr.sh 
### Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ###

[064bit prompt]> ls
code00.py  object.cpp  object.hpp
[064bit prompt]> 
[064bit prompt]> gcc -shared -fPIC -o libobject.so object.cpp -lstdc++
[064bit prompt]> ls
code00.py  libobject.so  object.cpp  object.hpp
[064bit prompt]> 
[064bit prompt]> python ./code00.py 
Python 3.10.12 (main, Jun 11 2023, 05:26:28) [GCC 11.4.0] 064bit on linux

CPP - [object.cpp] 30 (objectNew)
CPP - [object.cpp] 11 (Object)
`Object` instance (as a `void *`): 0x0000557F47601910
Doing something
CPP - [object.cpp] 41 (objectDoSomething)
CPP - [object.cpp] 21 (doSomething)
do_something() returned: 2.718282
Deleting instance
CPP - [object.cpp] 35 (objectDelete)
CPP - [object.cpp] 16 (~Object)

Done.

An alternative to manually creating and destroying the (C++) object, is making it static (automatically initialized then the .dll is loaded, and destroyed then the .dll is unloaded).

Might want to look over:

Thank you, this is very helpful. I will try to provide a proper stand-alone example in the future, rather than pulling random bits from my code...
You're welcome! [SO]: How to create a Minimal, Complete, and Verifiable example (mcve).

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