Question

Consider the following python ctypes – c++ binding:

// C++
class A
{
public:
    void someFunc();
};

A* A_new() { return new A(); }
void A_someFunc(A* obj) { obj->someFunc(); }
void A_destruct(A* obj) { delete obj; }

# python
from ctypes import cdll

libA = cdll.LoadLibrary(some_path)

class A:
    def __init__(self):
        self.obj = libA.A_new()

    def some_func(self):
        libA.A_someFunc(self.obj)

What’s the best way to delete the c++ object, when the python object is not needed any more.

[edit]
I added the delete function that was suggested, however the problem remains by whom and when the function is called. It should be as convenient as possible.

Answer 1

You could implement the __del__ method, which calls a destructor function you would have to define:

C++

class A
{
public:
    void someFunc();
};

A* A_new() { return new A(); }
void delete_A(A* obj) { delete obj; }
void A_someFunc(A* obj) { obj->someFunc(); }

Python

from ctypes import cdll

libA = cdll.LoadLibrary(some_path)

class A:
    def __init__(self):
        fun = libA.A_new
        fun.argtypes = []
        fun.restype = ctypes.c_void_p
        self.obj = fun()

    def __del__(self):
        fun = libA.delete_A
        fun.argtypes = [ctypes.c_void_p]
        fun.restype = None
        fun(self.obj)

    def some_func(self):
        fun = libA.A_someFunc
        fun.argtypes = [ctypes.c_void_p]
        fun.restype = None
        fun(self.obj)

Also note that you left out the self parameter on the __init__ method. Futhermore, you have to specify the return type/argument type explicitly, because ctypes defaults to 32 bit integers, while a pointer is likely 64 bits on modern systems.

Some think __del__ is evil. As an alternative, you can use with syntax:

class A:
    def __init__(self):
        fun = libA.A_new
        fun.argtypes = []
        fun.restype = ctypes.c_void_p
        self.obj = fun()

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        fun = libA.delete_A
        fun.argtypes = [ctypes.c_void_p]
        fun.restype = None
        fun(self.obj)

    def some_func(self):
        fun = libA.A_someFunc
        fun.argtypes = [ctypes.c_void_p]
        fun.restype = None
        fun(self.obj)

with A() as a:
    # Do some work
    a.some_func()