Cannot call C++ class function in Python

Question

C++ code:

#include <iostream>
class DemoClass{
 private:
 int a, b;
 public:
 DemoClass(int v1, int v2){
 a = v1;
 b = v2;
 }
 void DemoFunction(){
 std::cout << "Hello C++!" << std::endl;
 std::cout << "output: a = " << a << ", b = " << b << std::endl;
 }
};
extern "C" {
 DemoClass* DemoCpp(int v1, int v2){
 return new DemoClass(v1, v2);
 }
 void DemoCppFunction(DemoClass* demo){
 demo->DemoFunction();
 }
}

compile c++ via g++ test.cpp -shared -fPIC -o test.so

Python script:

from ctypes import cdll
lib = cdll.LoadLibrary('./test.so')
class CppClass():
 def __init__(self, v1: int, v2: int):
 self.obj = lib.DemoCpp(v1, v2)
 def demoFunction(self):
 lib.DemoCppFunction(self.obj)
f = CppClass(2, 3)
f.demoFunction()

Here is what I get:

Hello C++!
Segmentation fault

I am pretty sure the parameters are passed to C++ class. I just want to know a way to call the function in a C++ class.

• n. m. could be an AI: Use pybind11 perhaps?
• David Feurle: Have you tried to debug?

Answer 1

Python's ctypes doesn't do a lot of heavy lifting for you. It assumes that the argument and return types for cdll functions are all int until proven otherwise. So while your C++ code knows it's returning a DemoClass*, Python is truncating that to an integer, and on modern 64-bit systems pointers don't fit into plain integers.

Since you don't plan to use this pointer in Python for anything other than calling DLL functions, we don't have to reconstruct our structure on the Python side. Instead, we can use void*, the type of pointers to arbitrary data. In Python, we call this type c_void_p.

At the top of your program, just after loading the DLL, consider

lib.DemoCpp.argtypes = (c_int, c_int)
lib.DemoCpp.restype = c_void_p
lib.DemoCppFunction.argtypes = (c_void_p,)
lib.DemoCppFunction.restype = None

(Note: We can get away without lines 1 and 4 here, since Python will happily assume integer argument types, and our DemoCppFunction doesn't return anything so we don't try to use its return value. But for consistency, we may as well be explicit.)

• Cino: Thanks for your reply. It works well!!!!! What about I want to use DemoClass* and DemoCppFunction will return something? like this: DemoCppFunction will modify the value of a and return. ``` DemoClass* DemoCppFunction(DemoClass* demo){ demo->DemoFunction(); demo->a = 100; return demo; } ``` And I want to get the modified instance in python ``` newDemo = f.demoFunction() print(newDemo.a) # I know this does not work. ```
• Silvio Mayolo: Anywhere that you return an (opaque) pointer to struct in C++, set the restype to c_void_p in Python.