[Tutor] c++ on python

[Stefan Behnel]

James Chapman, 13.03.2014 17:35:
> Perhaps I should look into Cython as I'm currently working on a
> project that utilises a C API.
> 
> I've been finding that getting the data types to be exactly what the C
> API is expecting to be the hardest part.
> 
> With the original question in mind, here's an example calling into a
> C++ external C API:
> 
> (Works if compiled is VisualStudio. The DLL produced by MinGW-G++ didn't work).
> 
> -------------------------------
> // main.h
> 
> #ifndef __MAIN_H__
> #define __MAIN_H__
> 
> #include <windows.h>
> 
> #define DLL_EXPORT __declspec(dllexport)
> 
> #ifdef __cplusplus
> extern "C"
> {
> #endif
> 
>     int DLL_EXPORT add(int a, int b);
> 
> #ifdef __cplusplus
> }
> #endif
> 
> #endif // __MAIN_H__
> -------------------------------
> 
> -------------------------------
> //main.cpp
> 
> #include "main.h"
> 
> // a sample exported function
> int DLL_EXPORT add(int a, int b)
> {
>     return(a + b);
> }
> 
> extern "C" DLL_EXPORT BOOL APIENTRY DllMain(HINSTANCE hinstDLL, DWORD
> fdwReason, LPVOID lpvReserved)
> {
>     switch (fdwReason)
>     {
>         case DLL_PROCESS_ATTACH:
>             // attach to process
>             // return FALSE to fail DLL load
>             break;
> 
>         case DLL_PROCESS_DETACH:
>             // detach from process
>             break;
> 
>         case DLL_THREAD_ATTACH:
>             // attach to thread
>             break;
> 
>         case DLL_THREAD_DETACH:
>             // detach from thread
>             break;
>     }
>     return TRUE; // succesful
> }
> -------------------------------
> 
> -------------------------------
> # -*- coding: utf-8 -*-
> # dll.py
> 
> import ctypes
> 
> 
> class DllInterface(object):
> 
>     dll_handle = None
> 
>     def __init__(self, dll_file):
>         self.dll_handle = ctypes.WinDLL(dll_file)
> 
>     def add_a_and_b(self, a=0, b=0):
>         return self.dll_handle.add(a, b)
> 
> 
> if __name__ == '__main__':
>     dll_file = 'PythonDLL.dll'
>     external_lib = DllInterface(dll_file)
>     int_a = ctypes.c_int(1)
>     int_b = ctypes.c_int(2)
>     result = external_lib.add_a_and_b(int_a, int_b)
>     print(result)

In Cython, that would essentially be

    cdef extern from "main.h":
        int add(int a, int b)

    print(add(1, 2))

It compiles down to C(++), i.e. it interfaces at the API level, not the ABI
level, as ctypes would.

Stefan