c pointers and ctypes

Question

So, I have C++ class that I wrap in C so I can use it in Python using ctypes. Declaration of C++ class:

// Test.h
class Test
{
public:
 static double Add(double a, double b);
};
//Test.cpp
#include "stdafx.h"
#include "Test.h"
double Test::Add(double a, double b)
{
 return a + b;
}

C wrap:

// cdll.h
#ifndef WRAPDLL_EXPORTS
#define WRAPDLL_API __declspec(dllexport) 
#else
#define WRAPDLL_API __declspec(dllimport) 
#endif
#include "Test.h"
extern "C"
{
 WRAPDLL_API struct TestC;
 WRAPDLL_API TestC* newTest();
 WRAPDLL_API double AddC(TestC* pc, double a, double b);
}
//cdll.cpp
#include "stdafx.h"
#include "cdll.h"
TestC* newTest()
{
 return (TestC*) new Test;
}
double AddC(TestC* pc, double a, double b)
{
 return ((Test*)pc)->Add(a, b);
}

Python script:

import ctypes
t = ctypes.cdll('../Debug/cdll.dll')
a = t.newTest()
t.AddC(a, 2, 3)

Result of t.AddC(a, 2, 3) is always some negative integer. There is a problem with a pointer, but I do not know what is a problem. Does anyone have any ideas?

• user4759923: Show us your complete C and C++ code.
• kjrkvc: I edited the question, now there is complete C and C++ code

Answer 1

As AddC is a static function the pointer is not your problem.

You need to pass double values to AddC, and get a double type back:

t.AddC.restype = c_double
t.AddC(a, c_double(2), c_double(3))

The documentation for ctype explains all this.

Answer 2

As stated in the documentation

By default functions are assumed to return the C int type. Other return types can be specified by setting the restype attribute of the function object.

So just add

t.AddC.restype = c_double
t.AddC(a, 2.0, 3.0)

and you'll get 5.0 instead.

• J.J. Hakala: 2 and 3 are probably passed as integers, so the result might not be 5.0.
• tynn: Oh right, that's true. Thanks. Maybe one should set argtypes as well.
• kjrkvc: Actually you have to do it like this