Build DLL (MyMathFuncs) for use in Python Ctypes

Question

As a complete novice with C++ I just created my first Dynamic Link Library following the MS tutorial found here

The header file reads as follows:

// MathFuncsDll.h

namespace MathFuncs
{
    class MyMathFuncs
    {
    public:
        // Returns a + b
        static __declspec(dllexport) double Add(double a, double b);

        // Returns a - b
        static __declspec(dllexport) double Subtract(double a, double b);

        // Returns a * b
        static __declspec(dllexport) double Multiply(double a, double b);

        // Returns a / b
        // Throws DivideByZeroException if b is 0
        static __declspec(dllexport) double Divide(double a, double b);
    };
}

Now, I want to read this file into Python ctypes. I do this using:

import ctypes as ctypes

MyDll = 'MathFuncsDll.dll'
MyFuncs = ctypes.cdll.LoadLibrary(MyDll)

Now, I'm struggling to actually access the functions. My intuition leads me to try

a = ctypes.c_double(54)
b = ctypes.c_double(12)

summation = MyFuncs.Add(a,b)

which returns the error

AttributeError: function 'Add' not found

Is my problem that the function is nested within the class MyMathFuncs which is also within namespace MathFuncs? How do I gain access to these functions?

For reference, below is the contents of the .cpp file used to generate the dll

// MathFuncsDll.cpp
// compile with: /EHsc /LD

#include "MathFuncsDll.h"

#include <stdexcept>

using namespace std;

namespace MathFuncs
{
    double MyMathFuncs::Add(double a, double b)
    {
        return a + b;
    }

    double MyMathFuncs::Subtract(double a, double b)
    {
        return a - b;
    }

    double MyMathFuncs::Multiply(double a, double b)
    {
        return a * b;
    }

    double MyMathFuncs::Divide(double a, double b)
    {
        if (b == 0)
        {
            throw new invalid_argument("b cannot be zero!");
        }

        return a / b;
    }
}

Viktor Kerkez Viktor Kerkez · Accepted Answer · 2013-08-08T19:03:40

You cannot access C++ classes and namespaces using the ctypes. There is no standard binary interface for C++, as there is for C. Every compiler outputs it's own (different) binaries for the same C++ dynamic libraries. You can read more about it here.

The thing you can do (if you have to do it in C++) is do all your work in C++ and then write a tiny layer of C that wraps it and exposes it. Then you'll be able to access it using ctypes. Again, you can read about it here

Build DLL (MyMathFuncs) for use in Python Ctypes

1 Answers