Exposing cython extension class to c++

[Casper van Elteren]

I have several cython extension classes that I wish to expose to c++. Apparantly there are some not well documented keywords such as `api` and `public` that can be used to automatically create headers for exposure.

I have wrote some trivial classes

```cython
# file: A.pyx
cdef public class A [object PyA, type PyA_t]:
    cdef public int a
    cdef public double test(self, double x):
        return x

```

that produces a header `A.h` that exposes a struct in which I can see that it exposes the variable correctly. A `vtab` is additionally created. However I am not sure how I access the class methods directly. In my simple cpp code it gives me errors when I try it:


``` cpp
//file main.cpp

#include <iostream>
#include "Python.h"
#include "A.h"
using namespace std;

int main(){
  auto b = PyA();
  b.a = 5;  //works
  double c = 1.;
  auto test = PyA();
  test->test(c);

}

```

[da-woods]

I don't think the `vtab` or the class methods in it are exported in a usable way, unfortunately. That's probably a bug/missing feature.

I think the easiest thing to do would be to create global functions that call your class methods:

cdef public double A_test(A a_instance, double x):

    return a_instance.test(x)

A simple wrapper like this will cost almost no performance.

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[Casper van Elteren]

Ah ok so the feature is MIA at the moment. 

Providing bindings that way seems a bit overkill tbh. How come the vtable is not accessible in the usual way?

[da-woods]

> How come the vtable is not accessible in the usual way?

I don't think there's a good reason why it couldn't/shouldn't be accessible. I think embedding Cython into C is not a widely used feature and probably most people expose a fairly small interface (maybe mainly a "start_my_cython_routine" function).  Obviously knowing this doesn't help you...

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[Casper van Elteren]

Indeed. I understand my workflow is a bit niche as the code base has undergone many changes from native python with numpy to cython, now I wish to write the rest of the codebase that interacts with these objects in c++/c. This unfortunately puts me in the position to either (a) write wrappers as proposed above or (b) rewrite the entire project in c++. I hope the cython dev team puts putting c++ class apis on their radar.

Thanks anyway ;-)

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