[C++-sig] How to implement a cross-module C-API

[Thomas Schipolowski]

Hello Stefan,

thank you for the quick response.

>I'm not quite
>sure I would embedd these vtbls into the objects you are wrapping, as that
>is a bit invasive. 
>
Uhm, I am not sure if this is what I meant to do. I intended to take the 
route suggested by the std. Python extending tutorial: 
http://docs.python.org/ext/using-cobjects.html So I want to put the 
whole table of API function ptrs in just one of these Python-API 
CObjects, to get the addresses across the C++/Python/C borders. This 
would leave the C++ objects which I intend to access with my API 
untouched. They would even be oblivious to the existence of the C-API, 
as the functions will only access the objects using their normal public 
interface.

But, yes, the CObject itself would be wrapped in order to get it out to 
python. I thought of something like:

static void* call_table[MY_API_PTRS];
.... put function pointers in table ...

object get_c_api() {
    return object(handle<>(*PyCObject_FromVoidPtr*((void*)call_table, 
NULL)));
}

Then I would export 'get_c_api' with the normal boost::python 
facilities. In the plain C part, I would call get_c_api via python, 
extract the void* ptr from the PyCObject* and cast it into the array of 
function pointers, whose prototypes are known thanks to a header file 
that describes the fixed layout of call_table.

>Instead, you may create a dictionary that maps python
>type objects to the structures holding your vtbl and provide a simple
>lookup mechanism that queries the PyTypeObject pointer from a given python
>object and look up the C vtbl with that.
>  
>
Ok, this would be a nice way to associate the functions with the 
corresponding objects. But still I need to use PyCObject_FromVoidPtr or 
something equivalent to be able to put the opaque C pointers, or the ptr 
to the vtable, into a python dict, do I? But don't worry, I will need 
only one vtable with a static structure for my few API functions. So it 
boils down to the problem of getting a single address safely from the 
C++ part to the C part. Each function will check with 
extract<expected_class>(...).check()  to make sure it is called with the 
C++ object type it is intended for.

Hope it still makes sense.
Regards,
Thomas.


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