How to transfer data structure or class from Python to C/C++?

Aaron "Castironpi" Brady castironpi at gmail.com
Sat Oct 18 01:03:44 CEST 2008


On Oct 16, 9:10 am, Hongtian <hongtian.i... at gmail.com> wrote:
> Not exactly.
>
> In my C/C++ application, I have following function or flow:
>
> void func1(....)
> {
>     call PyFunc(struct Tdemo, struct &Tdemo1);
>
> }
>
> I mean I want to invoke Python function 'PyFunc' and transfer a data
> structure 'Tdemo' to this function. After some process in Python, I
> want it return 'Tdemo1' back to the C/C++ application.
>
> I research boost.python and think it is not a reasonable solution
> because it make the C/C++ application too complex.
>
> Thanks.
snip

Solution produced here.  Includes dirty kludge, which will welcome
correction.

/C file:

#include <Python.h>

typedef struct {
    int a;
    float b;
} TypeA;

static PyObject *
methA(PyObject *self, PyObject *args) {
    TypeA a;
    TypeA b;
    PyObject* fun;
    PyObject* res;

    PyArg_ParseTuple( args, "O", &fun );
    a.a= 10;
    a.b= 20.5;

    res= PyObject_CallFunction( fun, "II", &a, &b );

    printf( "%i %f\n", b.a, b.b );
    Py_DECREF( res );

    return PyInt_FromLong( 0 );
}

static PyMethodDef module_methods[] = {
    {"methA", methA, METH_VARARGS, "No doc"},
    {NULL, NULL, 0, NULL}  /* Sentinel */
};


#ifndef PyMODINIT_FUNC  /* declarations for DLL import/export */
#define PyMODINIT_FUNC void
#endif
PyMODINIT_FUNC
initng27ext(void)
{
    PyObject* m;
    m = Py_InitModule3("ng27ext", module_methods,
                       "Custom.");
    if (m == NULL)
      return;
}

/Py file:

import ng27ext

import ctypes as c
class TypeA( c.Structure ):
    _fields_= [
        ( 'a', c.c_int ),
        ( 'b', c.c_float )
        ]

from _ctypes import _cast_addr
_data_cast= c.PYFUNCTYPE( c.py_object, c.c_void_p, c.py_object,
c.py_object)( _cast_addr ) #dirty kludge

def exposed( obj1, obj2 ):
    cob1= _data_cast( obj1, None, c.POINTER( TypeA ) )
    cob2= _data_cast( obj2, None, c.POINTER( TypeA ) )
    print cob1.contents.a, cob1.contents.b
    cob2.contents.a= c.c_int( 60 )
    cob2.contents.b= c.c_float( 70.5 )
    print cob2.contents.a, cob2.contents.b

print ng27ext.methA( exposed )

/Compile & link:

c:/programs/mingw/bin/gcc ng27ext.c -c -Ic:/programs/python25/include
c:/programs/mingw/bin/gcc -shared ng27ext.o -o ng27ext.pyd -Lc:/
programs/python25/libs -lpython25

/Output:

10 20.5
60 70.5
60 70.500000
0
Press any key to continue . . .




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