(standard naive-user disclaimer)

I want to use a C++ class from C.  The class services are re-entrant, but
otherwise duplicate the getmntent() family of LibC functions.  I added wrapper
functions with "C" linkage and thought I knew how to do it until I considered
the arguments to the wrapper functions and how to convert them into the
underlying objects.

I've got it working, using a union to violate strict type-checking.  Is there
a better way?

The constructor for the class initializes an object. It is analogous to
setmntent().  A member function retrieves the 'next' entry, analogous to
getmntent().  The destructor is analogous to endmntent()

I declared my wrapper functions like this:

    extern "C" {
 void                *C_setmntent(const char *file, const char *type);
 const struct mntent *C_getmntent(void *thing);
 int                  C_endmntent(void *thing);
    }

I want C_setmntent() to create a new class object via 'new' and return the
address of the object, cast as a (void *).  That part is fine.

Then I want to receive that (void *) in C_getmntent() and use it as a pointer
to an object.  I could not find a cast that converts a (void *) into an
(object *).  I wrote a constructor that receives a (void *) pointer
and assigns 'this', but again I failed to find a suitable cast.  A constructor
builds an object; I need a pointer to the constructed object.

Here is a union to subvert the type-checking.  Is there a better way?

    union cheat { void *vp; getmntent_reentrant *gp; };

    void *C_setmntent(const char *file, const char *type)
    {
      getmntent_reentrant *my_mnt;

 my_mnt = new getmntent_reentrant(file, type);
 return( (void *)my_mnt );
    }

    const struct mntent *C_getmntent(void *thing)
    {
      cheat cheater;

 cheater.vp = thing;
 return( (cheater.gp)->getmntent() );
    }


If possible, please email any replies.

thanks,

   -- Bill Torcaso
      sundial!wildbill@merk.com