"Generic" Data

[richw at ada-uts.UUCP]

Does anybody have any ideas for how one could implement "generic"
data types in C?  The term generic is borrowed from Ada;
"parameterized clusters" in CLU are similar.

For instance, I'd like to implement a "package" of routines which
deal with lists.  These lists can contain elements of any type
but share some operations which are independent of the type of the
elements.

A simple way to do this involves type casting, but loses because
type-checking is sacrificed.  Specifically, the following illustrates
the basic idea:

------------------------- list.h --------------------------
typedef struct List {
    char *car;
    struct List *cdr;
} List;

/**  I apologize for using the all-too-common Lispish terminology  **/
/**  "What do you mean, you don't know what `cdr' means?"          **/
/**  "Isn't it perfectly obvious???"                               **/

     :
extern List *cons();
     :
-----------------------------------------------------------
------------------------- list.c --------------------------
#include "list.h"

List *cons(car, cdr)
char *car;
List *cdr;
{
    List *result;

    result = (List *) malloc(sizeof(List));
    result->car = car;
    result->cdr = cdr;
    return result;
}
-----------------------------------------------------------

Users of such a package would need to caste the arguments and results
of these functions to and from (char *), e.g.:

#include "list.h"

          :
{
    List *int_list;
    List *char_list;

    int_list = cons((char *) 1, cons((char *) 2, NULL));
    char_list = cons((char *) 'a', NULL);
          :
}

Not only does the user play games with the type-checking, there also
is the problem of data-size -- machines whose char pointers are
shorter than ints or chars themselves (unlikely, I know, but...)
would die given the above.

Other ideas which use (ab-use?) the C preprocessor are possible,
but are much more involved.

-- Rich Wagner