Type-safe sum types in C

gistfile1.c

// needs -fnested-functions. Can be done without, but makes the whole thing a lot more cumbersome
// and less useful.
#include <stdio.h>

#define my_data_alts  ( void (*A)(int) , void (*B)(int, int) , void (*C)(char, int) )
#define make_my_data(n) void n my_data_alts
typedef void (*my_data) my_data_alts;


void show(my_data match) {
  void A(int n)         { printf("A %d", n);       }
  void B(int a, int b)  { printf("B %d %d", a, b); }
  void C(char a, int b) { printf("C %d %c", a, b); }
  match(A,B,C);
}

int main() {
  make_my_data (foo) { A(1); };
  show (foo);
  return 0;
}