#include <stdlib.h>
#include <stdio.h>
#include <string.h>

enum clazz {A, B};
typedef struct PrintI {
  enum clazz id;
  void (*print)(void);
} *PrintI;

void printA(void) { printf("I'm an A\n"); }
void printB(void) { printf("I'm a B\n"); }
PrintI newA(void) {
  PrintI o = malloc(sizeof(struct PrintI));
  o->id = A;
  o->print = printA;
  return o;
}
PrintI newB(void) {
  PrintI o = malloc(sizeof(struct PrintI));
  o->id = B;
  o->print = printB;
  return o;
}

int main(int argc, char *argv[]) {
  PrintI o = NULL;
  if (argc == 2 && strcmp(argv[1], "A") == 0) {
    o = newA();
  }
  else if (argc == 2 && strcmp(argv[1], "B") == 0) {
    o = newB();
  }
  else {
    printf("Usage: ooc A   or   ooc B\n");
    return 0;
  }
  o->print();  // dynamic dispatch!
  return 0;
}