temp.c

struct foo {
  int type;
#define TYPE_INT 1
#define TYPE_PAIR_OF_INTS 2
#define TYPE_STRING 3
  union {
    int i;        // If type == TYPE_INT.
    int pair[2];  // If type == TYPE_PAIR_OF_INTS.
    char *str;    // If type == TYPE_STRING.
  } u;
};