typedef Bool(*SPECIAL_FUNC)(KEY key);

typedef struct{
  int a;
  int b;
  SPECIAL_FUNC;
}OBJ;

Bool add(KEY key); //code...
Bool sub(KEY key); //code...

typedef Bool(*SPECIAL_FUNC)(KEY key);

/**
 * struct description
 */
typedef struct{
  int a;
  int b;

  /**
   * Function pointer to one of the following:
   * - add()
   * - sub()
   */
  SPECIAL_FUNC;
}OBJ;

/**
 * @relates OBJ
 * add function.
 */
Bool add(KEY key); //code...

/**
 * @relates OBJ
 * sub function.
 */
Bool sub(KEY key); //code...

#ifdef MY_DOXYGEN_FAKE
  KEY key;
  add(key);
  sub(key);
#endif