Comment by kazinator

Use of void * for information hiding is a very poor idea because you lose type checking. For example, the differ function in section 1.5 can easily be misused without any diagnostic:

  int x, y, a[3];
  /*...*/
  differ("foo", "bar");
  differ(&x, &y);
  differ(a, a);
  differ(&a, &a);

Do not hide so much information that even the compiler is left in the dark!

To not reveal anything about how an object is implemented, use a pointer to an incomplete struct, like

  typedef struct object object;

  int differ (const object *, const object *);