In article <FENSTER.95Apr3173706@ground.cs.columbia.edu>,
Sam Fenster <fenster@ground.cs.columbia.edu> wrote:
> You compare an iterator against an end iterator when it's
> traversing/searching a pre-existing range.  Pre-existing ranges *have*
> end iterators.  When someone hands you an iterator that helps *define* a
> range, it's serving a different purpose.  There may be no other range in
> sight to compare it to.  Then you need Null as a standard way to signal
> invalidity.

A range is a *minimal* collection.  If what you want requires more
semantics than the minimumn, it's easy enough to provide it:

  template <class Iterator>
    class MaybeBadRange {
      Iterator first;
      Iterator last;
      bool isgood;
     public:
      MaybeBadRange(Iterator f, Iterator e, bool ok);
      bool is_ok() { return isgood; }
      // plus all the standard collection members
    };

There you go, no charge. :-)
That's the whole point: you get what you want at amazingly little
expense, and those of us not interested don't pay.

Nathan Myers