[boost] Re: iterator_adaptor distance question

From: Neal D. Becker (ndbecker2_at_[hidden])
Date: 2004-04-12 15:27:12

David Abrahams wrote:

> "Neal D. Becker" <ndbecker2_at_[hidden]> writes:
> 
>> Is the preference for the former case over the latter intentional?
> 
> Not particularly; it just worked out that way.  I guess in the
> interest of efficiency it'd be better to take the one that doesn't
> involve a unary minus.
> 
>> This
>> caused some surprise when I tried to re-implement my
>> cycle_iterator_adapator (a kind of circular buffer adaptor).
>>
>> std::copy (b, a, ...)
>> will do:
>>   template<typename _RandomAccessIter, typename _OutputIter>
>>     inline _OutputIter
>>     __copy(_RandomAccessIter __first, _RandomAccessIter __last,
>>            _OutputIter __result,
>>            random_access_iterator_tag)
>>     {
>>       typedef typename
>>       iterator_traits<_RandomAccessIter>::difference_type
>>           _Distance;
>>       for (_Distance __n = __last - __first; __n > 0; --__n) {
>>         *__result = *__first;
>>         ++__first;
>>         ++__result;
>>       }
>>       return __result;
>>     }
>>
>> As I implemented cycle_iterator_adaptor distance_to, it was expecting
>> that given iterators [a,b], you probably want a positive distance from a
>> to b,
> 
> Correct, if b is reachable by applying increments to a.
> 
>> but the way iterator_adaptors is implemented, it wants a negative
>> distance.
> 
> Huh?  What do you mean "it wants"?
> 

Sorry for the confusion.  I'm implementing a kind of circular buffer.  You
can reach from a->b or b->a with a positive increment.  If the user asks to
std::copy (a, b, c), it winds up calling 
      template <class Facade1, class Facade2>
      static typename Facade1::difference_type distance_from(
          Facade1 const& f1, Facade2 const& f2, mpl::true_)
      {
          return -f1.distance_to(f2);
      }

So my distance_to has to be reversed:

    difference_type
    distance_to(circle_iterator<BaseIterator> const& y) const {
      difference_type d = y.base_reference()-base_reference();
      if (d < 0)
        return d;
      else
        return d - allocated;
    }
      
The returned value from distance_to has to be negative, that's what I meant
by "it wants a negative distance", because it will then invert the sign.