[boost] Re: iterator_adaptor distance question

From: David Abrahams (dave_at_[hidden])
Date: 2004-04-12 16:31:24

"Neal D. Becker" <ndbecker2_at_[hidden]> writes:

> 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.  

Whoa there.  A circular buffer must always have one invalid element.
Reaching a->b->a with positive increments should be impossible, or you
have a bug in your implementation.  At least, that was true the last
time I thought about it.  Otherwise, how can you tell the difference
between empty and full?

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com