Re: [boost] [range] Is it conceivable to have it extended?

Subject: Re: [boost] [range] Is it conceivable to have it extended?
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-07-13 20:00:28

On Monday 14 July 2014 07:10:23 Vladimir Batov wrote:
> boost::iterator_range is a great concept/class but it does not seem to
> play efficiently with certain input sequences and seem to be too strong
> a requirement for certain algorithms. I have an old-fashioned C string
> in mind (I am sure there are other examples) and traversing algorithms.
> 
> When I tried deploying boost::iterator_range for string-to-int
> conversion purposes, I quickly realized that I was traversing the string
> twice -- first to find the end and second to do the work.

Not sure why would you need to find the end - it can be found in the parsing 
process (that is assuming the integer may not be the whole string; otherwise 
the end iterator is already provided by the range).

> What I think is missing is the "parent" concept of a "sequence". Say,
> 
> template<iterator_type, sentry_type>
> struct sequence
> {
>      iterator_type begin();
>      sentry_type sentry();
> };
> 
> Then "range" would implement and refine the "sequence" concept  by
> 
> template<iterator_type>
> struct range : sequence<iterator_type, iterator_type>
> {
>      iterator_type begin();
>      iterator_type end();
>      sentry_type sentry() { return end; }
> }
> 
> I.e., as "range" implements/is the "sequence", the boost::iterator_range
> will have one additional "useless" sentry() method.
> 
> That way all the old code (using and/or needing end()) would still work
> but traversing algorithms might gradually adapt to only require
> "sequences" instead of "ranges":
> 
> template<typename Iterator, typename Sentry, typename Function>
> Function
> for_each(Iterator beg, Sentry sentry, Function fun)
> {
>      for (; beg != sentry; ++beg) fun(*beg);
>      return fun;
> }
> 
> That means that now all "open-ended" (with no known "end") "ranges"
> (they are not even "ranges" in the strict meaning of the word but rather
> "sequences") can be handled efficiently.

I don't see much difference with the current ranges, where end iterators are 
singular. Can you provide a strong motivating example for the sentry() method?