Re: boost::date_time : iterator model

From: klapshin klapshin_at_[hidden])
Date: 2002-12-26 21:33:48

--- In Boost-Users_at_[hidden], "Jeff Garland" <jeff_at_c...> wrote:
> > annoying problem I encountered is that date iterators conform to InputIterator model, 
> > rather then Random Access Iterator.
> 
> The reason for this is that it is unclear in all cases that an operation is 
> easily reversed.  In particular, the problem comes about with handling the 
> end of the month.  Say for example you start an iterator on Jan 30.  When you 
> iterate to February what happens?  Well the rule the supplied iterator is to 
> back up to the end of the February -- either the 27 or 28th for a leap year.  
Yes, I understand these complications. I was quite surprised, btw,
that starting month_iterator from Jan,30 and iterating twice yields
Mar,30, rather then Mar,28. So iterator appears to be smarter then
I expected, which is great. :)

> Given that these rules are complex and at least possibly non-reversible I 
> tried to limit the exposure of the iterator: hence input iterator.   In 
> retrospect, however, it could probably be RandomAccess because I can't 
> give you an actual case where making the iterator reverse is an actual
> issue.
I can live with InputIterator, but having RandomAccess (or, at least,
Bidirectional) will simplify life significantly. Is there any chance
you will consider this feature request for future development?

> If you want to use the iterator you can simply do the following:
> 
> month_iterator mi(d, 3); // configure iterator to add 3 months at a time.
> mi++; //adds 3 months.
This is nice, thanks for pointing out this feature.

> If you don't want to use the iterator there are a couple of 
> possible solutions.  One is to do the following:
>   int N = 3; //add 3 months
>   date d(...);// constructed however
>   date d1(d.year(), d.month()+N, d.day()); 
> Well, except that this doesn't handle year wrap arounds and 
> other the issues associated with the end of the month as described
> above.  So you have to write some supporting code to handle
That is exactly what I did as a temporary solution. I hope though,
that someday you will create more powerful iterator or some other
means to accomplish this task in simpler and cleaner way.

So bottom line for this problem is: I was wrong talking about
inefficiency -- there is a way to move few month at a time. The real
problem though that I can't move iterator backward.

One more thing: it is quite inconvinient that it is prohibited to
create a vector of dates. Do you have plans to make it vector
compliant?
Default constructor could initialize it to not_a_date for instance.

Another little issue: there are few typos on the boost web site.
Example "Print a month" looks garbled (probably transformation from
source code to html failed).
For instance:

   int
   std::cin >> year;

and

   int eom_day = gregorian_calendar::end_of_month_day(
   date endOfMonth(year,month,eom_day);

look odd.
Don't know about other examples; did not have chance to study them
carefully.
Also under "Build & Compiler information" there is a typo:
  /boost/date_time/gregoran           -- gregorian calendar system header files

should be
  /boost/date_time/gregorIan

Best regards,
Kirill Lapshin.