Re: [boost] difference_type size()?

Subject: Re: [boost] difference_type size()?
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2011-10-19 15:31:58

On Wed, Oct 19, 2011 at 12:21 PM, Olaf van der Spek <ml_at_[hidden]> wrote:

> On Wed, Oct 19, 2011 at 8:50 PM, Jeffrey Lee Hellrung, Jr.
> <jeffrey.hellrung_at_[hidden]> wrote:
> >> I too stumbled upon this more than once. The common understanding of a
> >> valid
> >> range is that end() must be reachable by incrementing begin() finite
> times.
> >> Making the size signed makes little sense to me and it also introduces a
> >> problem (rather theoretical, however) when iterator_range is constructed
> >> from
> >> a very large valid range, whose size exceeds
> >> numeric_limits<difference_type>::max().
> >>
> >
> > How would that be possible?  Doesn't that violate the conditions on
> > difference_type that it is able to represent the (signed) distance
> between
> > any two iterators within the same range?
> >
> > Also, assuming that size() should return an unsigned integer type (I'm
> not
> > debating the merits of this), *what* unsigned integer type should that
> be?
>
> How about using make_unsigned from type_traits?
>

See my reply to Andrey.  Note that I'm not confident that make_unsigned
*wouldn't* work, necessarily.

> I don't know of any general way to go from difference_type to a
> "compatible"
> > unsigned integer type.  I suspect this will end up being filed away as
> "nice
> > to have but ultimately unimplementable generically" given current range
> > interfaces.
>
> Isn't this problem caused by iterator_traits missing size_type?
>

Yes.  Changing iterator_traits (specifically, the collection of types
associated with an iterator) is where I'm leaning, if you really want an
unsigned iterator_range::size().

> What would the problem be of just using std::size_t?
>

Well, likewise, what would be the problem of always using std::ptrdiff_t for
difference_type?  And, aside from whether std::size_t would be wide enough,
I don't think there are any requirements that std::size_t be constructable
from difference_type.

- Jeff