lifetime of ranges vs. iterators

Hello,

some adaptor ranges need certain constant information about their underlying ranges. For example,
iterating over a forward "difference_range", the difference between two sorted ranges, requires
access to the ends of the underlying ranges. These two additional iterators are constant, they do not
change when the difference_iterator is incremented.

Now, where should this additional constant information be stored?

a) In the difference_range, and each iterator holds a reference to the difference_range? That requires
the difference_range to be alive as long as its iterators are alive. This may be difficult to ensure if the
difference_range is a temporary.

b) In the iterators, so the lifetime of adaptor iterators is independent of the adaptor range. Of course,
one can use shared_ptrs and so on, but still the iterators are ultimately responsible to keep that
information available.

This initially seemed more natural to me, but if implemented naively by storing the additional iterators
in the iterator itself, it leads to storage bloat: each difference_iterator needs 4 instead of 2 of its
underlying iterators. When stacking difference iterators, the difference in required storage grows
exponentially: instead of 2^N, you need 4^N storage...

I feel that in order to solidify the range concept, one needs to make a choice whether a) or b) is "standard",
because any user of ranges would need to know. Any thoughts on this?

Arno

--
Dr. Arno Schoedl · aschoedl <at> think-cell.com 
Technical Director 

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