Jeffrey Lee Hellrung, Jr. wrote:
What if we introduced a metafunction that dictated what reverse_range<R> should store its adapted range by, either R& (most of the time) or R (in the case that the adapted range is a directly or indirectly a moved_range/by_value_range).
Do you mean reverse_range<R> stores reverse_forwarder and R for the latter case?
Yes (I think so).
Then, compared to the current implementation of reverse_range, the implementations of your reverse_range and my moved_range are essentially the same, right?
Now I'm curious: what are the advantages and disadvantages of implementing reverse_range<R> as a pair of reverse_iterator< R::iterator >'s (I'm being sloppy here, but based on your above assertion, this is the present implementation) versus as a wrapper around an R (held by reference or value) directly? In the latter case, for example, reverse_range<R>::begin would return reverse_iterator< R::iterator >(boost::end(r)) (where r is the wrapped range of type R).
Below, I say boost::begin(r) and boost::end(r) as the underlying iterators. Your range adaptors are "lazy adaptors": * Pipe operators does not adapt the underlying iterators in effect. * The underlying iterators are adapted only when begin/end is called. And each time begin/end of your range adaptors is called, the underlying iterators needs to be adapted. But, in some range adaptors, adapting the underlying iterators is a bit expensive. For example * In filtered_range, adapting the begin iterator can be expensive, since it needs to advance the begin iterator to the first "unfiltered" iterator. * In oven's dropped_range (this ignores the first n elements in the range) on bidirectional range, adapting the begin iterator takes O(n) time. So your range adaptors are inefficient in these cases. (moved_range internally caches the adapted iterators to avoid this problem.) Regards, Michel