On 24/06/12 10:51, Michel Morin wrote:
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.
As a general idiom the use of lazily adapting upon the invocation of begin/end would mix two responsibilities. If one considers the complications involved with managing functor and predicate state being delayed until the invocation of begin/end it appears to be a considerably more complex solution. I do not perceive a compensating advantage for this approach. Of course, I may well be missing the advantage and invite correction.
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.)
Exactly so. FWIW I like the proposed solution but strongly prefer the explicit move. My reasoning is that I prefer to apply the zero overhead principle, and that historically choosing implicit has been associated with more design errors. Implicit moving while superficially appealing doesn't seem like a good idea intuitively to me. I am, of course, assuming that there isn't a zero-cost implicit move solution that is completely general.
Regards, Michel
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Neil Groves