Subject: Re: [boost] [Range] Range adaptor approach for temporary range lifetime issue
From: Michel Morin (mimomorin_at_[hidden])
Date: 2012-06-26 20:12:15
Jeffrey Lee Hellrung, Jr. wrote:
> My rebuttal is that:
[...]
> (b) Range adaptors can be free to cache begin and end iterators (in
> addition to the adapted range), if that would yield noticeably better
> performance for more than one begin/end calls (e.g., filtered_range and
> dropped_range), but I suspect for most range adaptors, it doesn't make a
> difference. Take, for example, a transformed_range. If it's implemented as
> a pair of transform_iterators, calling begin/end requires copying the
> underlying iterators in addition to the transform function. If
> transformed_range is implemented as the underlying range paired with a
> function, calling begin/end requires generating the underlying begin/end
> iterators and still copying the transformation function. If copying the
> underlying iterators is no faster than generating the begin/end iterators
> from the underlying range, there should be no difference in performance
> between the two implementations of transformed_range. Of course, there's a
> big assumption on the relative speed of begin/end versus iterator copying
> of the underlying range, but I think it's, again, the common case; it's
> something we can aim to guarantee for all provided range adaptors; and if
> it's a client range that fails this assumption, we can provide a facility
> (oh yeah, that's what iterator_range is for) to explicitly address this.
[...]
I don't know what the consequence of the following fact is,
but it's worth noting:
Repeatedly adapted ranges might have **very** large size.
For example, on Mac OS X 10.7 with gcc-4.7,
std::vector<int> v;
sizeof(v | uniqued) --> 32
sizeof(v | uniqued | uniqued) --> 80
sizeof(v | uniqued | uniqued | uniqued) --> 176
sizeof(v | uniqued | uniqued | uniqued | uniqued) --> 368
// OvenToBoost's taken adaptor
sizeof(v | taken(1)) --> 48
sizeof(v | taken(1) | taken(1)) --> 112
sizeof(v | taken(1) | taken(1) | taken(1)) --> 240
sizeof(v | taken(1) | taken(1) | taken(1) | taken(1)) --> 496
sizeof(v | strided(1)) --> 80
sizeof(v | strided(1) | strided(1)) --> 272
sizeof(v | strided(1) | strided(1) | strided(1)) --> 848
sizeof(v | strided(1) | strided(1) | strided(1) | strided(1)) --> 2576
Regards,
Michel