Re: [boost] [Range] Range adaptor approach for temporary range lifetime issue

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