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Subject: Re: [boost] [range] fat iterators?
From: TONGARI J (tongari95_at_[hidden])
Date: 2013-11-07 22:59:38

2013/11/7 Eric Niebler <eniebler_at_[hidden]>

> I wrote an article describing the "fat" std::istream_iterator and how it
> can be slimmed down by giving it an owning istream_range[1]. I see there
> already is an istream_range in Boost.Range, but it suffers from the
> fat-iterator problem I describe there. Is there a compelling reason why
> the implementation of boost::istream_range shouldn't be changed to be
> more like the one I describe? Also, as a commented pointed out, the same
> problem exists for the filtered range and the transformed range, too.
> The trick, of course, will be keeping intermediate temporary ranges
> alive long enough to avoid lifetime issues when we chain adaptors and
> assign the result to a local variable. I think a range library that's
> sensitive to the value category of range objects and makes copies of
> rvalue ranges would solve this problem. Thoughts?
> Is it time for Boost.Range 3.0?
> [1]:

That reminds me of the question I asked years ago (no one answered):

Surely all approaches have tradeoffs...

About the lifetime problem, could it be solved(?) with the approach Fusion
used? (i.e. the distinction between 'view' & 'sequence').

I'm willing to have a reusable range, rather than a disposable one.
The problem of a disposable one also described in my question.

And I wonder, if we could have 2-phase construction.
phase 1: A lazy range builder w/o begin/end
phase 2: A range w/ begin/end and probably do some work at construction, as
Eric suggested.

And there have to be a trait like:

template<class Range>
struct range_type
    typedef Range& type;

And specialized for those have 2 phases.

To implement the algorithms:

template<class Range>
void some_algo(Range const& rng, ...)
    typename range_type<Range const>::type r(rng);
    some_algo(begin(r), end(r), ...);

For range adaptors, they also store phase 1 object, and make the underlying
phase 2 object on the fly.

What do you think?

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