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Subject: Re: [boost] [range] fat iterators?
From: Neil Groves (neil_at_[hidden])
Date: 2013-11-07 13:29:58

On Thu, Nov 7, 2013 at 5:37 PM, Eric Niebler <eniebler_at_[hidden]> wrote:
> On 11/7/2013 6:27 PM, Nathan Ridge wrote:
>>>> 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?
>>> This is definitely something that needs to be added. The challenge is
>>> merely finding sufficient time to do so.
>> Jeffrey Yaskin posted a candidate design, implemented for filter_iterator,
>> here:
>> What do you think about this design?
> It's a step in the right direction, but it still is essentially an
> iterator_range of filter_iterators. Each filter_iterator is fat; they
> both store a copy of the filter predicate. Rather, filter_range should
> store the predicate, and the iterators returned from filter_range should
> just hold a pointer to the range. That's what I'm suggesting.

I definitely like the idea. I'd like to measure the impact in some
use-cases to determine the impact of the worsened locality-of-data. I
don't believe it is always a performance win. If I get some
performance numbers I think we can combine the approaches optimally. I
would have thought that we should, under some scenarios, be using EBO
for the filter predicate since they are often stateless.

> And to answer Neil's objection, it necessarily means the iterators can't
> outlive the range, but IMO it's wrong to assume otherwise. Nobody
> expects a vector iterator to outlive the vector, after all. It will most
> certainly be a breaking change, which is why I suggest we ship a C++11
> range library in parallel with the existing C++98 one.

I'm not objecting. I'm concerned that using iterators from a range
after a range has died is a significant design change. It isn't at all
silly or an unusual use-case. It is commonly required when using
ranges in your own code and interfacing to a library that only uses
iterators. With the new requirements I would no longer be able to use
ranges at all if I wanted to continue using the library. That's a big
drawback. We can provide a new interface and define that as "wrong"
but it most certainly isn't wrong at the moment. It's useful. However
I'm not objecting. I'm pointing out that there are some significant
drawbacks to consider. I think we can put a new interface together and
perhaps this is the correct trade-off for a new interface. From my
perspective the reduction in interoperability with iterator-based
libraries is a significant drawback. I wonder if we can be clever and
solve this with something cunning?

A vector not outliving it's iterators has always been a requirement.
This requirement had no practical disadvantage to interoperability on
common use-cases. This is not true for range lifetime. The range has
often had a shorter lifetime than the container in many practical
use-cases. It has always been a disposable wrapper. Adding a
dependency from iterator to range has significant implications. I
think the idea is worthy of exploration. It's just not completely
obvious to me that the initial proposal is obviously exactly the way
forward. I think this proposal is a move toward making the ranges
primitives in themselves and I wonder if we should be thinking
slightly more broadly. In particular Andrei's work might be a source
of inspiration.

> --
> Eric Niebler

I'm attempting to stimulate better answers rather than stifle change.
I hope that comes across. Your input has been really useful.

Neil Groves

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