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Subject: [boost] [range][iterator] iterators desctuction and proxies (was: Re: [range] Proposal: addition of front(), back(), at(), operator[])
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2014-03-24 08:46:52

Adam Wulkiewicz wrote:
> Hi Valentin,
> Valentin Ziegler wrote:
>> Hi Adam,
>>> return *begin(rng);
>>> For BidirectionalRange
>>> return *(--end(rng));
>> In 99% of all cases above implementations will work just fine.
>> However, there may be rare cases where the lifetime of the reference
>> is bound to the lifetime of the iterator:
>> [iterator.requirements.general] 9. Destruction of an iterator may
>> invalidate pointers and references previously obtained from that
>> iterator.
> Thanks for pointing this one out.
> So yes, iterator_wrapper/reference_proxy should be returned. With
> members: operator casting to reference and for non-mutable Range -
> copy assignment and probably move assignment, probably using
> Boost.Move move emulation.
a remark: of course the assignment operators should be defined for
mutable ranges or non-const iterators.

Is there a reason why move assignment isn't defined in proxies
implemented in Boost.Iterator?

> Btw, do you know the reason for this requirement? I can imagine that
> some iterator could store a temporary created from data gathered
> during the traversal. Or when dereferenced return some wrapper/proxy
> with a pointer to itself or one of its members. But this doesn't
> convince me. It should be a case when some external data or block of
> memory could "dissapear" after the destruction. So there could be a
> container e.g. loading data to the memory on the fly or some external
> memory mapped somehow. In this case the Iterator would behave like a
> shared_ptr<> since we'd be forced to track all Iterators pointing to
> the data which might "dissapear". Still it's too complicated for an
> Iterator. Is there some prosaic reason that I can't see?
> Regards,
> Adam

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