Subject: Re: [boost] [range] Proposal: addition of front(), back(), at(), operator
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2014-03-24 08:38:51
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
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?
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