[PATCH] D27874: ilist_iterator: Allow conversion between reverse and forward iterators

[Duncan P. N. Exon Smith via llvm-commits]

I thing we should have *something* that does the natural thing, and I'm extremely hesitant to change the semantics of getReverse() since that was extremely error prone last time.  Here's another possibility:
- Rename getReverse() to some-great-new-name-that-is-clear.
- Add conversions between forward/reverse do the same thing as std::reverse_iterator, with the off-by-1 thing.

Any suggestions for something that's clear?

> On 2017-Jan-30, at 09:49, David Blaikie <dblaikie at gmail.com> wrote:
> 
> Maybe - I'm not sure the explicit ctor is sufficiently explicit from a self-documenting code perspective:
> 
> reverse_iterator R(I);
> reverse_iterator R = I.getReverse();
> 
> I would be a bit surprised that those two things had different semantics.
> 
> On Mon, Jan 30, 2017 at 9:47 AM Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> I think adding:
> --
> explicit iterator(reverse_iterator I) : I(++I.getReverse().base()) {}
> explicit reverse_iterator(iterator I) : I(++I.getReverse().base()) {}
> --
> should fix the problem.
> - getReverse() keeps the current semantics of getting the reverse of whatever you're currently referencing.
> - conversions between forward/reverse do the same thing as std::reverse_iterator, with the off-by-1 thing.
> 
> 
> 
> > On 2017-Jan-30, at 09:43, David Blaikie <dblaikie at gmail.com> wrote:
> >
> > Yeah, I think it might be too subtle to have these reverse iterators behave differently from other reverse iterators. (I haven't checked the spec in detail to see if it's only stD::reverse_iterator that has this oddity, or if all reverse iterators must do so) 'getReverse' seems like a good/clear solution, if a little odd but at least visibly so.
> >
> > On Wed, Jan 25, 2017 at 3:33 PM Matthias Braun via Phabricator via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> > MatzeB abandoned this revision.
> > MatzeB added a comment.
> >
> > I wasn't aware of the subtle shift-by-1 semantics when converting to std::reverse_iterator (must have missed the getReverse() comment). If we go with that semantics I would need extra std::next/std::prev calls in my code anyway which doesn't make it much simpler compared to MyIterator->getReverseIterator(). Going for semantics different from STL is not a good idea IMO.
> >
> >
> > Repository:
> >   rL LLVM
> >
> > https://reviews.llvm.org/D27874
> >
> >
> >
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