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

[David Blaikie via llvm-commits]

On Mon, Jan 30, 2017 at 10:00 AM Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

> I thing we should have *something* that does the natural thing,


By 'natural thing' you mean the standard-esque off-by-one 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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> >
>
>
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