On Thu, 22 May 2025 at 16:48, Jonathan Wakely <[email protected]> wrote:
>
> On Thu, 22 May 2025 at 16:44, Jonathan Wakely <[email protected]> wrote:
> >
> > On Thu, 22 May 2025 at 16:25, Tomasz Kaminski <[email protected]> wrote:
> > >
> > >
> > >
> > > On Thu, May 22, 2025 at 5:15 PM Tomasz Kaminski <[email protected]>
> > > wrote:
> > >>
> > >>
> > >>
> > >> On Thu, May 22, 2025 at 5:04 PM Jonathan Wakely <[email protected]>
> > >> wrote:
> > >>>
> > >>> On Thu, 22 May 2025 at 15:50, Tomasz Kaminski <[email protected]>
> > >>> wrote:
> > >>> >
> > >>> >
> > >>> >
> > >>> > On Thu, May 22, 2025 at 1:42 PM Jonathan Wakely <[email protected]>
> > >>> > wrote:
> > >>> >>
> > >>> >> The current overload set for __unique_copy handles three cases:
> > >>> >>
> > >>> >> - The input range uses forward iterators, the output range does not.
> > >>> >> This is the simplest case, and can just compare adjacent elements
> > >>> >> of
> > >>> >> the input range.
> > >>> >>
> > >>> >> - Neither the input range nor output range use forward iterators.
> > >>> >> This requires a local variable copied from the input range and
> > >>> >> updated
> > >>> >> by assigning each element to the local variable.
> > >>> >>
> > >>> >> - The output range uses forward iterators.
> > >>> >> For this case we compare the current element from the input range
> > >>> >> with
> > >>> >> the element just written to the output range.
> > >>> >>
> > >>> >> There are two problems with this implementation. Firstly, the third
> > >>> >> case
> > >>> >> assumes that the value type of the output range can be compared to
> > >>> >> the
> > >>> >> value type of the input range, which might not be possible at all, or
> > >>> >> might be possible but give different results to comparing elements of
> > >>> >> the input range. This is the problem identified in LWG 2439.
> > >>> >>
> > >>> >> Secondly, the third case is used when both ranges use forward
> > >>> >> iterators,
> > >>> >> even though the first case could (and should) be used. This means
> > >>> >> that
> > >>> >> we compare elements from the output range instead of the input range,
> > >>> >> with the problems described above (either not well-formed, or might
> > >>> >> give
> > >>> >> the wrong results).
> > >>> >>
> > >>> >> The cause of the second problem is that the overload for the first
> > >>> >> case
> > >>> >> looks like:
> > >>> >>
> > >>> >> OutputIterator
> > >>> >> __unique_copy(ForwardIter, ForwardIter, OutputIterator, BinaryPred,
> > >>> >> forward_iterator_tag, output_iterator_tag);
> > >>> >>
> > >>> >> When the output range uses forward iterators this overload cannot be
> > >>> >> used, because forward_iterator_tag does not inherit from
> > >>> >> output_iterator_tag, so is not convertible to it.
> > >>> >>
> > >>> >> To fix these problems we need to implement the resolution of LWG
> > >>> >> 2439 so
> > >>> >> that the third case is only used when the value types of the two
> > >>> >> ranges
> > >>> >> are the same. This ensures that the comparisons are well behaved. We
> > >>> >> also need to ensure that the first case is used when both ranges use
> > >>> >> forward iterators.
> > >>> >>
> > >>> >> This change replaces a single step of tag dispatching to choose
> > >>> >> between
> > >>> >> three overloads with two step of tag dispatching, choosing between
> > >>> >> two
> > >>> >> overloads at each step. The first step dispatches based on the
> > >>> >> iterator
> > >>> >> category of the input range, ignoring the category of the output
> > >>> >> range.
> > >>> >> The second step only happens when the input range uses non-forward
> > >>> >> iterators, and dispatches based on the category of the output range
> > >>> >> and
> > >>> >> whether the value type of the two ranges is the same. So now the
> > >>> >> cases
> > >>> >> that are handled are:
> > >>> >>
> > >>> >> - The input range uses forward iterators.
> > >>> >> - The output range uses non-forward iterators or a different value
> > >>> >> type.
> > >>> >> - The output range uses forward iterators and has the same value
> > >>> >> type.
> > >>> >>
> > >>> >> For the second case, the old code used
> > >>> >> __gnu_cxx::__ops::__iter_comp_val
> > >>> >> to wrap the predicate in another level of indirection. That seems
> > >>> >> unnecessary, as we can just use a pointer to the local variable
> > >>> >> instead
> > >>> >> of an iterator referring to it.
> > >>> >>
> > >>> >> libstdc++-v3/ChangeLog:
> > >>> >>
> > >>> >> PR libstdc++/120386
> > >>> >> * include/bits/stl_algo.h (__unique_copy_1): New overloads
> > >>> >> for
> > >>> >> the case where the input range uses non-forward iterators.
> > >>> >> (__unique_copy): Replace three overloads with two, depending
> > >>> >> only on the iterator category of the input range. Dispatch to
> > >>> >> __unique_copy_1 for the non-forward case.
> > >>> >> (unique_copy): Only pass the input range category to
> > >>> >> __unique_copy.
> > >>> >> ---
> > >>> >>
> > >>> >> Tested x86_64-linux.
> > >>> >
> > >>> > LGTM. Only small suggestion, regarding the change of order of
> > >>> > arguments.
> > >>>
> > >>> I forgot to say that I need to add tests for each of the cases,
> > >>> especially the case that fails with the existing code!
> > >>>
> > >>> >>
> > >>> >>
> > >>> >> libstdc++-v3/include/bits/stl_algo.h | 80
> > >>> >> +++++++++++++++-------------
> > >>> >> 1 file changed, 44 insertions(+), 36 deletions(-)
> > >>> >>
> > >>> >> diff --git a/libstdc++-v3/include/bits/stl_algo.h
> > >>> >> b/libstdc++-v3/include/bits/stl_algo.h
> > >>> >> index f5361aeab7e2..c0bb17f9c8b2 100644
> > >>> >> --- a/libstdc++-v3/include/bits/stl_algo.h
> > >>> >> +++ b/libstdc++-v3/include/bits/stl_algo.h
> > >>> >> @@ -918,24 +918,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >>> >>
> > >>> >> __gnu_cxx::__ops::__iter_comp_iter(__binary_pred));
> > >>> >> }
> > >>> >>
> > >>> >> - /**
> > >>> >> - * This is an uglified
> > >>> >> - * unique_copy(_InputIterator, _InputIterator, _OutputIterator,
> > >>> >> - * _BinaryPredicate)
> > >>> >> - * overloaded for forward iterators and output iterator as
> > >>> >> result.
> > >>> >> - */
> > >>> >> + // Implementation of std::unique_copy for forward iterators.
> > >>> >> + // This case is easy, just compare *i with *(i-1).
> > >>> >> template<typename _ForwardIterator, typename _OutputIterator,
> > >>> >> typename _BinaryPredicate>
> > >>> >> _GLIBCXX20_CONSTEXPR
> > >>> >> _OutputIterator
> > >>> >> __unique_copy(_ForwardIterator __first, _ForwardIterator __last,
> > >>> >> _OutputIterator __result, _BinaryPredicate
> > >>> >> __binary_pred,
> > >>> >> - forward_iterator_tag, output_iterator_tag)
> > >>> >> + forward_iterator_tag)
> > >>> >> {
> > >>> >> _ForwardIterator __next = __first;
> > >>> >> *__result = *__first;
> > >>> >> while (++__next != __last)
> > >>> >> - if (!__binary_pred(__first, __next))
> > >>> >> + if (!__binary_pred(__next, __first))
> > >>> >
> > >>> > I would prefer if you will not do this change, and pass iterators
> > >>> > that were already seen as the first argument.
> > >>>
> > >>> The standard seems clear that it should be bool(pred(*i, *(i - 1)))
> > >>> In theory a predicate could depend on that.
> > >>
> > >> Oh, indeed.
> > >>>
> > >>>
> > >>> > Note that the forward-output overload, preserves this order:
> > >>> > *__result = *__first;
> > >>> > while (++__first != __last)
> > >>> > if (!__binary_pred(__result, __first))
> > >>>
> > >>> Ah yes, well I should have changed that too ;-)
> > >>>
> > >>> What's your reason for preferring the current order?
> > >>
> > >> That what I would intuitively expect, that left argument is an element
> > >> that is left to right argument.
> > >> And if you sorted range with predicate lt, then passing not_fn(lt) is
> > >> equivalent to checking equality.
> > >
> > > It seems that I have built my expectation based on unique, that check
> > > i-1, i.
> > > https://eel.is/c++draft/alg.unique#1
> >
> > The standard seems clear, however, all three of libstdc++, libc++ and
> > MSVC use the "wrong" order in unique_copy.
> >
> > So I think the standard should be changed.
>
> The docs for the SGI STL matched the standard:
> https://web.archive.org/web/20161012091520/http://www.sgi.com/tech/stl/unique_copy.html
>
> But the actual SGI STL implementation (which libstdc++ is based on)
> does not match its own docs:
> https://web.archive.org/web/20170429054511/http://www.sgi.com/tech/stl/stl_algo.h
>
> My copy of "The C++ Standard Template Library" by Plauger, Stepanov,
> Lee and Musser is in a box somewhere in storage, but what it describes
> probably matches the MSVC STL.
Our ranges::unique_copy is inconsistent, but I think we should do this
to make it consistently "wrong":
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -1250,8 +1250,8 @@ namespace ranges
while (++__first != __last)
{
if (!(bool)std::__invoke(__comp,
- std::__invoke(__proj, *__first),
- std::__invoke(__proj, __value)))
+ std::__invoke(__proj, __value)),
+ std::__invoke(__proj, *__first))
{
__value = *__first;
*++__result = __value;
MSVC ranges::unique_copy is "wrong", and libc++ ranges::unique_copy
uses the same impl as their std::unique_copy so is "wrong".
range-v3 is "wrong":
https://github.com/ericniebler/range-v3/blob/ca1388fb9da8e69314dda222dc7b139ca84e092f/include/range/v3/algorithm/unique_copy.hpp#L46
But cmcstl2 is "right", i.e. matches the standard
https://github.com/CaseyCarter/cmcstl2/blob/a7a714a9159b08adeb00a193e77b782846b3b20e/include/stl2/detail/algorithm/unique_copy.hpp#L30
