This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++11 status.

### 1241. `unique_copy` needs to require `EquivalenceRelation`

Section: 25.7.9 [alg.unique] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-17 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [alg.unique].

View all issues with C++11 status.

Discussion:

A lot of fixes were silently applied during concept-time and we should not lose them again. The Requires clause of 25.7.9 [alg.unique]/5 doesn't mention that `==` and the predicate need to satisfy an `EquivalenceRelation`, as it is correctly said for `unique`. This was intentionally fixed during conceptification, were we had:

>     template<InputIterator InIter, class OutIter>
>       requires OutputIterator<OutIter, RvalueOf<InIter::value_type>::type>
>             && EqualityComparable<InIter::value_type>
>             && HasAssign<InIter::value_type, InIter::reference>
>             && Constructible<InIter::value_type, InIter::reference>
>       OutIter unique_copy(InIter first, InIter last, OutIter result);

>     template<InputIterator InIter, class OutIter,
>              EquivalenceRelation<auto, InIter::value_type> Pred>
>       requires OutputIterator<OutIter, RvalueOf<InIter::value_type>::type>
>             && HasAssign<InIter::value_type, InIter::reference>
>             && Constructible<InIter::value_type, InIter::reference>
>             && CopyConstructible<Pred>
>       OutIter unique_copy(InIter first, InIter last, OutIter result, Pred pred);

Note that EqualityComparable implied an equivalence relation.

[ N.B. `adjacent_find` was also specified to require `EquivalenceRelation`, but that was considered as a defect in concepts, see 1000 ]

[ 2009-10-31 Howard adds: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Change 25.7.9 [alg.unique]/5 as indicated:

>     template<class InputIterator, class OutputIterator>
>       OutputIterator
>         unique_copy(InputIterator first, InputIterator last, OutputIterator result);

>     template<class InputIterator, class OutputIterator, class BinaryPredicate>
>       OutputIterator
>         unique_copy(InputIterator first, InputIterator last,
>                     OutputIterator result, BinaryPredicate pred);

> > Requires: The comparison function shall be an equivalence relation. The ranges `[first,last)` and `[result,result+(last-first))` shall not overlap. The expression `*result = *first` shall be valid. If neither `InputIterator` nor `OutputIterator` meets the requirements of forward iterator then the value type of `InputIterator` shall be `CopyConstructible` (34) and `CopyAssignable` (table 36). Otherwise `CopyConstructible` is not required.