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Section: 28.5.8 [alg.adjacent.find] Status: NAD Concepts Submitter: Chris Jefferson Opened: 2009-03-09 Last modified: 2017-02-03
Priority: Not Prioritized
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Addresses UK 296
adjacent_find in C++03 allows an arbitrary predicate, but in C++0x EqualityComparable/EquivalenceRelation is required. This forbids a number of use cases, including:
adjacent_find(begin, end, less<double>) Find the first place where a range is not ordered in decreasing order - in use to check for sorted ranges. adjacent_find(begin, end, DistanceBiggerThan(6) ) ) Find the first place in a range where values differ by more than a given value - in use to check an algorithm which produces points in space does not generate points too far apart.
A number of books use predicate which are not equivalence relations in examples, including "Thinking in C++" and "C++ Primer".
Adding the requirement that the predicate is an EquivalenceRelation does not appear to open up any possibility for a more optimised algorithm.
Change the definition of adjacent_find in the synopsis of 28 [algorithms] and 28.5.8 [alg.adjacent.find] to:
template<ForwardIterator Iter> requires
EqualityComparable<Iter::value_type> Iter adjacent_find(Iter first, Iter last); template<ForwardIterator Iter, EquivalenceRelation<auto, Iter::value_type> Pred> requires CopyConstructible<Pred> Iter adjacent_find(Iter first, Iter last, Pred pred);