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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

**View all other** issues in [alg.adjacent.find].

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**Discussion:**

**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.

**Proposed resolution:**

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~~HasEqualTo<Iter::value_type, Iter::value_type> Iter adjacent_find(Iter first, Iter last); template<ForwardIterator Iter,~~EquivalenceRelation~~Predicate<auto, Iter::value_type, Iter::value_type> Pred> requires CopyConstructible<Pred> Iter adjacent_find(Iter first, Iter last, Pred pred);