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### 2325. `minmax_element()`'s behavior differing from `max_element()`'s should be noted

Section: 25.8.9 [alg.min.max] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-30

Priority: 3

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Discussion:

25.8.9 [alg.min.max]/23 says that `max_element()` finds the first biggest element, while /25 says that `minmax_element()` finds the last biggest element. This significant difference is unusual — it means that `minmax_element(args)` is not equivalent to `make_pair(min_element(args), max_element(args))`, whereas the other major "two for one" algorithm `equal_range(args)` is equivalent to `make_pair(lower_bound(args), upper_bound(args))`. `minmax_element()`'s behavior is intentional — it is a fundamental consequence of the `3N/2` algorithm — but the Standardese does not draw attention to this in any way. This wording came from LWG 715's resolution (which changed the semantics but didn't mention it), citing CLRS for the algorithm — but CLRS doesn't mention the behavior for equivalent elements! The wording here deeply confused me (as an STL maintainer fixing an incorrect implementation) until I walked through the algorithm by hand and figured out the fundamental reason. It would be really nice for the Standard to provide a hint that something magical is happening here.

[2014-06-06 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3691.

1. Add a footnote to 25.8.9 [alg.min.max]/25 as indicated:

   >     template<class ForwardIterator>
   >       pair<ForwardIterator, ForwardIterator>
   >         minmax_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >       pair<ForwardIterator, ForwardIterator>
   >         minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);

   > > -25- Returns: `make_pair(first, first)` if `[first,last)` is empty, otherwise `make_pair(m, M)`, where `m` is the first iterator in `[first,last)` such that no iterator in the range refers to a smaller element, and where `M` is the last iterator [Footnote: This behavior intentionally differs from `max_element()`.] in `[first,last)` such that no iterator in the range refers to a larger element.