2369. constexpr max(initializer_list) vs max_element

Section: 28.7.8 [alg.min.max] Status: C++17 Submitter: Marc Glisse Opened: 2014-02-21 Last modified: 2017-07-30

Priority: 3

View all other issues in [alg.min.max].

View all issues with C++17 status.

Discussion:

As part of the resolution for LWG issue 2350, max(initializer_list) was marked as constexpr. Looking at two implementations of this function (libstdc++ and libc++), both implement it in terms of max_element, which is not marked as constexpr. This is inconsistent and forces some small amount of code duplication in the implementation. Unless we remove constexpr from this overload of max, I believe we should add constexpr to max_element.

[2015-02 Cologne]

AM: Can we implement this with the C++14 constexpr rules? JM: Yes. AM: Ready? [Yes]

Accepted.

Proposed resolution:

This wording is relative to N3936.

  1. In 28.1 [algorithms.general], header <algorithm> synopsis, and 28.7.8 [alg.min.max], change as indicated (add constexpr to every signature from the first min_element to the second minmax_element)::

    template<class ForwardIterator>
    constexpr ForwardIterator min_element(ForwardIterator first, ForwardIterator last);
    template<class ForwardIterator, class Compare>
    constexpr ForwardIterator min_element(ForwardIterator first, ForwardIterator last,
                                          Compare comp);
    […]
    template<class ForwardIterator>
    constexpr ForwardIterator max_element(ForwardIterator first, ForwardIterator last);
    template<class ForwardIterator, class Compare>
    constexpr ForwardIterator max_element(ForwardIterator first, ForwardIterator last,
                                          Compare comp);
    […]
    template<class ForwardIterator>
    constexpr pair<ForwardIterator, ForwardIterator>
    minmax_element(ForwardIterator first, ForwardIterator last);
    template<class ForwardIterator, class Compare>
    constexpr pair<ForwardIterator, ForwardIterator>
    minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);