std::ranges::minmax_element,std::ranges::minmax_element_result(3)

NAME
std::ranges::minmax_element,std::ranges::minmax_element_result - std::ranges::minmax_element,std::ranges::minmax_element_result

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::forward_iterator I, std::sentinel_for<I> S, class Proj =
   std::identity,
             std::indirect_strict_weak_order<std::projected<I, Proj>> Comp (1) (since
   = ranges::less > C++20)
   constexpr minmax_element_result<I>
       minmax_element( I first, S last, Comp comp = {}, Proj proj = {} );
   template< ranges::forward_range R, class Proj = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<ranges::iterator_t<R>, Proj>> Comp = (2) (since
   ranges::less > C++20)
   constexpr minmax_element_result<ranges::borrowed_iterator_t<R>>
       minmax_element( R&& r, Comp comp = {}, Proj proj = {} );

Helper types
   template< class I > (3) (since
   using minmax_element_result = ranges::min_max_result<I>; C++20)
   1) Finds the smallest and largest elements in the range [first, last).
   2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as
   first and ranges::end(r) as last.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first, last - iterator-sentinel pair denoting the range to examine
   r - the range to examine
   comp - comparison to apply to the projected elements
   proj - projection to apply to the elements.

Return value
   An object consisting of an iterator to the smallest element as the first element and
   an iterator to the greatest element as the second. Returns {first, first} if the
   range is empty. If several elements are equivalent to the smallest element, the
   iterator to the first such element is returned. If several elements are equivalent
   to the largest element, the iterator to the last such element is returned.

Complexity
   At most std::max(std::floor(1.5 * (N − 1)), 0.0) applications of the comparison and
   twice as many applications of the projection, where N = ranges::distance(first,
   last).

Possible implementation
struct minmax_element_fn {
    template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less>
    constexpr ranges::minmax_element_result<I>
        operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
    {
        auto min = first, max = first;
        if (first == last || ++first == last)
            return {min, max};
        if (std::invoke(comp, std::invoke(proj, *first),
                        std::invoke(proj, *min)))
            min = first;
        else
            max = first;
        while (++first != last)
        {
            auto i = first;
            if (++first == last)
            {
                if (std::invoke(comp, std::invoke(proj, *i),
                        std::invoke(proj, *min)))
                    min = i;
                else if (!(std::invoke(comp, std::invoke(proj, *i),
                        std::invoke(proj, *max))))
                    max = i;
                break;
            }
            else
            {
                if (std::invoke(comp, std::invoke(proj, *first),
                        std::invoke(proj, *i)))
                {
                  if (std::invoke(comp, std::invoke(proj, *first),
                        std::invoke(proj, *min)))
                      min = first;
                  if (!(std::invoke(comp, std::invoke(proj, *i),
                        std::invoke(proj, *max))))
                      max = i;
                }
                else
                {
                    if (std::invoke(comp, std::invoke(proj, *i),
                        std::invoke(proj, *min)))
                        min = i;
                    if (!(std::invoke(comp, std::invoke(proj, *first),
                        std::invoke(proj, *max))))
                        max = first;
                }
            }
        }
        return {min, max};
    }
    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
    constexpr ranges::minmax_element_result<ranges::borrowed_iterator_t<R>>
        operator()(R&& r, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::ref(comp), std::ref(proj));
    } }; inline constexpr minmax_element_fn minmax_element;

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <iterator>
 namespace ranges = std::ranges;
 int main()
 {
     const auto v = {3, 9, 1, 4, 1, 2, 5, 9};
     const auto [min, max] = ranges::minmax_element(v);
     std::cout
         << "min = " << *min << ", at [" << ranges::distance(v.begin(), min) << "]\n"
         << "max = " << *max << ", at [" << ranges::distance(v.begin(), max) << "]\n";
 }

Output:
 min = 1, at [2]
 max = 9, at [7]

See also
   ranges::min_element returns the smallest element in a range
   (C++20) (niebloid)
   ranges::max_element returns the largest element in a range
   (C++20) (niebloid)
   ranges::minmax returns the smaller and larger of two elements
   (C++20) (niebloid)
   minmax_element returns the smallest and the largest elements in a range
   (C++11) (function template)