std::ranges::max_element(3)

NAME
std::ranges::max_element - std::ranges::max_element

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 I max_element( I first, S last, Comp comp = {}, Proj proj = {}
   );
   template< ranges::forward_range R, class Proj = std::identity,
   std::indirect_strict_weak_order< (since
   std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less > (2) C++20)
   constexpr ranges::borrowed_iterator_t<R>
   max_element( R&& r, Comp comp = {}, Proj proj = {} );
   1) Finds the greatest element 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 may not be specified when calling any of them.
     * None of them is visible to argument-dependent lookup.
     * When one of them is found by normal unqualified lookup for the name to the left
       of the function-call operator, it inhibits argument-dependent lookup.
   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
   Iterator to the greatest element in the range [first, last). If several elements in
   the range are equivalent to the greatest element, returns the iterator to the first
   such element. Returns first if the range is empty.

Complexity
   Exactly max(N-1,0) comparisons, where N = ranges::distance(first, last).

Possible implementation
  struct max_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 I operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
    {
      if (first == last) {
        return last;
      }
      auto largest = first;
      ++first;
      for (; first != last; ++first) {
          if (std::invoke(comp, std::invoke(proj, *largest), std::invoke(proj, *first))) {
              largest = first;
          }
      }
      return largest;
    }
    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::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 max_element_fn max_element;

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <vector>
 #include <cmath>
 int main()
 {
     std::vector<int> v{ 3, 1, -14, 1, 5, 9 };
     namespace ranges = std::ranges;
     auto result = ranges::max_element(v.begin(), v.end());
     std::cout << "max element at: " << ranges::distance(v.begin(), result) << '\n';
     auto abs_compare = [](int a, int b) { return (std::abs(a) < std::abs(b)); };
     result = ranges::max_element(v, abs_compare);
     std::cout << "max element (absolute) at: " << ranges::distance(v.begin(), result) << '\n';
 }

Output:
 max element at: 5
 max element (absolute) at: 2

See also
   ranges::min_element returns the smallest element in a range
   (C++20) (niebloid)
   ranges::minmax_element returns the smallest and the largest elements in a range
   (C++20) (niebloid)
   ranges::max returns the greater of the given values
   (C++20) (niebloid)
   max_element returns the largest element in a range
                        (function template)