std::ranges::min_element(3)

NAME
std::ranges::min_element - std::ranges::min_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 min_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>
   min_element( R&& r, Comp comp = {}, Proj proj = {} );
   1) Finds the smallest 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 smallest element in the range [first, last). If several elements in
   the range are equivalent to the smallest 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 min_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 smallest = first;
    ++first;
    for (; first != last; ++first) {
        if (!std::invoke(comp, std::invoke(proj, *smallest), std::invoke(proj, *first))) {
            smallest = first;
        }
    }
    return smallest;
  }
  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 min_element_fn min_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::min_element(v.begin(), v.end());
     std::cout << "min element at: " << ranges::distance(v.begin(), result) << '\n';
     auto abs_compare = [](int a, int b) { return (std::abs(a) < std::abs(b)); };
     result = ranges::min_element(v, abs_compare);
     std::cout << "min element (absolute) at: " << ranges::distance(v.begin(), result) << '\n';
 }

Output:
 min element at: 2
 min element (absolute) at: 1

See also
   ranges::max_element returns the largest 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)
   min_element returns the smallest element in a range
                        (function template)