std::ranges::min(3)

NAME
std::ranges::min - std::ranges::min

Synopsis
   Defined in header <algorithm>
   Call signature
   template< class T, class Proj = std::identity,
             std::indirect_strict_weak_order< (since
                 std::projected<const T*, Proj>> Comp = ranges::less > (1) C++20)
   constexpr const T&
       min( const T& a, const T& b, Comp comp = {}, Proj proj = {} );
   template< std::copyable T, class Proj = std::identity,
             std::indirect_strict_weak_order< (since
                 std::projected<const T*, Proj>> Comp = ranges::less > (2) C++20)
   constexpr T
       min( std::initializer_list<T> r, Comp comp = {}, Proj proj = {} );
   template< ranges::input_range R, class Proj = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<ranges::iterator_t<R>, Proj>> Comp =
   ranges::less > (3) (since
   requires std::indirectly_copyable_storable<ranges::iterator_t<R>, C++20)
                        ranges::range_value_t<R>*>
   constexpr ranges::range_value_t<R>
       min( R&& r, Comp comp = {}, Proj proj = {} );
   Returns the smaller of the given projected elements.
   1) Returns the smaller of a and b.
   2) Returns the first smallest element in the initializer list r.
   3) Returns the first smallest value in the range r.
   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
   a, b - the values to compare
   r - the range of values to compare
   comp - comparison to apply to the projected elements
   proj - projection to apply to the elements

Return value
   1) The smaller of a and b, according to the projection. If they are equivalent,
   returns a.
   2,3) The smallest element in r, according to the projection. If several values are
   equivalent to the smallest, returns the leftmost one. If the range is empty (as
   determined by ranges::distance(r)), the behavior is undefined.

Complexity
   1) Exactly one comparison.
   2,3) Exactly ranges::distance(r) - 1 comparisons.

Possible implementation
  struct min_fn
  {
      template<class T, class Proj = std::identity,
               std::indirect_strict_weak_order<
                   std::projected<const T*, Proj>> Comp = ranges::less>
      constexpr
      const T& operator()(const T& a, const T& b, Comp comp = {}, Proj proj = {}) const
      {
          return std::invoke(comp, std::invoke(proj, b), std::invoke(proj, a)) ? b : a;
      }
      template<std::copyable T, class Proj = std::identity,
               std::indirect_strict_weak_order<
                   std::projected<const T*, Proj>> Comp = ranges::less>
      constexpr
      T operator()(std::initializer_list<T> r, Comp comp = {}, Proj proj = {}) const
      {
          return *ranges::min_element(r, std::ref(comp), std::ref(proj));
      }
      template<ranges::input_range R, class Proj = std::identity,
               std::indirect_strict_weak_order<
                    std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
      requires std::indirectly_copyable_storable<ranges::iterator_t<R>,
                        ranges::range_value_t<R>*>
      constexpr
      ranges::range_value_t<R> operator()(R&& r, Comp comp = {}, Proj proj = {}) const
      {
          using V = ranges::range_value_t<R>;
          if constexpr (ranges::forward_range<R>)
              return
                  static_cast<V>(*ranges::min_element(r, std::ref(comp), std::ref(proj)));
          else
          {
              auto i = ranges::begin(r);
              auto s = ranges::end(r);
              V m(*i);
              while (++i != s)
                  if (std::invoke(comp, std::invoke(proj, *i), std::invoke(proj, m)))
                      m = *i;
              return m;
          }
      }
  };
  inline constexpr min_fn min;

Notes
   Capturing the result of std::ranges::min by reference produces a dangling reference
   if one of the parameters is a temporary and that parameter is returned:
 int n = -1;
 const int& r = std::ranges::min(n + 2, n * 2); // r is dangling

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <string>
 int main()
 {
     namespace ranges = std::ranges;
     using namespace std::string_view_literals;
     std::cout << "smaller of 1 and 9999: " << ranges::min(1, 9999) << '\n'
               << "smaller of 'a', and 'b': '" << ranges::min('a', 'b') << "'\n"
               << "shortest of \"foo\", \"bar\", and \"hello\": \""
               << ranges::min({"foo"sv, "bar"sv, "hello"sv}, {},
                        &std::string_view::size) << "\"\n";
 }

Output:
 smaller of 1 and 9999: 1
 smaller of 'a', and 'b': 'a'
 shortest of "foo", "bar", and "hello": "foo"

See also
   ranges::max returns the greater of the given values
   (C++20) (niebloid)
   ranges::minmax returns the smaller and larger of two elements
   (C++20) (niebloid)
   ranges::min_element returns the smallest element in a range
   (C++20) (niebloid)
   ranges::clamp clamps a value between a pair of boundary values
   (C++20) (niebloid)
   min returns the smaller of the given values
                       (function template)