std::ranges::minmax,std::ranges::minmax_result(3)

NAME
std::ranges::minmax,std::ranges::minmax_result - std::ranges::minmax,std::ranges::minmax_result

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 ranges::minmax_result<const T&>
   minmax( 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 ranges::minmax_result<T>
   minmax( 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 > (since
   requires std::indirectly_copyable_storable<ranges::iterator_t<R>, (3) C++20)
   ranges::range_value_t<R>*>
   constexpr ranges::minmax_result<ranges::range_value_t<R>>
   minmax( R&& r, Comp comp = {}, Proj proj = {} );

Helper types
   template< class T > (4) (since
   using minmax_result = ranges::min_max_result<T>; C++20)
   Returns the smallest and the greatest of the given projected values.
   1) Returns references to the smaller and the greater of a and b.
   2) Returns the smallest and the greatest of the values in the initializer list r.
   3) Returns the smallest and the greatest of the values in the range r.
   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
   a, b - the values to compare
   r - a non-empty range of values to compare
   comp - comparison to apply to the projected elements
   proj - projection to apply to the elements.

Return value
   1) {b, a} if, according to their respective projected value, b is smaller than a;
   otherwise it returns {a, b}.
   2-3) {s, l}, where s and l are respectively the smallest and largest values in r,
   according to their projected value. If several values are equivalent to the smallest
   and largest, returns the leftmost smallest value, and the rightmost largest value.
   If the range is empty (as determined by ranges::distance(r)), the behavior is
   undefined.

Complexity
   1) Exactly one comparison and two applications of the projection.
   2-3) At most 3 / 2 * ranges::distance(r) comparisons and twice as many applications
   of the projection.

Possible implementation
   struct minmax_fn {
     template<class T, class Proj = std::identity,
              std::indirect_strict_weak_order<
                  std::projected<const T*, Proj>> Comp = ranges::less>
     constexpr ranges::minmax_result<const T&> operator()(
         const T& a, const T& b, Comp comp = {}, Proj proj = {}) const
     {
         if (std::invoke(comp, std::invoke(proj, b), std::invoke(proj, a))) {
             return {b, a};
         }
         return {a, b};
     }
     template<std::copyable T, class Proj = std::identity,
              std::indirect_strict_weak_order<
                  std::projected<const T*, Proj>> Comp = ranges::less>
     constexpr ranges::minmax_result<T> operator()(
         std::initializer_list<T> r, Comp comp = {}, Proj proj = {}) const
     {
       auto result = ranges::minmax_element(r, std::ref(comp), std::ref(proj));
       return {*result.min, *result.max};
     }
     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::minmax_result<ranges::range_value_t<R>> operator()(
         R&& r, Comp comp = {}, Proj proj = {}) const
     {
         auto result = ranges::minmax_element(r, std::ref(comp), std::ref(proj));
         return {std::move(*result.min), std::move(*result.max)};
     }
   };
   inline constexpr minmax_fn minmax;

Notes
   For overload (1), if one of the parameters is a temporary, the reference returned
   becomes a dangling reference at the end of the full expression that contains the
   call to minmax:
 int n = 1;
 auto p = std::ranges::minmax(n, n+1);
 int m = p.min; // ok
 int x = p.max; // undefined behavior
 // Note that structured bindings have the same issue
 auto [mm, xx] = std::ranges::minmax(n, n+1);
 xx; // undefined behavior

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <random>
 #include <array>
 int main()
 {
     namespace ranges = std::ranges;
     constexpr std::array v{3, 1, 4, 1, 5, 9, 2, 6, 5};
     std::random_device rd;
     std::mt19937_64 generator(rd());
     std::uniform_int_distribution<> distribution(0, ranges::distance(v)); // [0..9]
     // auto bounds = ranges::minmax(distribution(generator), distribution(generator));
     // UB: dangling references: bounds.min and bounds.max have the type `const int&`.
     const int x1 = distribution(generator);
     const int x2 = distribution(generator);
     auto bounds = ranges::minmax(x1, x2); // OK: got references to lvalues x1 and x2
     std::cout << "v[" << bounds.min << ":" << bounds.max << "]: ";
     for (int i = bounds.min; i < bounds.max; ++i) {
         std::cout << v[i] << ' ';
     }
     std::cout << '\n';
     auto [min, max] = ranges::minmax(v);
     std::cout << "smallest: " << min << ", " << "largest: " << max << '\n';
 }

Possible output:
 v[3:9]: 1 5 9 2 6 5
 smallest: 1, largest: 9

See also
   ranges::min returns the smaller of the given values
   (C++20) (niebloid)
   ranges::max returns the greater of the given values
   (C++20) (niebloid)
   ranges::minmax_element returns the smallest and the largest elements in a range
   (C++20) (niebloid)
   ranges::clamp clamps a value between a pair of boundary values
   (C++20) (niebloid)
   minmax returns the smaller and larger of two elements
   (C++11) (function template)