std::minmax_element(3)

NAME
std::minmax_element - std::minmax_element

Synopsis
   Defined in header <algorithm>
   template< class ForwardIt >
   std::pair<ForwardIt,ForwardIt> (since C++11)
                        (until C++17)
   minmax_element( ForwardIt first, ForwardIt last
   );
   template< class ForwardIt >
   constexpr std::pair<ForwardIt,ForwardIt> (since C++17)
   minmax_element( ForwardIt first, ForwardIt last
   );
   template< class ExecutionPolicy, class ForwardIt
   >
   std::pair<ForwardIt,ForwardIt> (2) (since C++17)
   minmax_element( ExecutionPolicy&& policy,
   ForwardIt first, ForwardIt last ); (1)
   template< class ForwardIt, class Compare >
   std::pair<ForwardIt,ForwardIt> (since C++11)
                        (until C++17)
   minmax_element( ForwardIt first, ForwardIt last,
   Compare comp );
   template< class ForwardIt, class Compare >
   constexpr std::pair<ForwardIt,ForwardIt> (since C++17)
                        (3)
   minmax_element( ForwardIt first, ForwardIt last,
   Compare comp );
   template< class ExecutionPolicy, class
   ForwardIt, class Compare >
   std::pair<ForwardIt,ForwardIt> (4) (since C++17)
   minmax_element( ExecutionPolicy&& policy,
   ForwardIt first, ForwardIt last, Compare comp );
   Finds the smallest and greatest element in the range [first, last).
   1) Elements are compared using operator<.
   3) Elements are compared using the given binary comparison function comp.
   2,4) Same as (1,3), but executed according to policy. These overloads do not
   participate in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.

Parameters
   first, last - forward iterators defining the range to examine
   policy - the execution policy to use. See execution policy for details.
                 comparison function object (i.e. an object that satisfies the
                 requirements of Compare) which returns true if if *a is less than
                 *b.
                 The signature of the comparison function should be equivalent to the
                 following:
                 bool cmp(const Type1 &a, const Type2 &b);
   cmp -
                 While the signature does not need to have const &, the function must
                 not modify the objects passed to it and must be able to accept all
                 values of type (possibly const) Type1 and Type2 regardless of value
                 category (thus, Type1 & is not allowed
                 , nor is Type1 unless for Type1 a move is equivalent to a copy
                 (since C++11)).
                 The types Type1 and Type2 must be such that an object of type
                 ForwardIt can be dereferenced and then implicitly converted to both of
                 them.

Type requirements
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.

Return value
   a pair consisting of an iterator to the smallest element as the first element and an
   iterator to the greatest element as the second. Returns std::make_pair(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 max(floor((3/2)*(N−1)), 0) applications of the predicate, where N =
   std::distance(first, last).

Exceptions
   The overloads with a template parameter named ExecutionPolicy report errors as
   follows:
     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Notes
   This algorithm is different from std::make_pair(std::min_element(),
   std::max_element()), not only in efficiency, but also in that this algorithm finds
   the last biggest element while std::max_element finds the first biggest element.

Possible implementation

First version
   template<class ForwardIt>
   std::pair<ForwardIt, ForwardIt>
       minmax_element(ForwardIt first, ForwardIt last)
   {
       using value_type = typename std::iterator_traits<ForwardIt>::value_type;
       return std::minmax_element(first, last, std::less<value_type>());
   }

Second version
   template<class ForwardIt, class Compare>
   std::pair<ForwardIt, ForwardIt>
       minmax_element(ForwardIt first, ForwardIt last, Compare comp)
   {
       auto min = first, max = first;
       if (first == last || ++first == last)
           return {min, max};
       if (comp(*first, *min)) {
           min = first;
       } else {
           max = first;
       }
       while (++first != last) {
           auto i = first;
           if (++first == last) {
               if (comp(*i, *min)) min = i;
               else if (!(comp(*i, *max))) max = i;
               break;
           } else {
               if (comp(*first, *i)) {
                   if (comp(*first, *min)) min = first;
                   if (!(comp(*i, *max))) max = i;
               } else {
                   if (comp(*i, *min)) min = i;
                   if (!(comp(*first, *max))) max = first;
               }
           }
       }
       return {min, max};
   }

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

Output:
 min = 1, max = 9

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