std::minmax(3)

NAME
std::minmax - std::minmax

Synopsis
   Defined in header <algorithm>
   template< class T > (since
   std::pair<const T&,const T&> minmax( const C++11)
   T& a, const T& b ); (until
                        C++14)
   template< class T > (since
   constexpr std::pair<const T&,const T&> C++14)
   minmax( const T& a, const T& b );
   template< class T, class Compare >
                        (since
   std::pair<const T&,const T&> minmax( const C++11)
   T& a, const T& b, (until
                        C++14)
   Compare comp );
   template< class T, class Compare >
   constexpr std::pair<const T&,const T&> (since
   minmax( const T& a, const T& b, (1) C++14)
   Compare comp );
   template< class T > (since
   std::pair<T,T> minmax( (2) C++11)
   std::initializer_list<T> ilist); (until
                        C++14)
   template< class T > (since
   constexpr std::pair<T,T> minmax( C++14)
   std::initializer_list<T> ilist);
   template< class T, class Compare > (3) (since
   std::pair<T,T> minmax( C++11)
   std::initializer_list<T> ilist, Compare comp (until
   ); (4) C++14)
   template< class T, class Compare >
   constexpr std::pair<T,T> minmax( (since
   std::initializer_list<T> ilist, Compare comp C++14)
   );
   Returns the lowest and the greatest of the given values.
   1-2) Returns references to the smaller and the greater of a and b.
   3-4) Returns the smallest and the greatest of the values in initializer list ilist.
   The (1,3) versions use operator< to compare the values, whereas the (2,4) versions
   use the given comparison function comp.

Parameters
   a, b - the values to compare
   ilist - initializer list with the values to compare
             comparison function object (i.e. an object that satisfies the requirements
             of Compare) which returns true if the first argument is less than the
             second.
             The signature of the comparison function should be equivalent to the
             following:
             bool cmp(const Type1 &a, const Type2 &b);
   comp -
             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 T can be
             implicitly converted to both of them.

Type requirements
   -
   T must meet the requirements of LessThanComparable in order to use overloads (1,3).
   -
   T must meet the requirements of CopyConstructible in order to use overloads (3,4).

Return value
   1-2) Returns the result of std::pair<const T&, const T&>(a, b) if a<b or if a is
   equivalent to b. Returns the result of std::pair<const T&, const T&>(b, a) if b<a.
   3-4) A pair with the smallest value in ilist as the first element and the greatest
   as the second. If several elements are equivalent to the smallest, the leftmost such
   element is returned. If several elements are equivalent to the largest, the
   rightmost such element is returned.

Complexity
   1-2) Exactly one comparison
   3-4) At most ilist.size() * 3 / 2 comparisons

Possible implementation

First version
   template<class T>
   constexpr std::pair<const T&, const T&> minmax( const T& a, const T& b )
   {
       return (b < a) ? std::pair<const T&, const T&>(b, a)
                      : std::pair<const T&, const T&>(a, b);
   }

Second version
   template<class T, class Compare>
   constexpr std::pair<const T&, const T&> minmax( const T& a, const T& b, Compare comp )
   {
       return comp(b, a) ? std::pair<const T&, const T&>(b, a)
                        : std::pair<const T&, const T&>(a, b);
   }
                        Third version
   template< class T >
   constexpr std::pair<T, T> minmax( std::initializer_list<T> ilist )
   {
       auto p = std::minmax_element(ilist.begin(), ilist.end());
       return std::pair(*p.first, *p.second);
   }
                        Fourth version
   template< class T, class Compare >
   constexpr std::pair<T, T> minmax( std::initializer_list<T> ilist, Compare comp )
   {
       auto p = std::minmax_element(ilist.begin(), ilist.end(), comp);
       return std::pair(*p.first, *p.second);
   }

Notes
   For overloads (1,2), 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::minmax(n, n+1);
 int m = p.first; // ok
 int x = p.second; // undefined behavior
 // Note that structured bindings have the same issue
 auto [mm, xx] = std::minmax(n, n+1);
 xx; // undefined behavior

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <vector>
 #include <cstdlib>
 #include <ctime>
 int main()
 {
     std::vector<int> v {3, 1, 4, 1, 5, 9, 2, 6};
     std::srand(std::time(0));
     std::pair<int, int> bounds = std::minmax(std::rand() % v.size(),
                        std::rand() % v.size());
     std::cout << "v[" << bounds.first << "," << bounds.second << "]: ";
     for (int i = bounds.first; i < bounds.second; ++i) {
         std::cout << v[i] << ' ';
     }
     std::cout << '\n';
 }

Possible output:
 v[2,7]: 4 1 5 9 2

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