std::ranges::max(3)

NAME
std::ranges::max - std::ranges::max

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&
       max( 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
       max( 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>
       max( R&& r, Comp comp = {}, Proj proj = {} );
   Returns the greater of the given projected values.
   1) Returns the greater of a and b.
   2) Returns the first greatest value in the initializer list r.
   3) Returns the first greatest 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 greater of a and b, according to their respective projected values. If they
   are equivalent, returns a.
   2,3) The greatest value in r, according to the projection. If several values are
   equivalent to the greatest, 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 max_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, a), std::invoke(proj, b)) ? 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::max_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::max_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, m), std::invoke(proj, *i)))
                      m = *i;
              return m;
          }
      }
  };
  inline constexpr max_fn max;

Notes
   Capturing the result of std::ranges::max 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::max(n + 2, n * 2); // r is dangling

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <string>
 static_assert(std::ranges::max({0B10, 0X10, 010, 10}) == 16); // overload (2)
 int main()
 {
     namespace ranges = std::ranges;
     using namespace std::string_view_literals;
     std::cout << "larger of 1 and 9999: " << ranges::max(1, 9999) << '\n'
               << "larger of 'a', and 'b': '" << ranges::max('a', 'b') << "'\n"
               << "longest of \"foo\", \"bar\", and \"hello\": \""
               << ranges::max({"foo"sv, "bar"sv, "hello"sv}, {},
                        &std::string_view::size) << "\"\n";
 }

Output:
 larger of 1 and 9999: 9999
 larger of 'a', and 'b': 'b'
 longest of "foo", "bar", and "hello": "hello"

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