std::lexicographical_compare_three_way(3)

NAME
std::lexicographical_compare_three_way - std::lexicographical_compare_three_way

Synopsis
   Defined in header <algorithm>
   template< class InputIt1, class InputIt2, class Cmp >
   constexpr auto lexicographical_compare_three_way
       ( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 (1) (since C++20)
   last2,
         Cmp comp ) -> decltype(comp(*first1, *first2));
   template< class InputIt1, class InputIt2 >
   constexpr auto lexicographical_compare_three_way (2) (since C++20)
       ( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2
   last2 );
   Lexicographically compares two ranges [first1, last1) and [first2, last2) using
   three-way comparison and produces a result of the strongest applicable comparison
   category type.
   1) Returns the order between the first non-equivalent pair of elements according to
   comp in both ranges if any, otherwise (if one ranges is equivalent to the prefix of
   another according to comp), returns the order between the length of both ranges.
   2) Equivalent to return std::lexicographical_compare_three_way(
       first1, last1, first2, last2, std::compare_three_way());
   If the return type is not one of the three comparison category types, the program is
   ill-formed:
     * std::strong_ordering
     * std::weak_ordering
     * std::partial_ordering

Parameters
   first1, last1 - the first range of elements to examine
   first2, last2 - the second range of elements to examine
   comp - a function object

Type requirements
   -
   InputIt1, InputIt2 must meet the requirements of LegacyInputIterator.

Return value
   The value of a comparison category type specified above.

Complexity
   Given \(\scriptsize N_1\)N
   1 as std::distance(first1, last1) and \(\scriptsize N_2\)N
   2 as std::distance(first2, last2):
   1) At most \(\scriptsize \min(N_1,N_2)\)min(
   1,N
   2) applications of comp.
   2) At most \(\scriptsize \min(N_1,N_2)\)min(N
   1,N
   2) applications of std::compare_three_way().

Possible implementation
   template<class I1, class I2, class Cmp>
   constexpr auto lexicographical_compare_three_way(I1 f1, I1 l1, I2 f2, I2 l2, Cmp comp)
       -> decltype(comp(*f1, *f2))
   {
       using ret_t = decltype(comp(*f1, *f2));
       static_assert(std::disjunction_v<
                        std::is_same<ret_t, std::strong_ordering>,
                        std::is_same<ret_t, std::weak_ordering>,
                        std::is_same<ret_t, std::partial_ordering>>,
                     "The return type must be a comparison category type.");
       bool exhaust1 = (f1 == l1);
       bool exhaust2 = (f2 == l2);
       for (; !exhaust1 && !exhaust2; exhaust1 = (++f1 == l1), exhaust2 = (++f2 == l2))
           if (auto c = comp(*f1, *f2); c != 0)
               return c;
       return !exhaust1 ? std::strong_ordering::greater:
              !exhaust2 ? std::strong_ordering::less:
                        std::strong_ordering::equal;
   }

Example
// Run this code
 #include <algorithm>
 #include <cctype>
 #include <compare>
 #include <iomanip>
 #include <iostream>
 #include <string_view>
 #include <utility>
 using namespace std::literals;
 void show_result(std::string_view s1, std::string_view s2, std::strong_ordering o)
 {
     std::cout << std::quoted(s1) << " is ";
     std::is_lt(o) ? std::cout << "less than ":
     std::is_gt(o) ? std::cout << "greater than ":
                     std::cout << "equal to ";
     std::cout << std::quoted(s2) << '\n';
 }
 std::strong_ordering cmp_icase(unsigned char x, unsigned char y)
 {
     return std::toupper(x) <=> std::toupper(y);
 };
 int main()
 {
     for (const auto& [s1, s2] :
     {
         std::pair{"one"sv, "ONE"sv}, {"two"sv, "four"sv}, {"three"sv, "two"sv}
     })
     {
         const auto res = std::lexicographical_compare_three_way(
             s1.cbegin(), s1.cend(), s2.cbegin(), s2.cend(), cmp_icase);
         show_result(s1, s2, res);
     }
 }

Output:
 "one" is equal to "ONE"
 "two" is greater than "four"
 "three" is less than "two"
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3410 C++20 extraneous comparisons between such requirement removed
                       iterators were required

See also
                        returns true if one range is lexicographically less
   lexicographical_compare than another
                        (function template)
   compare_three_way constrained function object implementing x <=> y
   (C++20) (class)
   ranges::lexicographical_compare returns true if one range is lexicographically less
   (C++20) than another
                        (niebloid)