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std::lexico...e_three_way(3)   C++ Standard Libary  std::lexico...e_three_way(3)

NAME
     std::lexicographical_compare_three_way	 -     std::lexicographical_com-
     pare_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_or-
     dering 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	retroac-
     tively to
	previously published C++ standards.

	   DR	  Applied to	      Behavior as published		 Correct
     behavior
	LWG 3410 C++20	    extraneous	comparisons  between	       such  re-
     quirement removed
			    iterators were required

See also
					returns  true  if  one	range is lexico-
     graphically 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  lexico-
     graphically less
	(C++20) 			than another
					(niebloid)

http://cppreference.com 	   2024.06.10	    std::lexico...e_three_way(3)
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