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std::compare_three_way(3)      C++ Standard Libary     std::compare_three_way(3)

NAME
     std::compare_three_way - std::compare_three_way

Synopsis
	Defined in header <compare>
	Defined in header <functional>
	struct compare_three_way;	(since C++20)

	Function  object for performing comparisons. Deduces the parameter types
     and the
	return type of the function call operator.

Member functions
	operator() obtains the result of three-way comparison on both arguments
		   (public member function)

     std::compare_three_way::operator()

	template< class T, class U >
	constexpr auto operator()( T&& t, U&& u ) const;

	Given the expression std::forward<T>(t) <=> std::forward<U>(u) as expr:

	  * If expr results in a call to built-in operator<=>  comparing  point-
     ers, given the
	    composite pointer type of t and u as P:

	  *  Compares  the two converted pointers (of type P) in the implementa-
     tion-defined
	    strict total order over pointers:

	  * If t precedes u, returns std::strong_ordering::less.
	  * If u precedes t, returns std::strong_ordering::greater.
	  * Otherwise, returns std::strong_ordering::equal.
	  * If the conversion sequence from T to P or  the  conversion	sequence
     from U to P is
	    not equality-preserving, the behavior is undefined.
	  * Otherwise:

	  * Returns the result of expr.
	  * If std::three_way_comparable_with<T, U> is not modeled, the behavior
     is
	    undefined.

	This overload participates in overload resolution only if
	std::three_way_comparable_with<T, U> is satisfied.

Example
     // Run this code

      #include <compare>
      #include <iostream>

      struct Rational
      {
	  int num;
	  int den; // > 0

	  // Although the comparison X <=> Y will work, a direct call
	  // to std::compare_three_way{}(X, Y) requires the operator==
	  // be defined, to satisfy the std::three_way_comparable_with.
	  constexpr bool operator==(Rational const&) const = default;
      };

      constexpr std::weak_ordering operator<=>(Rational lhs, Rational rhs)
      {
	  return lhs.num * rhs.den <=> rhs.num * lhs.den;
      }

      void print(std::weak_ordering value)
      {
	  value < 0 ? std::cout << "less\n" :
	  value > 0 ? std::cout << "greater\n" :
		      std::cout << "equal\n";
      }

      int main()
      {
	  Rational a{6, 5};
	  Rational b{8, 7};
	  print(a <=> b);
	  print(std::compare_three_way{}(a, b));
      }

Output:
      greater
      greater

	Defect reports

	The  following	behavior-changing  defect  reports were applied retroac-
     tively to
	previously published C++ standards.

	   DR	 Applied to	  Behavior as published 	     Correct be-
     havior
	LWG 3530 C++20	    syntactic checks were relaxed     only semantic  re-
     quirements are
			    while comparing pointers	      relaxed

See also
	ranges::equal_to      constrained function object implementing x == y
	(C++20) 	      (class)
	ranges::not_equal_to  constrained function object implementing x != y
	(C++20) 	      (class)
	ranges::less	      constrained function object implementing x < y
	(C++20) 	      (class)
	ranges::greater       constrained function object implementing x > y
	(C++20) 	      (class)
	ranges::less_equal    constrained function object implementing x <= y
	(C++20) 	      (class)
	ranges::greater_equal constrained function object implementing x >= y
	(C++20) 	      (class)

http://cppreference.com 	   2024.06.10	       std::compare_three_way(3)
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