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std::compar...er_fallback(3)   C++ Standard Libary  std::compar...er_fallback(3)

NAME
     std::compare_weak_order_fallback - std::compare_weak_order_fallback

Synopsis
	Defined in header <compare>
	inline namespace /* unspecified */ {

	    inline	    constexpr	       /*	  unspecified	      */
     (since C++20)
		compare_weak_order_fallback = /* unspecified */;

	}
	Call signature
	template< class T, class U >

	    requires /* see below */
	constexpr					      std::weak_ordering
     (since C++20)

	    compare_weak_order_fallback( T&& t, U&& u ) noexcept(/* see below
	*/);

	Performs  three-way  comparison on subexpressions t and u and produces a
     result of
	type std::weak_ordering, even if the operator <=> is unavailable.

	If std::decay_t<T> and std::decay_t<U> are the same type,
	std::compare_weak_order_fallback(t, u) is expression-equivalent to:

	  * std::weak_order(t, u), if it is a well-formed expression; otherwise,
	  * t == u ? std::weak_ordering::equivalent :
	    t_<_ _ _ _t_:_w_a__ordering::less :
	    a a a a a a a a a std::weak_ordering::greater, if the expressions  t
     == u and t < u are
	    both  well-formed  and  each of decltype(t == u) and decltype(t < u)
     models
	    boolean-testable, except that t and u are evaluated only once.

	In all	other  cases,  std::compare_weak_order_fallback(t,  u)	is  ill-
     formed, which can
	result	in substitution failure when it appears in the immediate context
     of a
	template instantiation.

Example
     // Run this code

      #include <compare>
      #include <iostream>

      // does not support <=>
      struct Rational_1
      {
	  int num;
	  int den; // > 0
      };

      inline constexpr bool operator<(Rational_1 lhs, Rational_1 rhs)
      {
	  return lhs.num * rhs.den < rhs.num * lhs.den;
      }

      inline constexpr bool operator==(Rational_1 lhs, Rational_1 rhs)
      {
	  return lhs.num * rhs.den == rhs.num * lhs.den;
      }

      // supports <=>
      struct Rational_2
      {
	  int num;
	  int den; // > 0
      };

      inline constexpr std::weak_ordering operator<=>(Rational_2 lhs, Rational_2
     rhs)
      {
	  return lhs.num * rhs.den <=> rhs.num * lhs.den;
      }

      inline constexpr bool operator==(Rational_2 lhs, Rational_2 rhs)
      {
	  return lhs <=> rhs == 0;
      }

      void print(int id, std::weak_ordering value)
      {
	  std::cout << id << ") ";
	  if (value == 0)
	      std::cout << "equal\n";
	  else if (value < 0)
	      std::cout << "less\n";
	  else
	      std::cout << "greater\n";
      }

      int main()
      {
	  Rational_1 a{1, 2}, b{3, 4};
      //  print(0, a <=> b); // does not work
	  print(1, std::compare_weak_order_fallback(a, b)); //	works,	defaults
     to < and ==

	  Rational_2 c{6, 5}, d{8, 7};
	  print(2, c <=> d); // works
	  print(3, std::compare_weak_order_fallback(c, d)); // works

	  Rational_2 e{2, 3}, f{4, 6};
	  print(4, e <=> f); // works
	  print(5, std::compare_weak_order_fallback(e, f)); // works
      }

Output:
      1) less
      2) greater
      3) greater
      4) equal
      5) equal

	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  2114  C++20      the fallback mechanism only required   constraints
     strengthened
	(P2167R3)	     return types to be convertible to bool

See also
	weak_order performs 3-way comparison and produces a result of type
	(C++20)    std::weak_ordering
		   (customization point object)

http://cppreference.com 	   2024.06.10	    std::compar...er_fallback(3)
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