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std::compar...der_fallback(3) C++ Standard Libarystd::compar...der_fallback(3)

NAME
       std::compare_partial_order_fallback  - std::compare_partial_order_fall-
       back

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

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

	  }
	  Call signature
	  template< class T, class U >

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

	      compare_partial_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::partial_ordering, even if the operator <=> is unavailable.

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

	    * std::partial_order(t, u),	if it is a well-formed expression;
	    * otherwise, t == u	? std::partial_ordering::equivalent :
	      t	< u  ? std::partial_ordering::less :
	      u	< t  ? std::partial_ordering::greater :
	      std::partial_ordering::unordered

	  if  the  expressions t == u, t < u and u < t are all 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_partial_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
	   This	section	is incomplete
	   Reason: no example

	  Defect reports

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

	     DR	    Applied to	       Behavior	as published		  Cor-
       rect behavior
	  LWG 2114  C++20      the fallback  mechanism	only  required	  con-
       straints	strengthened
	  (P2167R3)	       return types to be convertible to bool
	  LWG 3465  C++20      the fallback mechanism did not require required
			       u < t to	be well-formed

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

Category:
	    * Todo no example

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

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