std::compare_weak_order_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
   compare_weak_order_fallback(T&& t, U&& u) noexcept(/* see below */);
   Performs three-way comparison on t and u and produces a result of type
   std::weak_ordering, even if the operator <=> is unavailable.
   Let t and u be expressions and T and U denote decltype((t)) and decltype((u))
   respectively, std::compare_weak_order_fallback(t, u) is expression-equivalent to:
     * If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true:
          * std::weak_order(t, u), if it is a well-formed expression;
          * otherwise,
 t == u ? std::weak_ordering::equivalent :
 t < u ? std::weak_ordering::less :
          std::weak_ordering::greater
                        if t == u and t < u are both well-formed and convertible to
                        bool, 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.
  Expression-equivalent
   Expression e is expression-equivalent to expression f, if
     * e and f have the same effects, and
     * either both are constant subexpressions or else neither is a constant
       subexpression, and
     * either both are potentially-throwing or else neither is potentially-throwing
       (i.e. noexcept(e) == noexcept(f)).
  Customization point objects
   The name std::compare_weak_order_fallback denotes a customization point object,
   which is a const function object of a literal semiregular class type. For exposition
   purposes, the cv-unqualified version of its type is denoted as
   __compare_weak_order_fallback_fn.
   All instances of __compare_weak_order_fallback_fn are equal. The effects of invoking
   different instances of type __compare_weak_order_fallback_fn on the same arguments
   are equivalent, regardless of whether the expression denoting the instance is an
   lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified
   instance is not required to be invocable). Thus, std::compare_weak_order_fallback
   can be copied freely and its copies can be used interchangeably.
   Given a set of types Args..., if std::declval<Args>()... meet the requirements for
   arguments to std::compare_weak_order_fallback above,
   __compare_weak_order_fallback_fn models
     * std::invocable<__compare_weak_order_fallback_fn, Args...>,
     * std::invocable<const __compare_weak_order_fallback_fn, Args...>,
     * std::invocable<__compare_weak_order_fallback_fn&, Args...>, and
     * std::invocable<const __compare_weak_order_fallback_fn&, Args...>.
   Otherwise, no function call operator of __compare_weak_order_fallback_fn
   participates in overload resolution.

Example
// Run this code
 #include <iostream>
 #include <compare>
 // 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
     bool operator==(Rational_2 const&) const = default;
 };
 inline constexpr std::weak_ordering operator<=>(Rational_2 lhs, Rational_2 rhs)
 {
     return lhs.num * rhs.den <=> rhs.num * lhs.den;
 }
 void print(std::weak_ordering value)
 {
     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};
     Rational_1 b{3, 4};
 // print(a <=> b); // doesn't work
     print(std::compare_weak_order_fallback(a, b)); // works, defaults to < and ==
     Rational_2 c{6, 5};
     Rational_2 d{8, 7};
     print(c <=> d); // works
     print(std::compare_weak_order_fallback(c, d)); // works
 }

Output:
 less
 greater
 greater

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