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 pointers, given the
       composite pointer type of t and u as P:
     * Compares the two converted pointers (of type P) in the implementation-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 retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3530 C++20 syntactic checks were relaxed only semantic requirements 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)