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.
  Implementation-defined strict total order over pointers
   The function call operator yields the implementation-defined strict total order over
   pointers if the <=> operator between arguments invokes a built-in comparison
   operator for a pointer, even if the built-in <=> operator does not.
   The implementation-defined strict total order is consistent with the partial order
   imposed by built-in comparison operators (<=>, <, >, <=, and >=), and consistent
   among following standard function objects:
     * std::less, std::greater, std::less_equal, and std::greater_equal, when the
       template argument is a pointer type or void
     * std::ranges::equal_to, std::ranges::not_equal_to, std::ranges::less,
       std::ranges::greater, std::ranges::less_equal, std::ranges::greater_equal, and
       std::compare_three_way

Member types
   Member type Definition
   is_transparent /* unspecified */

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 >
   requires std::three_way_comparable_with<T, U> // with different semantic
   requirements
   constexpr auto operator()( T&& t, U&& u ) const;
   Compares t and u, equivalent to return std::forward<T>(t) <=> std::forward<U>(u);,
   except when that expression resolves to a call to a builtin operator<=> comparing
   pointers.
   When a call would not invoke a built-in operator comparing pointers, the behavior is
   undefined if std::three_way_comparable_with<T, U> is not modeled.
   When a call would invoke a built-in operator comparing pointers of type P, the
   result is instead determined as follows:
     * Returns std::strong_ordering::less if the (possibly converted) value of the
       first argument precedes the (possibly converted) value of the second argument in
       the implementation-defined strict total ordering over all pointer values of type
       P. This strict total ordering is consistent with the partial order imposed by
       the builtin operators <, >, <=, and >=.
     * Otherwise, returns std::strong_ordering::greater if (possibly converted) value
       of the second argument precedes the (possibly converted) value of the first
       argument in the same strict total ordering.
     * Otherwise, returns std::strong_ordering::equal.
   The behavior is undefined unless the conversion sequences from both T and U to P are
   equality-preserving (see below).
  Equality preservation
   An expression is equality preserving if it results in equal outputs given equal
   inputs.
     * The inputs to an expression consist of its operands.
     * The outputs of an expression consist of its result and all operands modified by
       the expression (if any).
   In specification of standard concepts, operands are defined as the largest
   subexpressions that include only:
     * an id-expression, and
     * invocations of std::move, std::forward, and std::declval.
   The cv-qualification and value category of each operand is determined by assuming
   that each template type parameter denotes a cv-unqualified complete non-array object
   type.
   Every expression required to be equality preserving is further required to be
   stable: two evaluations of such an expression with the same input objects must have
   equal outputs absent any explicit intervening modification of those input objects.

Example
// Run this code
 #include <iostream>
 #include <compare>
 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 while only semantic requirements
                       comparing pointers relaxed

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