std::weak_order(3)

NAME
std::weak_order - std::weak_order

Synopsis
   Defined in header <compare>
   inline namespace /* unspecified */ {
   inline constexpr /* unspecified */ weak_order = /* unspecified */; (since C++20)
   }
   Call signature
   template< class T, class U >
   requires /* see below */
   constexpr std::weak_ordering weak_order(T&& t, U&& u) noexcept(/* see
   below */);
   Compares two values using 3-way comparison and produces a result of type
   std::weak_ordering
   Let t and u be expressions and T and U denote decltype((t)) and decltype((u))
   respectively, std::weak_order(t, u) is expression-equivalent to:
     * If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true:
          * std::weak_ordering(weak_order(t, u)), if it is a well-formed expression
            with overload resolution performed in a context that does not include a
            declaration of std::weak_order,
          * otherwise, if T is a floating-point type:
               * if std::numeric_limits<T>::is_iec559 is true, performs the weak
                 ordering comparison of floating-point values (see below) and returns
                 that result as a value of type std::weak_ordering,
               * otherwise, yields a value of type std::weak_ordering that is
                 consistent with the ordering observed by T's comparison operators,
          * otherwise, std::weak_ordering(std::compare_three_way()(t, u)), if it is
            well-formed,
          * otherwise, std::weak_ordering(std::strong_order(t, u)), if it is
            well-formed.
     * In all other cases, the expression 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::weak_order 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 __weak_order_fn.
   All instances of __weak_order_fn are equal. The effects of invoking different
   instances of type __weak_order_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::weak_order 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::weak_order above, __weak_order_fn models
     * std::invocable<__weak_order_fn, Args...>,
     * std::invocable<const __weak_order_fn, Args...>,
     * std::invocable<__weak_order_fn&, Args...>, and
     * std::invocable<const __weak_order_fn&, Args...>.
   Otherwise, no function call operator of __weak_order_fn participates in overload
   resolution.

Notes
    Strict weak order of IEEE floating-point types
   Let x and y be values of same IEEE floating-point type, and weak_order_less(x, y) be
   the boolean result indicating if x precedes y in the strict weak order defined by
   the C++ standard.
     * If neither x nor y is NaN, then weak_order_less(x, y) == true if and only if x <
       y, i.e. all representations of equal floating-point value are equivalent;
     * If x is negative NaN and y is not negative NaN, then weak_order_less(x, y) ==
       true;
     * If x is not positive NaN and y is positive NaN, then weak_order_less(x, y) ==
       true;
     * If both x and y are NaNs with the same sign, then (weak_order_less(x, y) ||
       weak_order_less(y, x)) == false, i.e. all NaNs with the same sign are
       equivalent.

Example
    This section is incomplete
    Reason: no example

See also
   weak_ordering the result type of 3-way comparison that supports all 6
   (C++20) operators and is not substitutable
                        (class)
   strong_order performs 3-way comparison and produces a result of type
   (C++20) std::strong_ordering
                        (customization point object)
   partial_order performs 3-way comparison and produces a result of type
   (C++20) std::partial_ordering
                        (customization point object)
   compare_weak_order_fallback performs 3-way comparison and produces a result of type
   (C++20) std::weak_ordering, even if operator<=> is unavailable
                        (customization point object)