std::strong_order(3)

NAME
std::strong_order - std::strong_order

Synopsis
   Defined in header <compare>
   inline namespace /* unspecified */ {
   inline constexpr /* unspecified */ strong_order = /* unspecified */; (since C++20)
   }
   Call signature
   template< class T, class U >
   requires /* see below */
   constexpr std::strong_ordering strong_order( T&& t, U&& u )
   noexcept(/* see below */);
   Compares two values using 3-way comparison and produces a result of type
   std::strong_ordering
   Let t and u be expressions and T and U denote decltype((t)) and decltype((u))
   respectively, std::strong_order(t, u) is expression-equivalent to:
     * If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true:
          * std::strong_ordering(strong_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::strong_order,
          * otherwise, if T is a floating-point type:
               * if std::numeric_limits<T>::is_iec559 is true, performs the
                 ISO/IEC/IEEE 60559 totalOrder comparison of floating-point values and
                 returns that result as a value of type std::strong_ordering (note:
                 this comparison can distinguish between the positive and negative zero
                 and between the NaNs with different representations),
               * otherwise, yields a value of type std::strong_ordering that is
                 consistent with the ordering observed by T's comparison operators,
          * otherwise, std::strong_ordering(std::compare_three_way()(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::strong_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 __strong_order_fn.
   All instances of __strong_order_fn are equal. The effects of invoking different
   instances of type __strong_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::strong_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::strong_order above, __strong_order_fn models
     * std::invocable<__strong_order_fn, Args...>,
     * std::invocable<const __strong_order_fn, Args...>,
     * std::invocable<__strong_order_fn&, Args...>, and
     * std::invocable<const __strong_order_fn&, Args...>.
   Otherwise, no function call operator of __strong_order_fn participates in overload
   resolution.

Notes
    Strict total order of IEEE floating-point types
   Let x and y be values of same IEEE floating-point type, and total_order_less(x, y)
   be the boolean result indicating if x precedes y in the strict total order defined
   by totalOrder in ISO/IEC/IEEE 60559.
   (total_order_less(x, y) || total_order_less(y, x)) == false if and only if x and y
   have the same bit pattern.
     * if neither x nor y is NaN:
          * if x < y, then total_order_less(x, y) == true;
          * if x > y, then total_order_less(x, y) == false;
          * if x == y,
               * if x is negative zero and y is positive zero, total_order_less(x, y)
                 == true,
               * if x is not zero and x's exponent field is less than y's, then
                 total_order_less(x, y) == (x > 0) (only meaningful for decimal
                 floating-point number);
     * if either x or y is NaN:
          * if x is negative NaN and y is not negative NaN, then total_order_less(x, y)
            == true,
          * if x is not positive NaN and y is positive NaN, then total_order_less(x, y)
            == true,
          * if both x and y are NaNs with the same sign and x's mantissa field is less
            than y's, then total_order_less(x, y) == !std::signbit(x).

Example
    This section is incomplete
    Reason: no example

See also
   strong_ordering the result type of 3-way comparison that supports all
   (C++20) 6 operators and is substitutable
                        (class)
   weak_order performs 3-way comparison and produces a result of
   (C++20) type std::weak_ordering
                        (customization point object)
   partial_order performs 3-way comparison and produces a result of
   (C++20) type std::partial_ordering
                        (customization point object)
                        performs 3-way comparison and produces a result of
   compare_strong_order_fallback type std::strong_ordering, even if operator<=> is
   (C++20) unavailable
                        (customization point object)