std::strong_order

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

• 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
    • 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) )
• 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.

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

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...>
• std::invocable < const __strong_order_fn&, Args...> .
  

Otherwise, no function call operator of __strong_order_fn participates in overload resolution.

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

(total_order_less(x, y) || total_order_less(y, x) ) == false if and only if x and y

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