std:: weak_order

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.

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.

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