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 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.

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.

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 (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)