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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.


[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

[edit] 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.

[edit] Notes

[edit] 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).

[edit] Example

[edit] See also

the result type of 3-way comparison that supports all 6 operators and is substitutable
(class) [edit]
performs 3-way comparison and produces a result of type std::weak_ordering
(customization point object) [edit]
performs 3-way comparison and produces a result of type std::partial_ordering
(customization point object) [edit]
performs 3-way comparison and produces a result of type std::strong_ordering, even if operator<=> is unavailable
(customization point object) [edit]