< cpp‎ | utility
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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>> == 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,
    • other the expression is ill-formed, which can result in substitution failure when it appears in the immediate context of a template instantiation.
  • Otherwise, std::weak_order(t, u) is ill-formed.


[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::weak_order denotes a customization point object, which is a const function object of a literal semiregular class type (denoted, for exposition purposes, as weak_order_ftor). All instances of weak_order_ftor are equal. 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_ftor will satisfy std::invocable<const weak_order_ftor&, Args...>. Otherwise, no function call operator of weak_order_ftor participates in overload resolution.

[edit] Notes

[edit] 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.

[edit] Example

[edit] See also

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