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. See CustomizationPointObject for details.

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

See also