< cpp‎ | utility
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

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:


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

Otherwise, no function call operator of __weak_order_fn participates in overload resolution.

[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]