Defined in header <compare>
inline namespace /* unspecified */ {
    inline constexpr /* unspecified */
        compare_weak_order_fallback = /* unspecified */;
(since C++20)
Call signature
template< class T, class U >
    requires /* see below */
constexpr std::weak_ordering
    compare_weak_order_fallback( T&& t, U&& u ) noexcept(/* see below */);
(since C++20)

Performs three-way comparison on subexpressions t and u and produces a result of type std::weak_ordering, even if the operator <=> is unavailable.

If std::decay_t<T> and std::decay_t<U> are the same type, std::compare_weak_order_fallback(t, u) is expression-equivalent to:

  • std::weak_order(t, u), if it is a well-formed expression;
  • otherwise, t == u ? std::weak_ordering::equivalent :t < u ? std::weak_ordering::less : std::weak_ordering::greater,
if the expressions t == u and t < u are both well-formed and each of decltype(t == u) and decltype(t < u) models boolean-testable, except that t and u are evaluated only once.

In all other cases, std::compare_weak_order_fallback(t, u) 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::compare_weak_order_fallback 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 __compare_weak_order_fallback_fn.

All instances of __compare_weak_order_fallback_fn are equal. The effects of invoking different instances of type __compare_weak_order_fallback_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::compare_weak_order_fallback 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::compare_weak_order_fallback above, __compare_weak_order_fallback_fn models

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


#include <compare>
#include <iostream>
// does not support <=>
struct Rational_1
    int num;
    int den; // > 0
inline constexpr bool operator<(Rational_1 lhs, Rational_1 rhs)
    return lhs.num * rhs.den < rhs.num * lhs.den;
inline constexpr bool operator==(Rational_1 lhs, Rational_1 rhs)
    return lhs.num * rhs.den == rhs.num * lhs.den;
// supports <=>
struct Rational_2
    int num;
    int den; // > 0
inline constexpr std::weak_ordering operator<=>(Rational_2 lhs, Rational_2 rhs)
    return lhs.num * rhs.den <=> rhs.num * lhs.den;
inline constexpr bool operator==(Rational_2 lhs, Rational_2 rhs)
    return lhs <=> rhs == 0;
void print(int id, std::weak_ordering value)
    std::cout << id << ") ";
    if (value == 0)
        std::cout << "equal\n";
    else if (value < 0)
        std::cout << "less\n";
        std::cout << "greater\n";
int main()
    Rational_1 a{1, 2}, b{3, 4};
//  print(1, a <=> b); // does not work
    print(2, std::compare_weak_order_fallback(a, b)); // works, defaults to < and ==
    Rational_2 c{6, 5}, d{8, 7};
    print(3, c <=> d); // works
    print(4, std::compare_weak_order_fallback(c, d)); // works
    Rational_2 e{2, 3}, f{4, 6};
    print(5, e <=> f); // works
    print(6, std::compare_weak_order_fallback(e, f)); // works


2) less
3) greater
4) greater
5) equal
6) equal

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2114
C++20 the fallback mechanism only required
return types to be convertible to bool
constraints strengthened

See also

performs 3-way comparison and produces a result of type std::weak_ordering
(customization point object)

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