< cpp‎ | utility
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 */);

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

Let t and u be expressions and T and U denote decltype((t)) and decltype((u)) respectively, std::compare_weak_order_fallback(t, u) is expression-equivalent to:

  • If std::is_same_v<std::decay_t<T>, std::decay_t<U>> == true:
    • the expression is expression-equivalent to std::weak_order(t, u), if it is a well-formed expression;
    • Otherwise, if t == u and t < u are both well-formed and convertible to bool, the expression is expression-equivalent to
t == u ? std::weak_ordering::equal :
t < u  ? std::weak_ordering::less :
except that t and u are evaluated only once.
  • In all other cases, std::compare_weak_order_fallback(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::compare_weak_order_fallback denotes a customization point object, which is a function object of a literal semiregular class type (denoted, for exposition purposes, as compare_weak_order_fallback_ftor). All instances of compare_weak_order_fallback_ftor are equal. 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_ftor will satisfy std::invocable<const compare_weak_order_fallback_ftor&, Args...>. Otherwise, no function call operator of compare_weak_order_fallback_ftor participates in overload resolution.

[edit] Example

#include <iostream>
#include <compare>
// 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;
void print(std::weak_ordering value)
    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};
    Rational_1 b{3, 4};
//  print(a <=> b);                // doesn't work
    print(std::compare_weak_order_fallback(a, b)); // works, defaults to < and ==
    Rational_2 c{6, 5};
    Rational_2 d{8, 7};
    print(c <=> d);                // works
    print(std::compare_weak_order_fallback(c, d)); // works



[edit] See also

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