std::compare_weak_order_fallback

inline namespace /* unspecified */ {
    inline constexpr /* unspecified */
        compare_weak_order_fallback = /* unspecified */;
}

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 and 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>> is true:
  • 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

• 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.

Expression-equivalent

Expression e is expression-equivalent to expression f, if.

• e and f have the same effects, and
• either both are constant subexpressions or else neither is a constant subexpression, and
• either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

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
.

• std::invocable<__compare_weak_order_fallback_fn, Args...>,
• std::invocable<const __compare_weak_order_fallback_fn, Args...>,
• std::invocable<__compare_weak_order_fallback_fn&, Args...>, and
• std::invocable<const __compare_weak_order_fallback_fn&, Args...>.
  

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

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
 
    bool operator==(Rational_2 const&) const = default;
};
 
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";
    else
        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
}

less
greater
greater

See also