NAME

std::compare_weak_order_fallback - std::compare_weak_order_fallback

Synopsis

Defined in header <compare>
inline namespace /* unspecified */ {

inline constexpr /* unspecified */ (since C++20)
compare_weak_order_fallback = /* unspecified */;

}
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 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

if t == u and t < u are both well-formed and convertible to
bool, 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.

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

// Run this code

#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
}

Output:

less
greater
greater

See also

weak_order performs 3-way comparison and produces a result of type
(C++20) std::weak_ordering
(customization point object)