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 (since C++20)

compare_weak_order_fallback( T&& t, U&& u ) noexcept(/* see below
*/);

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.

Example¶

// Run this code

#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";
else
std::cout << "greater\n";
}

int main()
{
Rational_1 a{1, 2}, b{3, 4};
// print(0, a <=> b); // does not work
print(1, std::compare_weak_order_fallback(a, b)); // works, defaults to < and ==

Rational_2 c{6, 5}, d{8, 7};
print(2, c <=> d); // works
print(3, std::compare_weak_order_fallback(c, d)); // works

Rational_2 e{2, 3}, f{4, 6};
print(4, e <=> f); // works
print(5, std::compare_weak_order_fallback(e, f)); // works
}

Output:¶

1) less
2) greater
3) greater
4) equal
5) 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 constraints strengthened
(P2167R3) return types to be convertible to bool

See also¶

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