std::compare_weak_order_fallback - cppreference.com (original) (raw)

| Defined in header | | | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | ------------- | | 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. See CustomizationPointObject for details.

[edit] Example

#include #include   // 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

[edit] Defect reports

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

[edit] See also

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