std::compare_weak_order_fallback

 

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Defined in header <compare>
inlinenamespace/* unspecified */{     inlineconstexpr/* unspecified */         compare_weak_order_fallback =/* unspecified */; }		(since C++20)
Call signature
template<class T, class U >     requires /* see below */ constexprstd::weak_ordering     compare_weak_order_fallback( T&& t, U&& u )noexcept(/* see below */);		(since C++20)

Performs three-way comparison on subexpressionst 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 literalsemiregular class type. See CustomizationPointObject for details.

Example

Run this code

#include <compare>#include <iostream>   // does not support <=>struct Rational_1 {int num;int den;// > 0};   inlineconstexprbool operator<(Rational_1 lhs, Rational_1 rhs){return lhs.num* rhs.den< rhs.num* lhs.den;}   inlineconstexprbool 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};   inlineconstexprstd::weak_ordering operator<=>(Rational_2 lhs, Rational_2 rhs){return lhs.num* rhs.den<=> rhs.num* lhs.den;}   inlineconstexprbool 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";elseif(value <0)std::cout<<"less\n";elsestd::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 (P2167R3)	C++20	the fallback mechanism only required return types to be convertible to bool	constraints strengthened

See also

weak_order (C++20)

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