std:: compare_weak_order_fallback

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. 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 <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
}

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.

See also