std::compare_weak_order_fallback

#include <compare>
#include <iostream>
 
// 不支持 <=>
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;
}
 
// 支持 <=>
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;
}
 
void print(int id, std::weak_ordering value)
{
    std::cout << id << ") ";
    if (value == 0)
        std::cout << "等于\n";
    else if (value < 0)
        std::cout << "小于\n";
    else
        std::cout << "大于\n";
}
 
int main()
{
    Rational_1 a{1, 2}, b{3, 4};
//  print(1, a <=> b); // 不起效
    print(2, std::compare_weak_order_fallback(a, b)); // 起效，默认到 < 和 ==
 
    Rational_2 c{6, 5}, d{8, 7};
    print(3, c <=> d); // 起效
    print(4, std::compare_weak_order_fallback(c, d)); // 起效
 
    Rational_2 e{2, 3}, f{4, 6};
    print(5, e <=> f); // 起效
    print(6, std::compare_weak_order_fallback(e, f)); // 起效
}