class Base {

public:

auto operator<=>(const Base&) const = default;

};

std::strong_ordering operator <=>(const std::string& a, const std::string& b) {

int cmp = a.compare(b);

if (cmp < 0) return std::strong_ordering::less;

else if (cmp > 0) return std::strong_ordering::greater;

else return std::strong_ordering::equivalent;

}

class TotallyOrdered : Base {

std::string tax_id;

std::string first_name;

std::string last_name;

public:

TotallyOrdered(const std::string& id, const std::string& first, const std::string& last)

:tax_id(id), first_name(first), last_name(last) {}

// 定制 operator<=>，因为我们想先比较姓

std::strong_ordering operator<=>(const TotallyOrdered& that) const {

if (auto cmp = (Base&)(*this) <=> (Base&)that; cmp != 0) return cmp;

if (auto cmp = last_name <=> that.last_name; cmp != 0) return cmp;

if (auto cmp = first_name <=> that.first_name; cmp != 0) return cmp;

return tax_id <=> that.tax_id;

}

// ……非比较函数……

};

void test_compare02() {

// 编译器生成全部四个关系运算符

TotallyOrdered to1{ "1", "first1", "last1" }, to2{ "2", "first2", "last2" };

std::set<TotallyOrdered> s; // ok

s.insert(to1); // ok

s.insert(to2);

if (to1 <= to2) { // ok，调用一次 <=>

std::cout << "to1 <= to2\n";

}

else {

std::cout << "!(to1 <= to2)\n";

}

}