C++20草案中的宇宙飞船运算符(<=>,spaceship operator)

Herb Sutter提议的新三路运算符<=>已经被合入C++20草案中。

宇宙飞船运算符（hh）形式如lhs<=>rhs。
比如a与b是整型，那么a<=>b返回std::strong_ordering类型的纯右值(prvalue,不能取地址那种)：

如果a<b,(a<=>b)返回std::strong_ordering::less
如果a>b,(a<=>b)返回std::strong_ordering::greater
如果a与b全等/相等,(a<=>b)返回std::strong_ordering::equal

如果a b是浮点值，就返回的是std::partial_ordering::less/greater/equivalent或者std::partial_ordering::unordered(如果ab中有一个NaN)

标准还有更多规定，比如a与b是指针，a与b是枚举等，详细内容请参见更多

最后，std::strong_ordering和std::partial_ordering的值定义如下:

+--------------------------------------------------------------------+
|                  |          Numeric  values          | Non-numeric |
|     Category     +-----------------------------------+             |
|                  | -1   | 0          | +1            |   values    |
+------------------+------+------------+---------------+-------------+
| strong_ordering  | less | equal      | greater       |             |
| weak_ordering    | less | equivalent | greater       |             |
| partial_ordering | less | equivalent | greater       | unordered   |
| strong_equality  |      | equal      | nonequal      |             |
| weak_equality    |      | equivalent | nonequivalent |             |
+------------------+------+------------+---------------+-------------+

也就是说，返回的是0/1/-1,还记得strcmp(stra,strb)吧，<=>设计理念之一正是基于这种函数，如果a<=>b且a==3 && b==3,那么a<=>b就等于0

http://open-std.org/JTC1/SC22/WG21/docs/papers/2017/p0515r0.pdf
https://en.cppreference.com/w/cpp/language/operator_comparison