C++中的<=>("spaceship",三向比较)运算符是什么？

Question

当我尝试学习 C++ 运算符时，我偶然发现了一个关于 cppreference.com、^* 的奇怪比较运算符在看起来像这样的 table 中：

“好吧，如果这些是 C++ 中的常用运算符，我最好学习它们”，我想。但是我试图阐明这个谜团的所有尝试都没有成功。即使在这里，在 Stack Overflow 上我的搜索也没有成功。

<=>和C++有联系吗?

如果有，这个运算符到底做了什么？

_{* 同时 cppreference.com 更新了该页面，现在包含有关 <=> 运营商的信息。}

Answer 1

在 2017-11-11, the ISO C++ committee adopted Herb Sutter's proposal for the <=> "spaceship" three-way comparison operator as one of the new features that were added to C++20. In the paper titled Consistent comparison 上，Sutter、Maurer 和 Brown 演示了新设计的概念。有关该提案的概述，以下是文章的摘录：

The expression a <=> b returns an object that compares <0 if a < b, compares >0 if a > b, and compares ==0 if a and b are equal/equivalent.

Common case: To write all comparisons for your type X with type Y, with memberwise semantics, just write:
auto X::operator<=>(const Y&) =default;
Advanced cases: To write all comparisons for your type X with type Y, just write operator<=> that takes a Y, can use =default to get memberwise semantics if desired, and returns the appropriate category type:

Return an _ordering if your type naturally supports <, and we’ll efficiently generate symmetric <, >, <=, >=, ==, and !=; otherwise return an _equality, and we’ll efficiently generate symmetric == and !=.

Return strong_ if for your type a == b implies f(a) == f(b) (substitutability, where f reads only comparison-salient state that is accessible using the public const members), otherwise return weak_.

比较类别

五个比较类别被定义为 std:: 类型，每个都有以下预定义值：

+--------------------------------------------------------------------+
|                  |          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 |             |
+------------------+------+------------+---------------+-------------+

这些类型之间的隐式转换定义如下：

less

equal

greater

strong_ordering 隐式转换为：
- weak_ordering，值为 {less、equivalent、greater}
- partial_ordering，值为 {less、equivalent、greater}
- strong_equality，值为 {unequal、equal、unequal}
- weak_equality，值为 {nonequivalent、equivalent、nonequivalent}

less

equivalent

greater

weak_ordering 隐式转换为：
- partial_ordering，值为 {less、equivalent、greater}
- weak_equality，值为 {nonequivalent、equivalent、nonequivalent}

less

equivalent

greater

unordered

partial_ordering 隐式转换为：
- weak_equality 的值为 {nonequivalent、equivalent、nonequivalent、nonequivalent}
strong_equality 与值 {equal, unequal} 隐式转换为：
- weak_equality 值 {equivalent, nonequivalent}

三路比较

引入了<=>令牌。在旧源代码中，字符序列<=>标记为<= >。例如，X<&Y::operator<=>需要加一个space来保留其意义。

可重载运算符<=>是一个三向比较函数，优先级高于<低于<<。它 return 是一种可以与文字 0 进行比较的类型，但允许使用其他 return 类型，例如支持表达式模板。在语言和标准库中定义的所有<=>运算符return上述5种std::比较类别类型之一。

对于语言类型，提供了以下内置<=>同类型比较。所有都是 constexpr，除非另有说明。不能使用标量 promotions/conversions.

异构调用这些比较

对于bool、整数和指针类型，<=>returnsstrong_ordering。
对于指针类型，允许不同的cv-qualifications和derived-to-base转换调用一个homogeneous built-in<=>，并且还有heterogeneous built-inoperator<=>(T*, nullptr_t)。只有指向相同 object/allocation 的指针的比较才是常量表达式。
对于基本浮点类型，<=> returnspartial_ordering，并且可以通过将参数扩展到更大的浮点类型来异构调用。
对于枚举，<=> return与枚举的基础类型相同<=>。
对于nullptr_t,<=> returnsstrong_ordering并且总是产生equal.
对于可复制数组，T[N] <=> T[N]return与T<=>的类型相同，并执行字典顺序元素比较。其他数组没有<=>
对于void没有<=>.

_{为了更好地理解这个运算符的内部工作原理，请阅读原文paper。这正是我使用搜索引擎发现的。}

Answer 2

这称为三向比较运算符。

根据 P0515 论文提案：

There’s a new three-way comparison operator, <=>. The expression a <=> b returns an object that compares <0 if a < b, compares >0 if a > b, and compares ==0 if a and b are equal/equivalent.

To write all comparisons for your type, just write operator<=> that returns the appropriate category type:

Return an _ordering if your type naturally supports <, and we’ll efficiently generate <, >, <=, >=, ==, and !=; otherwise return an _equality, and we’ll efficiently generate == and !=.

Return strong if for your type a == b implies f(a) == f(b) (substitutability, where f reads only comparison-salient state accessible using the nonprivate const interface), otherwise return weak.

cppreference 说：

The three-way comparison operator expressions have the form
lhs <=> rhs   (1)  
The expression returns an object that

compares <0 if lhs < rhs

compares >0 if lhs > rhs

and compares ==0 if lhs and rhs are equal/equivalent.

Answer 3

由于引用的网页已更改，此答案已变得无关紧要

web page you are referencing 坏了。那天它被编辑了很多，不同的部分不同步。我看的时候状态是：

在页面顶部，它列出了当前存在的比较运算符（在 C++14 中）。那里没有<=>。

在页面底部，他们本应列出相同的运算符，但他们犯了错误并添加了这个未来的建议。

gcc 还不知道 <=>（-std=c++14，永远不会），所以它认为你的意思是 a <= > b。这解释了错误消息。

如果你在五年后尝试同样的事情，你可能会得到更好的错误信息，比如 <=> not part of C++14.

Answer 4

默认 <=> 自动免费提供 ==, !=, <, >, <=, >=

C++20 有一个新的“默认比较”功能设置，因此默认 <=> 可以免费提供所有其他功能。我相信这是添加 operator<=>.

背后的主要动机

改编自https://en.cppreference.com/w/cpp/language/default_comparisons：

main.cpp

#include <cassert>
#include <compare>
#include <set>

struct Point {
    int x;
    int y;
    auto operator<=>(const Point&) const = default;
};

int main() {
    Point pt1{1, 1}, pt2{1, 2};

    // Just to show it Is enough for `std::set`.
    std::set<Point> s;
    s.insert(pt1);

    // All of these are automatically defined for us!
    assert(!(pt1 == pt2));
    assert( (pt1 != pt2));
    assert( (pt1 <  pt2));
    assert( (pt1 <= pt2));
    assert(!(pt1 >  pt2));
    assert(!(pt1 >= pt2));
}

编译并运行:

sudo apt install g++-10
g++-10 -ggdb3 -O0 -std=c++20 -Wall -Wextra -pedantic -o main.out main.cpp
./main.out

上面的一个等效的更明确的版本是：

struct Point {
    int x;
    int y;
    auto operator<=>(const Point& other) const {
        if (x < other.x) return -1;
        if (x > other.x) return 1;
        if (y < other.y) return -1;
        if (y > other.y) return 1;
        return 0;
    }
    bool operator==(const Point& other) const = default;
};

在这种情况下，我们需要显式设置 bool operator==(const Point& other) const = default;，因为如果 operator<=> 未默认（例如，如上所述），则 operator== 不会自动默认：

Per the rules for any operator<=> overload, a defaulted <=> overload will also allow the type to be compared with <, <=, >, and >=.

If operator<=> is defaulted and operator== is not declared at all, then operator== is implicitly defaulted.

上面的示例使用与默认 operator<=> 相同的算法，如 cppreference 所解释的那样：

The default operator<=> performs lexicographical comparison by successively comparing the base (left-to-right depth-first) and then non-static member (in declaration order) subobjects of T to compute <=>, recursively expanding array members (in order of increasing subscript), and stopping early when a not-equal result is found

在 C++20 之前，您不能做类似 operator== = default 的事情，并且定义一个运算符不会导致定义其他运算符，例如以下无法使用 -std=c++17 编译：

#include <cassert>

struct Point {
    int x;
    int y;
    auto operator==(const Point& other) const {
        return x == other.x && y == other.y;
    };
};

int main() {
    Point pt1{1, 1}, pt2{1, 2};

    // Do some checks.
    assert(!(pt1 == pt2));
    assert( (pt1 != pt2));
}

有错误：

main.cpp:16:18: error: no match for ‘operator!=’ (operand types are ‘Point’ and ‘Point’)
   16 |     assert( (pt1 != pt2));
      |              ~~~ ^~ ~~~
      |              |      |
      |              Point  Point

上面的代码确实在 -std=c++20 下编译。

C++中的<=>("spaceship",三向比较)运算符是什么？

What is the <=> ("spaceship", three-way comparison) operator in C++?

c++

operators

c++-faq

spaceship-operator

c++20

<=>和C++有联系吗?

比较类别

三路比较