为什么在C ++ 20中从标准库容器中删除了比较运算符? [英] Why have comparison operators been removed from standard library containers in C++ 20?

问题描述

我正在浏览 cppreference ，并看到了 vector的比较操作已在C ++ 20中删除，并且引入了飞船运算符(< => ).对于许多其他标准库容器，例如 set 和 map .

I was browsing cppreference and saw that vector's comparison operations have been removed in C++20, and the spaceship operator (<=>) has been introduced. The same thing can be seen for many other standard library containers like set and map.

如何在新标准中进行比较?另外，C ++ 20是否会开始在较旧的代码上出现错误?

How do I do the comparisons in the new standard? Also, will C++20 start giving errors on older code?

推荐答案

如果您继续在参考站点上稍微浏览一下，则可能会来到

If you continue to browse on the reference site a little, you might come to the section on default comparisons, which simply states that:

简而言之，定义 operator< => 的类会自动获取编译器生成的运算符< ，< = ，> 和> = .

In brief, a class that defines operator<=> automatically gets compiler-generated operators <, <=, >, and >=.

因此，如果某个类存在"spaceship"运算符，则编译器将使用< => 运算符的结果自动生成其余的比较运算符.

So, if the "spaceship" operator exists for a class, the compiler will auto-generate the remaining comparison operators using the result of the <=> operator.

请注意， == 运算符不是 生成的(即使可能)，但是 std :: vector 会保持重载的 operator == .

Note that the == operator is not generated (even though it should be possible), but std::vector keeps an overload of operator==.

至:

C ++ 20将开始在较旧的代码上出现错误吗?

will C++ 20 start giving errors on older codes ?

不，不会.

当您使用C ++ 20编译器进行编译时，与其一起使用的标准库也应该针对C ++ 20制作，从而实现< => 运算符，该运算符将按上述说明使用.

When you build with a C++20 compiler, the standard library used with it should also be made for C++20 and thus implement the <=> operator, which will then be used as explained above.

但是，如果您使用C ++ 20编译器来构建较旧的标准库，则该较旧的标准库仍将实现较旧的比较运算符.

However, if you use a C++20 compiler to build with an older standard library, that older standard library will still have the older comparison operators implemented.

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