为什么我们不能在C ++中进行三向比较? [英] Why can't we do three-way comparison in C++?

问题描述

为了快速总结，为什么 2为什么<x ＜9 等于 2<x&&x ＜9 ?

To summarize it quickly, why isn't 2 < x < 9 equal to 2 < x && x < 9?

这是我编写的测试代码:

This is the test code I've written:

#include <iostream>

int main()
{
    int nums[] = { 5 , 1, 10};

    // We are gonna check if the number is in the range 2 - 9 
    for (auto e : nums)
    {
        if (2 < e < 9)
            std::cout << "2 < " << e << " < 9" << std::endl;

        if(2 < e && e < 9)
            std::cout << "2 < " << e << " and " << e << " < 9" << std::endl;
    }

    std::cin.get();
} 

这是我得到的输出:

2 < 5 < 9
2 < 5 and 5 < 9
2 < 1 < 9
2 < 10 < 9

看起来只有 2<e&&e<9 正常工作.

推荐答案

表达式

2 < x < 9

分组为

(2 < x) < 9

并且由于 2<x 为 false (0)或 true (1)，且两者均小于9，始终为 true .

And since 2 < x is either false (0) or true (1), and both are less than 9, it's always true.

因此，除非您对非内置类型 x 使用重载运算符(否则，如果 2 返回要定义< 的 proxy 对象的实例)，如果要测试 x 是否为在(2，9)间隔中，您需要按照自己的方式编写.

So unless you use overloaded operators for a non-built-in type x (then a 3-way comparison would be possible if 2 < x were to return an instance of a proxy object on which < is defined), if you want to test if x is in the interval (2, 9) you need to write it the way you have.

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