链接布尔值给出相反的结果预期 [英] Chaining Bool values give opposite result to expected

问题描述

不假思索我写了一些code，检查一个结构的所有值设置为0。要做到这一点，我使用的：

Unthinkingly I wrote some code to check that all the values of a struct were set to 0. To accomplish this I used:

bool IsValid() {
    return !(0 == year == month == day == hour == minute == second);
}

，所有结构成员无符号短。我用code作为一个大型测试的一部分，但注意到它正在恢复为都等于零值假从零不同的值，和真正的 - 与我期望的相反

where all struct members were of type unsigned short. I used the code as part of a larger test but noticed that it was returning false for values differing from zero, and true for values that were all equal to zero - the opposite of what I expected.

我改变了code阅读：

I changed the code to read:

bool IsValid() {
    return (0 != year) || (0 != month) || (0 != day) || (0 != hour) || (0 != minute) || (0 != second);
}

不过，想知道是什么引起的古怪行为。它是precedence的结果？我试图谷歌这个答案却一无所获，如果有任何术语来形容我很想知道它的结果。

But would like to know what caused the odd behaviour. Is it a result of precedence? I've tried to Google this answer but found nothing, if there's any nomenclature to describe the result I'd love to know it.

我编译使用VS9和VS8的code。

I compiled the code using VS9 and VS8.

推荐答案

== 组由左到右，因此，如果所有值都为零，那么：

== groups from left to right, so if all values are zero then:

0 == year // true
(0 == year) == month // false, since month is 0 and (0 == year) converts to 1
((0 == year) == month) == day // true

等。

在一般情况下， X == ==Ÿž是的不的等同于 X == Y'放大器;＆放;点¯x==ž你似乎期待。

In general, x == y == z is not equivalent to x == y && x == z as you seem to expect.

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