分配作为条件 [英] assignment works as a condition

问题描述

考虑下面的代码，

int i;
while(i=0)
    printf("Hello");

现在一般来说 i=0 是一个 assignment 而不是 condition 供 while 检查.但是 GCC 编译器让它发出警告，甚至正确评估它(不执行打印语句).

Now Generally speaking i=0 is an assignment and not a condition for while to check. But the GCC compiler lets it go with a warning and even evaluates it correctly (does not execute the print statement).

为什么?为了真值，我通常会用括号做，但我的后辈觉得我错了，括号里没有真正的理由！

Why? I usually would do with parenthesis for the truth value but my juniors feel that I am wrong and there is no real reason for the parenthesis in this!

将实际"疑问归零，请考虑以下测试用例

Zeroing down on the 'actual' doubt, Please consider the following test case

int callme(){
    return 0;
}

int main(int argc,char*argv[]){
    int c;
    while(c = callme()){
        printf("Calling...\n"); 
    }
    return 0;    
}

推荐答案

表达式 i = 0 做了两件事:

The expression i = 0 does 2 things:

• 有在i
中存储o的副作用
• 产生值 0

我通常会用括号来表示真值，但我的后辈觉得我错了，括号里没有真正的理由在这个

I usually would do with parenthesis for the truth value but my juniors feel that i am wrong and there is no real reason for the parenthesis in this

这通常是对编译器的一个提示，意思是我真的想要这个，我没有忘记 =，闭嘴".

It's usually a hint to the compiler meaning "I actually want this, I didn't forget a =, shut up".

对于您的特定情况，没有理由编写 if (i = 0):您已经知道 if (0) 的作用.但是当用作:

For your specific case there's no reason to write if (i = 0): you already know what if (0) does. But it's pretty useful when used as:

if ((i = some_function()))
    ...

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