C语言宏中需要括号 [英] The need for parentheses in macros in C

问题描述

我尝试在以下代码中使用宏 SQR 的定义:

I tried to play with the definition of the macro SQR in the following code:

#define SQR(x) (x*x)
int main()
{
    int a, b=3;
    a = SQR(b+5);      // Ideally should be replaced with (3+5*5+3), though not sure.
    printf("%d
",a);
    return 0;
}

它打印 23.如果我将宏定义更改为 SQR(x) ((x)*(x))，则输出如预期的那样，64.我知道在 C 中调用宏会将调用替换为宏的定义，但我仍然无法理解，它是如何计算 23 的.

It prints 23. If I change the macro definition to SQR(x) ((x)*(x)) then the output is as expected, 64. I know that a call to a macro in C replaces the call with the definition of the macro, but I still can’t understand, how it calculated 23.

推荐答案

预处理器宏在代码编译之前执行文本替换，所以SQR(b+5) 转换为(b+5*b+5) = (6b+5) = 6*3+5 = 23

Pre-processor macros perform text-replacement before the code is compiled so SQR(b+5) translates to (b+5*b+5) = (6b+5) = 6*3+5 = 23

常规函数调用会在将参数 (b+3) 传递给函数之前计算其值，但由于宏是预编译替换，因此运算的代数顺序变得非常重要.

Regular function calls would calculate the value of the parameter (b+3) before passing it to the function, but since a macro is pre-compiled replacement, the algebraic order of operations becomes very important.

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