某些宏语句在 C++ 中可能会产生意想不到的结果?

2022-01-11 00:00:00 c macros c++

哪个宏语句可能导致意外结果?

Which macro statement may cause an unexpected results ? 

#define YEAR_LENGTH   365
#define MONTH_LENGTH  30
 #define DAYCALC(y, m, d) ((y * YEAR_LENGTH) + (m * MONTH_LENGTH) + d)

 int main()
 {
    int x = 5, y = 4 , z = 1;
    cout << DAYCALC(x *3 , y %3 , z) << endl ;
    cout << DAYCALC(x +12 , y  , 300) << endl ;
    cout << DAYCALC(x , 40 - y , 3+z) << endl ;
    cout << DAYCALC(x  , y  , (z+50)) << endl ;
    cout << DAYCALC(x  , y %3 , z) << endl ;
    cout << DAYCALC(4 % x , y++ , z) << endl;
    return 0;
 }

我运行程序非常好，没有任何意外结果.

I run the program very well w/o any unexpected results.

是否有一些隐藏的例外?

Are there some hidden exceptions ?

推荐答案

您遇到了运算符优先级问题.宏实际上扩展为文本复制和粘贴.

You have an operator precendence problem. Macros are literally expanded as text copy and paste.

例如:

DAYCALC(x , 40 - y , 3+z)

扩展为:

((40 - y * YEAR_LENGTH) + (x * MONTH_LENGTH) + 3+z)

请注意，由于运算符优先级，40 - y * YEAR_LENGTH 不是您想要的.

Note that 40 - y * YEAR_LENGTH, is not what you want due to operator precedence.

所以你需要将 () 放在宏中的参数周围:

So you need to put () around your parameters in the macro:

#define DAYCALC(y, m, d)     (((y) * YEAR_LENGTH) + ((m) * MONTH_LENGTH) + (d))

一般来说，如果一个宏参数在宏中出现不止一次，诸如y++(在你的最后一条语句中)之类的副作用也会被应用不止一次.所以这是需要小心的.

In general, if a macro parameter appears more than once in the macro, side effects such as y++ (in your last statement) will also be applied more than once. So it's something to be careful of.

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