为什么这个 C 宏 function 计算 2 个数字的和会产生错误？

Question

I have a piece of code to calculate sum of 2 numbers using #define below:

#include <stdio.h>

#define sum(a, b) \
    int res = 0;    \
    res += (a + b); \
    (res ? res : 0)

int main () {
    printf("%d ", sum(3, 4));
    return 0;
}

When I compiled upper code, I caught this error:

error: expected expression before 'int'
     int res = 0;    \
note: in expansion of macro 'sum'     
     printf("%d ", sum(3, 4));

So, what did this error mean? And, how can I fix this?

Thank you

Answer 1

If you run your program through C preprocessor like gcc -E program.c then you will have following towards end:

int main () {
    printf("%d ", int res = 0; res += (3 + 4); (res ? res : 0));
    return 0;
}

which is invalid code.

To fix this you need to fix the macro for sum

#include <stdio.h>

#define sum(a, b) ((a) + (b))

int main () {
    printf("%d ", sum(3, 4));
    return 0;
}

You need to use parentheses to counter against potential misuse of operator precedence for operators like ++ or --. It is not a problem here but in general it is a good practice.

A better choice would be to write an inline function for this kind of work.

Answer 2

If you use GCC then it has a non-standard and therefore non-portable extension that allows statement expressions , which allows you to put multiple statements in an expression.

I don't really recommend its use, but it is possible to do what you want as a macro:

#define sum(a, b) \
    ({ int res = 0;    \
       res += (a + b); \
       (res ? res : 0) })

But the proper solution is to use functions instead, and rely on the compiler to properly inline it (with a little "hint" from you):

inline int sum(int a, int b)
{
    return a + b;
}