Help me understand this short chunk of code

Question

I understand this code calculates the sum of the args of a variable, however, I don't understand how it works. It looks really abstract to me. Can someone explain how the below works?

Thanks!

#include <stdio.h>

#define sum(...) \
    _sum(sizeof((int []){ __VA_ARGS__ }) / sizeof(int), (int []){ __VA_ARGS__ })

int _sum(size_t count, int values[])
{
    int s = 0;
    while(count--) s += values[count];
    return s;
}

int main(void)
{
    printf("%i", sum(1, 2, 3));
}

Answer 1

使用预处理器宏

#define sum(...) \
    _sum(sizeof((int []){ __VA_ARGS__ }) / sizeof(int), (int []){ __VA_ARGS__ })

被调用时sum(1,2,3),行被转换(简单的字符串替换,替换"__VA_ARGS__"为"1,2,3"):

_sum(sizeof((int []){1,2,3}) / sizeof(int), (int []){1,2,3})

这是一个函数调用_sum()传递两件事:

• 数组{1,2,3}中的整数数为3(通过将三个整数数组的大小除以单个整数的大小得到这个数).
• 指向数组本身的指针(或者包含相同值的完全不同的数组,具体取决于编译器的智能程度).

所有_sum()函数都是将每个整数添加到s(最初为零),直到计数用完为止.

上面的第一个要点有一些解释.如果您有一个N如下定义的元素数组:

tType x[22];

所述的尺寸数组是sizeof(x),所有元件的尺寸.该数组的单个元素sizeof(x[0])的大小是第一个元素的大小,尽管我经常更喜欢该sizeof(*x)变体.

因此,要计算元素的数量,只需使用以下之一将总大小除以元素的大小:

sizeof(x) / sizeof(x[0])
sizeof(x) / sizeof(*x)

而且,既然您已经要求对代码进行详细分析,那么我们开始:

// Needed for printf().

#include <stdio.h>

// Macro to convert sum(n1,n2,...,nN) to _sum(N,n1,n2,...,nN).
// This calculates the length of the array by dividing its size by the size
//   of an int and passes both the length and array through to the new
//   function.
// __VA_ARGS__ is replaced with the entire marcro argument list, '1,2,3' in
//   this case.

#define sum(...) \
    _sum(sizeof((int []){ __VA_ARGS__ }) / sizeof(int), (int []){ __VA_ARGS__ })

// Function to take size and pointer to int array, and return sum.

int _sum (size_t count, int values[]) {
    int s = 0;                // Initial sum of zero.
    while(count--)            // Until elements exhausted (count down).
        s += values[count];   // Add each array element to accumulator.
    return s;                 // Return sum.
}

int main (void) {
    printf ("%i", sum(1, 2, 3));   // Test it with 1, 2 and 3 (should print 6).
}