c - "&(epage->incoming[0]) = spage;"和 "&(epage->incoming[p2]) = spage;"之间的区别

Question

该特定代码块的最后一行产生错误“需要左值作为赋值的左操作数”。令人困惑的是为什么最后一行抛出此错误，而倒数第二行却没有。

      int p2 = 0;
      spage = find(in.startpage);
      spage->noutgoing++;
      spage->outgoing = (struct webpage *)realloc((spage->outgoing),((spage->noutgoing)*sizeof(struct webpage)));

      epage = find(in.endpage);
      epage->nincoming++;
      epage->incoming = (struct webpage *)realloc((epage->incoming),((epage->nincoming)*sizeof(struct webpage)));


      position = ((epage->nincoming));

      &(epage->incoming[0]) = spage;
      &(epage->incoming[p2]) = spage;

其中 spage、epage 是下面定义的结构:

struct webpage {
   char name;               /* name of page */
   struct webpage *outgoing;    //array of pointers pointing to outgoing webpages
   struct webpage *incoming ;   //array of pointers pointing to incoming webpages
   int noutgoing;
   int nincoming;
};

函数find返回一个指向结构网页的指针。

通过将结构 outgoing 和 incoming 更改为双指针并将最后一行更改为 (epage->incoming[p2]) = spage 解决了问题;.

仍然不知道为什么会发生错误...

Answer 1

这段代码:

struct S {
    int * p;
};

int main() {
    struct S * s;
    int n;
    &(s->p[42]) = 0;
    &(s->p[n]) = 0;
}

使用 GCC 4.5.1 编译，生成:

n.c:9:16: error: lvalue required as left operand of assignment
n.c:10:15: error: lvalue required as left operand of assignment

换句话说，这两种情况都会产生所需左值错误。你能用你的编译器尝试一下这个确切的代码吗？