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代码如下：请多多解惑。困惑好几天。度娘那里也没找到answer

typedef struct _MenuNode
{
   unsigned int  MenuNodeID;
   unsigned char *MenuString;
   unsigned char MenuStatus;
   struct _MenuNode *First_Node; //新定义的结构体内，就定义结构体指针。为什么这样表达？
   struct _MenuNode *Last_Node;
   struct _MenuNode *Prev_Node;
   struct _MenuNode *Next_Node;
   void(*Action)(unsigned char in);
}MenuNode;

MenuNode MenuNode1 ={1,"1",0,(void*)0,(void*)0,(void*)0,(void*)0,MenuNode1Action};
//为什么(void*)0这种形式来表述  struct _MenuNode *First_Node？？