说明：  指针是一个特殊的变量，它里面存储的数值被解释成为内存里的一个地址。 要搞清一个指针需要搞清指针的四方面的内容：指针的类型，指针所指向的 类型，指针的值或者叫指针所指向的内存区，还有指针本身所占据的内存区。
(Pointer is a special variable that stores the value which is interpreted as a memory address. Need to figure out a pointer to a pointer to find out the four aspects: the type of the pointer, the pointer points to the type of value or pointer is called the memory area pointed to by the pointer, and pointer memory area occupied by itself.)