pointer & iterator

pointer: address, use operator(*) to get/set the value

　　　　1) support operator(+,-), move to next positon accroding to the element type it points to

　　　　2) only should be used when mem is successive

iterator: class template, an iterator is an object of class Iter<element>

　　　　1) we can't asign an element to it directly

　　　　2) it stores a pointer of the elements it points to.

　　　　3) support operator(+,-), which will change the pointer value of the element it points to, return a reference of it's self (Iter)

　　　　4) and we can use operator(*) to get/set the value of the elements