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