今天写作业被这个const搞得很自闭，因此来总结一下，避免以后踩坑（此处不一定是int，可以是任意type）
const这个东西对于一般变量很简单，const int a and const a int are totally same. Both of them tell the compiler that the value stored in a can not be modified. But when it is combined with pointer, things go complicated
const int *
For C++, int* is not viewed as a type. Only *a be viewed as a mark telling the complier a is a pointer. So as we have already known that int const is same with const int, int const *a is also same with const int *a. Both of them means *a is a const, which means the value poined by a can not be modified but a, as a pointer, can point to different places.
int* const
By contrast, the const in int* const is a mark for a, so a, as a pointer, can only point to one specific place(determined when it is initialized), but the value pointed by it can be changed.
Besides, we can combine two usages of consttogether: const int* const a--a, as a pointer, can not be modified and the value pointed by a cannot be modified.
