顶层const和底层const

As we’ve seen, a pointer is an object that can point to a different object. As a result,
we can talk independently about whether a pointer is const and whether the objects
to which it can point are const. We use the term top-level const to indicate that the
pointer itself is a const. When a pointer can point to a const object, we refer to
that const as a low-level const.

这是英文的介绍，我感觉清晰明了比那些遮遮掩掩的中文好多了。