Question: What does `const` at "top level" qualifier mean in C++?

And what are other levels?

For example:

```
int const *i;
int *const i;
int const *const i;

```

Answer: 
A top-level const qualifier affects the object itself. Others are only
relevant with pointers and references. They do not make the object
const, and only prevent modification through a path using the pointer or
reference. Thus:

```
char x;
char const* p = &x;

```

This is not a top-level const, and none of the objects are immutable.
The expression `*p` cannot be used to modify `x`, but other expressions
can be; `x` is not const. For that matter

```
*const_cast<char*>( p ) = 't'

```

is legal and well defined.

But

```
char const x = 't';
char const* p = &x;

```

This time, there is a top-level const on `x`, so `x` is immutable. No
expression is allowed to change it (even if `const_cast` is used). The
compiler may put `x` in read-only memory, and it may assume that the
value of `x` never changes, regardless of what other code may do.

To give the pointer top-level `const`, you'd write:

```
char x = 't';
char *const p = &x;

```

In this case, `p` will point to `x` forever; any attempt to change this 
is undefined behavior (and the compiler may put `p` in read-only memory,
or assume that `*p` refers to `x`, regardless of any other code).
`int *const i` puts `const` at the top-level, whereas `int const *i` does not.

The first says that the pointer `i` itself is immutable, whereas the second says that the memory the pointer points to is immutable.

Whenever `const` appears immediately before or after the type of the identifier, that is considered a top-level qualifier.