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).
The way it was explained to me, given:

```
[const] TYPE * [const] VARIABLE

```

*VARIABLE* is used to point to *data* of type *TYPE* through `*VARIABLE`

Draw a line through the `*` or multiple `*`s

1. If there is a `const` to the **left** of the `*` it applies to the
**data** and the **data** cannot be changed: `*VARIABLE` cannot be assigned, except at initialization
2. If there is a `const` to the **right** of the `*` it applies to the
*VARIABLE* and what the *VARIABLE* points to cannot be changed: `VARIABLE` cannot be assigned, except at initialization

So:

```
          |              left  right
int       *       i1;
          |              no    no     can change *i1 and i1

int const *       i2;     
          |              yes   no     cannot change *i2 but can change i2

int       * const i3;
          |              no    yes    can change *i3 but i3 cannot be changed

int const * const i4;
          |              yes   yes    cannot change *i4 or i4

```