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 1

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).