Top-level const vs Low-level const

• Top-level const: the object itself is const; if it is a pointer, the pointer itself is const. E.g. int *const ptr.
• Low-level const: the object being pointed or referenced is const. E.g. const int *ptr.

const int * const has both top-level const and low-level const.

For example, for a non-pointer variable

// x is a top-level const integer
const int x = 10;

// Error: Cannot modify a const variable
x = 20;

For a pointer, top-level const:

// ptr is a top-level const pointer
int *const ptr = &x;

// OK: Modifies the value pointed to by ptr
*ptr = 20;

// Error: Cannot change the address stored in ptr
ptr = &y;

A pointer, low-level const:

// ptr is a low-level const pointer
const int *ptr = &x;

// Error: Cannot modify the value pointed to by ptr
*ptr = 20;

// OK: Can change the address stored in ptr
ptr = &y;