A bit on cpp qualifiers.

CV Qualifiers

Any thing that is const, or volatile, or both.

Top level cv_qualifiers

What is const int * x. What is constant here, the pointer or the underlying data? It is the underlying data that is constant, the pointer can change its value to point to any other const int.

By extension, int * const x will have a pointer which can be dereferenced and the underlying data's value can be changed, but the pointer itself cannot change to point to something else.

Here the second const is referred to as the top level cv-qualifier. Because it is a constness on the highest level, the pointer itself.

References cannot have top level cv-qualifiers

A reference is not an object in itself. It is an alias, an alternative name for an object. Therefore there is no such thing as int & const x. You cannot change what a reference is an alias for. However const int & x is still valid as it is an alias to a const int.