That's not an answer. The STL has to have a non-const version that returns some kind of reference. Because C++ strings are mutable and even something like

    s[3] = 'h';

involves forming a mutable reference and then assigning its pointee.

The way I think about it, to make CoW strings really usable in C++, you basically have to make strings immutable, like many newer languages like Python. That ship has sailed by the time standardization for C++ happened.