Giving this further thought, I think languages with mutable strings can overload the increment and decrement operators as a way to resize buffers, or move the fill pointer. In C++ this would actually make perfect sense, for implementing high-performance strings. The ++ can be a synonym for realloc, and the base pointer to the string, along with its length and encoding can be stored in an opaque "struct string" handle. Fast, secure pascal-style strings.]
Indeed, but I said "for implementing high-performance strings."
I would love to hear about the OP's ideas for faster variable-length and adjustable string implementation using C++ member or friend functions (the C++ implementation of "methods".)
When it comes to performance, the same design I detailed is used by all the "high-per" libraries that I know of. If we're talking API aesthetics, fast implementations can always be wrapped with something more attractive.
P.S. Let me make it clear that I am not advocating operator overloading for resizing strings and that the OP actually does have a point. (sorry if I came off as a douche in that regard.) But implementing fast-strings as classes + methods is not really fastest way in C++.
I'm getting the impression you don't know C++ that well. A "member" function or "method" in C++ is just a function that has access to other members of its class. If you don't add the virtual keyword there doesn't have to be any difference between it and a normal function (besides the mentioned scoping rules) and it could even be inlined. Overloaded operators aren't any faster than functions.
[Edit:
Giving this further thought, I think languages with mutable strings can overload the increment and decrement operators as a way to resize buffers, or move the fill pointer. In C++ this would actually make perfect sense, for implementing high-performance strings. The ++ can be a synonym for realloc, and the base pointer to the string, along with its length and encoding can be stored in an opaque "struct string" handle. Fast, secure pascal-style strings.]