"However, the object to which it is a reference is not const and can change its value, and msg is not immutable."

That is accurate. The object reference by immutable msg is hello_world which is mutable. If the value of hello_world changes, then the value of msg changes too. In the simple example, it won't happen, but the point is that it could, especially when multi-threaded.

I don't buy it, by calling that function, the caller is saying the value won't change underneath you (because you are being called in multithreaded code). The object itself cannot change itself, though of course there can be mutable members, which tend to be syncronisation mechanisms (mutexes) anyway.

No, the caller is saying that the callee isn't permitted to change the value! That's what const means. No promise is made about the actual object itself. There's no general way to specify that in C++.

This isn't so much an issue for functions, where the caller is stopped while the callee runs, but it needs to be borne in mind for longer-lived objects that take const references - or, indeed, const pointers - to objects that have longer lifetimes again. (I don't think multithreading need be introduced for this to be an issue.)

There's quite a difference between an object that could change and one that won't, if you're considering caching values across method calls, or setting up the referring object based on the referred object's current state, etc.

Ok, I know what the spec says, but it is utterly rediculous to expect people to write code in which parameters are allowed to change in an undefined way.

Right, multithreading is needed for it to be an issue with functions, not with containers.

> I don't buy it, by calling that function, the caller is saying the value won't change underneath you

You really do not understand what "const" means in C++.

If what you said were true, a whole class of optimizations would be possible that are not currently possible. For example, consider this silly function:

  void f(const int *x) {
    int y = *x;
    g();
    if (y == *x) h();
  }

If what you said were true, this would be able to optimize away the call to h() completely, but if you compile the function with maximum optimization you will see that this is not the case. Why not? Because the caller could look like this:

  int val = 0;
  void g() { val = 1; }
  int main() { f(&val); }