Sharing memory between two threads, but address of data structures changes

I am using promises in C++, and have this really simple program. I create a promise, and pass it to a thread function by reference.

Inside the thread, I create a string and then set the promise's value to that string. Then in the main program, I retrieve the promise's value using a future. I'm confused about why the address of the string doesn't remain the same? Ideally it shouldn't vary between the threads since it belongs to shared memory right?

  void tfunc(promise<string> & prms) {
    string str("Hello from thread");
    cout << (void *)str.data() << endl;
    prms.set_value(str);
  }

  int main() {

    promise<string> prms;
    future<string> ftr = prms.get_future();
    thread th(&tfunc, ref(prms));
    string str = ftr.get();
    // This address should be the same as that inside the thread, right?
    cout << (void *)str.data() << endl;
    th.join();
    return 0;
  }