irritating corner-case c++ question

Is there any portable way to read [as in the system call] from a file directly into a C++ standard string? One would like to do something like

  str.reserve(filesize);
  got = ::read(fd, str.data(), str.capacity());
  str.enlarge_to_cover(got);

(error handling and loop until EOF omitted for clarity) but I can't actually find a method that does enlarge_to_cover(). Also, data() returns a read-only pointer.

... why am I still awake?