Yep. stringstreams make copies of passed strings too. I had to hack around that once like this:

  struct ZeroCopyBuf : public std::streambuf
  {
   ZeroCopyBuf(const std::string &s) : ZeroCopyBuf(s.c_str(), s.length()) {}
   ZeroCopyBuf(const char *c, std::size_t l) : ZeroCopyBuf(const_cast<char*>(c), l) {}
   ZeroCopyBuf(char *c, std::size_t l) { setg(c, c, c + l); }
  };
  ...
  std::string s ...;
  ZeroCopyBuf buf(s);
  std::istream is(&buf);

Terrible. C++-20 makes it better by adding move semantics to most methods... Still.