The compiler is free to elide this test, since (this == NULL) can never be true; applying an ordinary member function to NULL invokes undefined behavior. This function needs to be changed.

char* Metadata::print_value_string() const {
  char buf[256];
  stringStream st(buf, sizeof(buf));
  if (this == NULL) {
    st.print("NULL");
  } else {
    print_value_on(&st);
  }
  return st.as_string();
}