Strings with known encoding that can transfer into other strings with a known encoding easily and readable (and in some cases without any interaction) would be useful.
For instance,
Stdio.FILE x = ...; x->set_encoding("utf8");
string s = "räksmörgås"; String t = String.JP2022("\33(BHello, world!"); x->write(s); x->write(t); x->write(s+t);