Calling the function tie() without any argument returns a pointer to the output stream that is
currently tied to a stream. To tie a new output stream to a stream, a pointer to that output stream
must be passed as the argument to tie(). The argument is defined to be a pointer because you can
also pass nullptr (or 0 or NULL) as an argument. This argument means “no tie” and unties any tied
output stream. If no output stream is tied, tie() returns nullptr or 0. For each stream, you can
have only one output stream that is tied to this stream. However, you can tie an output stream to
different streams.