[ostream.seeks]
31 Input/output library [input.output]
31.7 Formatting and manipulators [iostream.format]
31.7.6 Output streams [output.streams]
31.7.6.2 Class template basic_ostream [ostream]
31.7.6.2.5 Seek members [ostream.seeks]
Each seek member function begins execution by constructing an object of class sentry.
It returns by destroying the sentry object.
Returns: If fail() != false, returns pos_type(-1) to indicate failure.
Otherwise, returns rdbuf()->pubseekoff(0, cur, out).
basic_ostream& seekp(pos_type pos);
Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::out).
In case of failure, the function calls setstate(failbit) (which may throw ios_base::failure).
basic_ostream& seekp(off_type off, ios_base::seekdir dir);