void clearField(int fieldno);

The function clearField() removes all data from a certain field and fills it with zeroes (0x00) or spaces (0x20).
You can call this function before calling setField(), setMemoBuf().

{
    // your code
    dbf->clearField(0);
    dbf->setField(0, "some value");
    dbf->writeField(row, 0);
    // your code
}