Storage of data in memory is temporary. Files are used for data persistence—permanent retention of data. Computers store files on secondary storage devices, such as hard disks, CDs, DVDs, flash drives and tapes. In this chapter, we explain how to build C++ programs that create, update and process data files. We consider both sequential files and random-access files. We compare formatted-data file processing and raw-data file processing. We’ve already shown how to output data to a string in memory using an ostringstream—Chapter 21 continues that discussion and demonstrates how to read data from strings in memory using an istringstream.