So far, we have seen how to create sequential files and search them to locate information. Sequential files are inappropriate for instant-access applications, in which a particular record must be located immediately. Common instant-access applications are airline reservation systems, banking systems, point-of-sale systems, automated teller machines and other kinds of transaction-processing systems that require rapid access to specific data. A bank might have hundreds of thousands (or even millions) of other customers, yet, when a customer uses an automated teller machine, the program checks that customer's account in a few seconds or less for sufficient funds. This kind of instant access is made possible with random-access files. Individual records of a random-access file can be accessed directly (and quickly) without having to search other records.
As we have said, C++ does not impose structure on a file. So the application that wants to use random-access files must create them. A variety of techniques can be used. Perhaps the easiest method is to require that all records in a file be of the same fixed length. Using same-size, fixed-length records makes it easy for a program to calculate (as a function of the record size and the record key) the exact location of any record relative to the beginning of the file. We soon will see how this facilitates immediate access to specific records, even in large files.
Figure 17.9 illustrates C++'s view of a random-access file composed of fixed-length records (each record, in this case, is 100 bytes long). A random-access file is like a railroad train with many same-size carssome empty and some with contents.
Figure 17.9. C++ view of a random-access file.
Data can be inserted into a random-access file without destroying other data in the file. Data stored previously also can be updated or deleted without rewriting the entire file. In the following sections, we explain how to create a random-access file, enter data into the file, read the data both sequentially and randomly, update the data and delete data that is no longer needed.