17. Input, Output, and Files

In this chapter you’ll learn about the following:

• The C++ view of input and output

• The iostream family of classes

• Redirection

• ostream class methods

• Formatting output

• istream class methods

• Stream states

• File I/O

• Using the ifstream class for input from files

• Using the ofstream class for output to files

• Using the fstream class file input and output

• Command-line processing

• Binary files

• Random file access

• Incore formatting

Discussing C++ input and output (I/O) poses a problem. On the one hand, practically every program uses input and output, and learning how to use them is one of the first tasks facing someone learning a computer language. On the other hand, C++ uses many of its more advanced language features to implement input and output, including classes, derived classes, function overloading, virtual functions, templates, and multiple inheritance. Thus, to really understand C++ I/O, you must know a lot of C++. To get you started, the early chapters of this book outline the basic ways for using the istream class object cin and the ostream class object cout for input and output, and, to a lesser degree, using ifstream and ofstream objects for file input and output. This chapter takes a longer look at C++’s input and output classes, showing how they are designed and explaining how to control the output format. (If you’ve skipped a few chapters just to learn advanced formatting, you can read the sections on formatting, noting the techniques and ignoring the explanations.)

The C++ facilities for file input and output are based on the same basic class definitions that cin and cout are based on, so this chapter uses the discussion of console I/O (keyboard and screen) as a springboard to investigating file I/O.

The ANSI/ISO C++ standards committee has worked to make C++ I/O more compatible with existing C I/O, and this has produced some changes from traditional C++ practices.

An Overview of C++ Input and Output

Most computer languages build input and output into the language itself. For example, if you look through the lists of keywords for languages such as BASIC and Pascal, you see that PRINT statements, writeln statements, and the like are part of the language vocabulary. But neither C nor C++ has built input and output into the language. If you look through the keywords for these languages, you find for and if but nothing relating to I/O. C originally left I/O to compiler implementers. One reason for this was to give implementers the freedom to design I/O functions that best fit the hardware requirements of the target computer. In practice, most implementers based I/O on a set of library functions originally developed for the Unix environment. ANSI C formalized recognition of this I/O package, called the Standard Input/Output package, by making it a mandatory component of the standard C library. C++ also recognizes this package, so if you’re familiar with the family of C functions declared in the stdio.h file, you can use them in C++ programs. (Newer implementations use the cstdio header file to support these functions.)

However, C++ relies on a C++ solution rather than a C solution to I/O, and that solution is a set of classes defined in the iostream (formerly iostream.h) and fstream (formerly fstream.h) header files. This class library is not part of the formal language definition (cin and istream are not keywords); after all, a computer language defines rules for how to do things, such as create classes, and doesn’t define what you should create by following those rules. But just as C implementations come with a standard library of functions, C++ comes with a standard library of classes. At first, that standard class library was an informal standard consisting solely of the classes defined in the iostream and fstream header files. The ANSI/ISO C++ committee decided to formalize this library as a standard class library and to add a few more standard classes, such as those discussed in Chapter 16, “The string Class and the Standard Template Library.” This chapter discusses standard C++ I/O. But first, let’s examine the conceptual framework for C++ I/O.