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12 Stream I/O In this chapter: Stream Classes The ios Class ios State Variables The istream and ostream Classes Unformatted Input Functions Unformatted Output Functions Stream Manipulators Stream Classes The C++ programming language does not include any input/output facilities. These are supplied by using standard libraries. We have used the directive #include <iostream.h> in every program that does I/O. This includes the iostream.h header file that includes the definitions for the I/O library function. This chapter describes in more detail the contents of this library and how it is used.
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The I/O library defines a hierarchies of stream classes.
The iostream class is the one that we usually use for ordinary I/O. Note that it is a subclass of both the istream and the ostream classes, both of which are subclasses of the ios base class. The classes with "fstream" in their name are used for file processing. The ios Class The ios class serves as the base class for the other stream classes. Its primary purpose is to control the buffer for whatever stream object has been instantiated. This means that the stream controls how many characters are inserted into or extracted from the buffer. To do that, the ios object maintains a collection of data elements that control I/O behavior. They include such things as the number base (octal, decimal, hexadecimal) that is used, the width of the display field, the number of digits displayed for floating point numbers, etc. We shall examine how to interface with the ios class. ios State Variables Every stream has a _state data member that is defined in the ios class. The _state member is a bit string that holds several Boolean variables. These state variables are specified in the enum definition: enum { goodbit=0, // all ok
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eofbit = 01, failbit = 02, badbit = 04 };
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// end of file // last operation failed // invalid operation
A stream's format flags can only be changed explicitly, and only by means of the access functions described below. In contrast, a stream's state variables are changed implicitly, a result of I/O operations. For example, when a user inputs Control-Z to indicate end-of-file, the cin's eof flag is set, and we say that the stream is an eof state.
A stream's four state variables (goodbit, eofbit, failbit, and badbit) can be accessed individually by their access functions (good(), eof(), fail (), and bad ()). State variables are generally used to read the current state of the stream. The stream conversion operator () is overloaded to return 0 if the state is nonzero. So for example, if in is an input stream, then the expression (in) will evaluate to true if none of the flags are set (i. e., there is still more input), and false otherwise. The second of these access functions overloads the negation operator. It simply calls fail () and returns its return value, which will be nonzero unless both the failbit and the badbit are clear. The advantage of this alternate form for determining whether the stream can be used any more is that, like the conversion operator above, this form can be used conveniently in conditional expressions. Example 12.1 Operator operator void* () for Loop Control int n, sum = 0; cin >> n; while (cin) { // loop continues while _state sum += n; cin >> n; }
44 11 22 ^Z sum = 77
Using Control-Z to terminate input is simple and convenient. Pressing this key sets the eofbit in the input stream. If you want to use it again in the program, it has to be cleared first. This is done with the member function clear (), as: cin. clear ();
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The istream and ostream Classes The istream and ostream classes both inherit from the iosclass: class istream : virtual public ios { //. }; class ostream : virtual public ios { // . Making ios a virtual base class facilitates the multiple inheritance that the iostream class has from both the istream and ostream classes by preventing multiple copies of the ios class to be made for the lostream class. The istream class defines the cin object and the stream extraction operator >> for formatted input. The ostream class defines the cout, cerr, and clog objects and the stream insertion operator << for formatted output. The familiar I/O operations that use the extraction and insertion operators are called formatted I/0 because they recognize the types of the objects accessed and format the data accordingly. For example, if n is an integer with value 22, then cout < n prints the value 22 in integer format. The istream and ostream classes also define a set of member functions for unformatted I/O desribed briefly in the next section that handles data simply as a sequence of bytes. The istream class defines the stream extraction operator >> which reads data from istream objects, which are usually the standard input device cin (i.e., the keyboard). If successful, this operator returns a reference to the object so that calls can be chained like cin >> x >> y >> z; If cin is unsuccessful, it returns 0. Under normal operation, cin skips white space characters (blanks, tabs, newlines, etc.). The >> operator will return 0 when it encounters the end-of-file character. This can be used to control an input loop: Example 12.2 Controlling an Input Loop
int n, sum = 0; while (cin >> n) sum += n; cout <<"The sum is " <<sum <<endl;
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