C++ Primer Plus

The ISO/ANSI C++98 Standard revised I/O a couple ways. First, there’s the change from ostream.h to ostream, with ostream placing the classes in the std namespace. Second, the I/O classes have been rewritten. To be an international language, C++ has to be able to handle international character sets that require a 16-bit or wider character type. So the language added the wchar_t (or “wide”) character type to the traditional 8-bit char (or “narrow”) type. C++11 adds char16_t and char32_t types. Each type needs its own I/O facilities. Rather than develop two (or, now, four) separate sets of classes, the standards committee developed a template set of I/O classes, including basic_istream > and basic_ostream >. The traits template, in turn, is a template class that defines particular traits for a character type, such as how to compare for equality and its EOF value. The C++11 Standard provides char and wchar_t specializations of the I/O classes. For example, istream and ostream are typedefs for char specializations. Similarly, wistream and wostream are wchar_t specializations. For example, there is a wcout object for outputting wide character streams. The ostream header file contains these definitions.

Certain type-independent information that used to be kept in the ios base class has been moved to the new ios_base class. This includes the various formatting constants such as ios::fixed, which is now ios_base::fixed. Also ios_base contains some options that aren’t available in the old ios.

The C++ iostream class library takes care of many details for you. For example, including the iostream file in a program creates eight stream objects (four for narrow character streams and four for wide character streams) automatically:

• The cin object corresponds to the standard input stream. By default, this stream is associated with the standard input device, typically a keyboard. The wcin object is similar but works with the wchar_t type.

• The cout object corresponds to the standard output stream. By default, this stream is associated with the standard output device, typically a monitor. The wcout object is similar but works with the wchar_t type.

• The cerr object corresponds to the standard error stream, which you can use for displaying error messages. By default, this stream is associated with the standard output device, typically a monitor, and the stream is unbuffered. This means that information is sent directly to the screen, without waiting for a buffer to fill or for a newline character. The wcerr object is similar but works with the wchar_t type.

• The clog object also corresponds to the standard error stream. By default, this stream is associated with the standard output device, typically a monitor, and the stream is buffered. The wclog object is similar but works with the wchar_t type.

What does it mean to say that an object represents a stream? Well, for example, when the iostream file declares a cout object for a program, that object has data members holding information relating to output, such as the field widths to be used in displaying data, the number of places after the decimal to use, what number base to use for displaying integers, and the address of a streambuf object that describes the buffer used to handle the output flow. A statement such as the following places the characters from the string "Bjarne free" into the buffer managed by cout via the pointed-to streambuf object:

cout << "Bjarne free";

The ostream class defines the operator<<() function used in this statement, and the ostream class also supports the cout data members with a variety of other class methods, such as the ones this chapter discusses later. Furthermore, C++ sees to it that the output from the buffer is directed to the standard output, usually a monitor, provided by the operating system. In short, one end of a stream is connected to the program, the other end is connected to the standard output, and the cout object, with the help of a type streambuf object, manages the flow of bytes through the stream.

Redirection