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17.3. Files and Streams
C++ views each file as a sequence of bytes (Fig. 17.2). Each file ends either with an end-of-file marker or at a specific byte number recorded in a system-maintained, administrative data structure. When a file is opened, an object is created, and a stream is associated with the object. In Chapter 15, we saw that objects cin, cout, cerr and clog are created when <iostream> is included. The streams associated with these objects provide communication channels between a program and a particular file or device. For example, the cin object (standard input stream object) enables a program to input data from the keyboard or from other devices, the cout object (standard output stream object) enables a program to output data to the screen or other devices, and the cerr and clog objects (standard error stream objects) enable a program to output error messages to the screen or other devices.
Figure 17.2. C++'s view of a file of n bytes.
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To perform file processing in C++, header files <iostream> and <fstream> must be included. Header <fstream> includes the definitions for the stream class templates basic_ifstream (for file input), basic_ofstream (for file output) and basic_fstream (for file input and output). Each class template has a predefined template specialization that enables char I/O. In addition, the fstream library provides a set of typedefs that provide aliases for these template specializations. For example, the typedef ifstream represents a specialization of basic_ifstream that enables char input from a file. Similarly, typedef ofstream represents a specialization of basic_ofstream that enables char output to files. Also, typedef fstream represents a specialization of basic_fstream that enables char input from, and output to, files.
Files are opened by creating objects of these stream template specializations. These templates "derive" from class templates basic_istream, basic_ostream and basic_iostream, respectively. Thus, all member functions, operators and manipulators that belong to these templates (which we described in Chapter 15) also can be applied to file streams. Figure 17.3 summarizes the inheritance relationships of the I/O classes that we have discussed to this point.
Figure 17.3. Portion of stream I/O template hierarchy.