Unit 4
Files - Introduction
A file is collection of data or information that has a name, called the filename. Files are
stored in secondary storage devices such as floppy disks and hard disks.
The main memories of a computer such as random access memory or read-only memory
are not used for the storage of files. This is because the main memory of a computer is limited and
cannot hold a large amount of data. Another reason is that the main memory is volatile; that is,
when the computer is switched off, the contents of RAM vanish.
Fig. Communication between program, file, and output device
As shown in Figure, the data read from the keyboard are stored in variables. Variables are
created in RAM (type of primary memory).. It is also possible to read data from secondary storage
devices. When data are read from such devices, they are placed in the RAM and then, console I/O
operations are used to transfer them to the screen. RAM is used to hold data temporarily.
1
, Data communication can be performed between programs and output devices or between
files and programs. File streams are used to carry the communication among the above-mentioned
devices. The stream is nothing but a flow of data in bytes in sequence. If data were received from
input devices in sequence, then it is called a source stream, and if the data were passed to output
devices, then it is called a destination stream. Figure shows the input and output streams. The
input stream brings data to the program, and the output stream collects data from the program. In
another way, the input stream extracts data from the file and transfers it to the program; whereas
the output stream stores the data in the file provided by the program.
Fig. Input and output streams
File Stream Classes
A stream is nothing but a flow of data. In the object-oriented programming, the streams are
controlled using the classes.
2
, The ios class is the base class. All other classes are derived from the ios class. These
classes contain several member functions that perform input and output operations. The
streambuf class has low-level routines and provides interface to physical devices.
The istream and ostream classes control input and output functions, respectively. The ios
is the base class of these two classes. The functions get(), getline(), and read() and overloaded
extraction operators (>>) are defined in the istream class. The functions put(), write(), and
overloaded insertion operators (<<) are defined in the ostream class. The iostream class is also a
derived class. It is derived from istream and ostream classes. The classes ifstream, ofstream and
fstream are derived from istream ,ostream and iostream respectively. These classes handle input
and output with the disk files. The header file fstream.h contains a declaration of ifstream,
ofstream, and fstream classes, including isotream.h file. This file should be included in the
program while doing disk I/O operations.
Details of File Stream Classes:
Class Description
Sets the file buffers to read and write. It holds constant openprot used in
filebuf function open() and close() as a member.
The fstreambase acts as a base class for fstream, ifstream, and ofstream.
fstreambase The functions such as open() and close() are defined in fstreambase
Provides input operations on files. Contains open() with default input
ifstream mode. Inherits the functions as get(), getline(), seekg(), tellg(), and
read() from istream class
Provides output operations on files. Contains open() with default output
ofstream mode. Inherits the functions as put(), seekp(), write(), and tellp()from
ostream class
Provides support for simultaneous input/output file stream class. Inherits
fstream
all functions from istream and ostream classes through iostream.
3
, Steps of File Operations
Before performing file operations, it is necessary to create a file. The operation of a file
involves the following basic activities:
Specifying suitable file name
Opening the file in desired mode
Reading or writing the file (file processing)
Detecting errors
Closing the file
File Opening: In order to perform operations, we have to create a file stream object and
connecting it with the file name. The classes ifstream, ofstream, and fstream can be used for
creating a file stream. The selection of the class is according to the operation that is to be carried
out with the file. The operation may be read or write. Two methods are used for the opening of a
file. They are as follows:
Constructor of the class
Member function open()
1. Constructor of the class:
When objects are created, a constructor is automatically executed, and objects are
initialized. In the same way, the file stream object is created using a suitable class, and it is
initialized with the file name. The constructor itself uses the file name as the fist argument and
opens the file. The class ofstream creates output stream objects, and the class ifstream creates
input stream objects.
Consider the following examples:
a) ofstream out (“text”);
b) ifstream in(“list”);
In the statement (a), out is an object of the class ofstream; file name text is opened, and
data can be written to this file. The file name text is connected with the object out. Similarly,
in the statement (b), in is an object of the class ifstream. The file list is opened for input and
4
Files - Introduction
A file is collection of data or information that has a name, called the filename. Files are
stored in secondary storage devices such as floppy disks and hard disks.
The main memories of a computer such as random access memory or read-only memory
are not used for the storage of files. This is because the main memory of a computer is limited and
cannot hold a large amount of data. Another reason is that the main memory is volatile; that is,
when the computer is switched off, the contents of RAM vanish.
Fig. Communication between program, file, and output device
As shown in Figure, the data read from the keyboard are stored in variables. Variables are
created in RAM (type of primary memory).. It is also possible to read data from secondary storage
devices. When data are read from such devices, they are placed in the RAM and then, console I/O
operations are used to transfer them to the screen. RAM is used to hold data temporarily.
1
, Data communication can be performed between programs and output devices or between
files and programs. File streams are used to carry the communication among the above-mentioned
devices. The stream is nothing but a flow of data in bytes in sequence. If data were received from
input devices in sequence, then it is called a source stream, and if the data were passed to output
devices, then it is called a destination stream. Figure shows the input and output streams. The
input stream brings data to the program, and the output stream collects data from the program. In
another way, the input stream extracts data from the file and transfers it to the program; whereas
the output stream stores the data in the file provided by the program.
Fig. Input and output streams
File Stream Classes
A stream is nothing but a flow of data. In the object-oriented programming, the streams are
controlled using the classes.
2
, The ios class is the base class. All other classes are derived from the ios class. These
classes contain several member functions that perform input and output operations. The
streambuf class has low-level routines and provides interface to physical devices.
The istream and ostream classes control input and output functions, respectively. The ios
is the base class of these two classes. The functions get(), getline(), and read() and overloaded
extraction operators (>>) are defined in the istream class. The functions put(), write(), and
overloaded insertion operators (<<) are defined in the ostream class. The iostream class is also a
derived class. It is derived from istream and ostream classes. The classes ifstream, ofstream and
fstream are derived from istream ,ostream and iostream respectively. These classes handle input
and output with the disk files. The header file fstream.h contains a declaration of ifstream,
ofstream, and fstream classes, including isotream.h file. This file should be included in the
program while doing disk I/O operations.
Details of File Stream Classes:
Class Description
Sets the file buffers to read and write. It holds constant openprot used in
filebuf function open() and close() as a member.
The fstreambase acts as a base class for fstream, ifstream, and ofstream.
fstreambase The functions such as open() and close() are defined in fstreambase
Provides input operations on files. Contains open() with default input
ifstream mode. Inherits the functions as get(), getline(), seekg(), tellg(), and
read() from istream class
Provides output operations on files. Contains open() with default output
ofstream mode. Inherits the functions as put(), seekp(), write(), and tellp()from
ostream class
Provides support for simultaneous input/output file stream class. Inherits
fstream
all functions from istream and ostream classes through iostream.
3
, Steps of File Operations
Before performing file operations, it is necessary to create a file. The operation of a file
involves the following basic activities:
Specifying suitable file name
Opening the file in desired mode
Reading or writing the file (file processing)
Detecting errors
Closing the file
File Opening: In order to perform operations, we have to create a file stream object and
connecting it with the file name. The classes ifstream, ofstream, and fstream can be used for
creating a file stream. The selection of the class is according to the operation that is to be carried
out with the file. The operation may be read or write. Two methods are used for the opening of a
file. They are as follows:
Constructor of the class
Member function open()
1. Constructor of the class:
When objects are created, a constructor is automatically executed, and objects are
initialized. In the same way, the file stream object is created using a suitable class, and it is
initialized with the file name. The constructor itself uses the file name as the fist argument and
opens the file. The class ofstream creates output stream objects, and the class ifstream creates
input stream objects.
Consider the following examples:
a) ofstream out (“text”);
b) ifstream in(“list”);
In the statement (a), out is an object of the class ofstream; file name text is opened, and
data can be written to this file. The file name text is connected with the object out. Similarly,
in the statement (b), in is an object of the class ifstream. The file list is opened for input and
4
