1. Data File Handling
in C++
Edited by:
Dr. Muhammad Tanvir Afzal
Computer Science
Muhammad Ali Jinnah University
Islamabad
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Topics - Agenda
• Introduction
• Opening & closing of files
• Stream state member functions
• File operations
• Binary file operations
• Random access file operations
• Conclusion

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Introduction
• Computer programs are associated to work
with files as it helps in storing data &
information permanently.
• File - itself a bunch of bytes stored on
some storage devices.
• In C++ this is achieved through a
component header file called fstream.h
• The I/O library manages two aspects- as
interface and for transfer of data.
• The library predefine a set of operations
for all file related handling through certain
classes.

4. The fstream.h header file
Streams act as an interface between files and
programs.
They represent as a sequence of bytes and deals with
the flow of data.
Every stream is associated with a class having member
functions and operations for a particular kind of data
flow.
File  Program ( Input stream) - reads
Program  File (Output stream) – write
All designed into fstream.h and hence needs to be
included in all file handling programs.
Diagrammatically as shown in next slide

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6. File Handling Classes
Hierarchy Diagram

7. Why to use Files:
•Convenient way to deal large quantities of data.
•Store data permanently (until file is deleted).
•Avoid typing data into program multiple times.
•Share data between programs.
We need to know:
how to "connect" file to program
how to tell the program to read data
how to tell the program to write data
error checking and handling EOF

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// Initial experience reading and writing files
#include <fstream.h>
#include <iostream.h>
#include <stdlib.h>
int main()
{ ifstream in_stream;
ofstream out_stream;
int num;
in_stream.open("numbers.dat");
if (in_stream.fail()) { cout << "Input file could not be opened.n";
exit(1); }
out_stream.open("squares.dat");
if (out_stream.fail()) { cout <<"Output file could not opened.n";
exit(1); }
in_stream >> num;
out_stream << "The square of " << num << " is " <<num * num;
in_stream.close();
out_stream.close();
}

9. File Handling Classes
• When working with files in C++, the following
classes can be used:
– ofstream – writing to a file
– ifstream – reading for a file
– fstream – reading / writing
• What does it all have to do with cout?
– When ever we include <iostream.h>, an ostream
object, pointing to stdout is automatically defined –
this object is cout.
• ofstream inherits from the class ostream
(standard output class).
• ostream overloaded the operator >> for standard
output.…thus an ofstream object can
use methods and operators defined in
ostream.

10. Opening & Closing a File
 A file can be open by the method “open()” or
immediately in the constructor (the natural and
preferred way).
void ofstream / ifstream::open(const char* filename, int mode);
 filename – file to open (full path or local)
 mode – how to open (1 or more of following – using | )
ios::app – append
ios::ate – open with marker at the end of the file
ios::in / ios::out – (the defaults of ifstream and
ofstream)
ios:nocreate / ios::noreplace – open only if the file
exists / doesn’t exist
ios::trunc – open an empty file
ios::binary – open a binary file (default is textual)
 Don’t forget to close the file using the method
“close()”

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1: To access file handling routines:
#include <fstream.h>
2: To declare variables that can be used to access file:
ifstream in_stream;
ofstream out_stream;
3: To connect your program's variable (its internal name) to
an external file (i.e., on the Unix file system):
in_stream.open("infile.dat");
out_stream.open("outfile.dat");
4: To see if the file opened successfully:
if (in_stream.fail())
{ cout << "Input file open failedn";
exit(1); // requires <stdlib.h>}

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5: To get data from a file (one option), must declare
a variable to hold the data and then read it using the
extraction operator:
int num;
in_stream >> num;
[Compare: cin >> num;]
6: To put data into a file, use insertion operator:
out_stream << num;
[Compare: cout << num;]
NOTE: Streams are sequential – data is read and
written in order – generally can't back up.
7: When done with the file:
in_stream.close();
out_stream.close();

13. Stream state member functions
•In C++, file stream classes inherit a stream state
member from the ios class, which gives out the
information regarding the status of the stream.
For e.g.:
–eof() –used to check the end of file character
–fail()- used to check the status of file at opening
for I/O
–bad()- used to check whether invalid file
operations or unrecoverable error .
–good()- used to check whether the previous file
operation has been successful

14. File operations
The following member functions are used
for reading and writing a character from a
specified file.
get()- is used to read an alphanumeric
character from a file.
put()- is used to write a character to a
specified file or a specified output stream

15. Reading /Writing from/to
Binary Files
• To write n bytes:
– write (const unsigned char* buffer, int n);
• To read n bytes (to a pre-allocated buffer):
– read (unsighed char* buffer, int num)
#include <fstream.h>
main()
{
int array[] = {10,23,3,7,9,11,253};
ofstream OutBinaryFile("my_b_file.txt“, ios::out |
ios::binary);
OutBinaryFile.write((char*) array, sizeof(array));
OutBinaryFile.close();
}

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C++ has some low-level facilities for character I/O.
char next1, next2, next3;
cin.get(next1);
Gets the next character from the keyboard. Does not skip over
blanks or newline (n). Can check for newline (next == 'n')
Example:
cin.get(next1);
cin.get(next2);
cin.get(next3);
Predefined character functions must #include <ctype.h> and can be
used to
convert between upper and lower case
test whether in upper or lower case
test whether alphabetic character or digit
test for space
CHARACTER I/O

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//#include, prototypes, void main() omitted for space
ifstream fin;
char Chem1, Chem2;
double ratio;
fin.open("input.dat"); // open error check omitted for space
fin.get(Chem1);
while (!fin.eof())
{
if (isdigit(Chem1))
cout << "Test Code: " << Chem1 << endl;
else
{
fin >> ratio;
fin.get(Chem2);
Chem2 = toupper(Chem2);
cout << "Ratio of " << Chem1 << " to " << Chem2 << " is " << ratio << endl;
}
new_line(fin);
fin.get(Chem1);
}
}
void new_line(istream& in)
{
char symbol;
do {
in.get(symbol);
} while (symbol != 'n');
}

18. Reading /Writing from/to Textual Files
• To write:
– put() – writing single
character
– << operator – writing
an object
• To read:
– get() – reading a
single character of a
buffer
– getline() – reading a
single line
– >> operator –
reading a object
#include <fstream.h>
main()
{
// Writing to file
ofstream OutFile("my_file.txt");
OutFile<<"Hello "<<5<<endl;
OutFile.close();
int number;
char dummy[15];
// Reading from file
ifstream InFile("my_file.txt");
InFile>>dummy>>number;
InFile.seekg(0);
InFile.getline(dummy,sizeof(dummy));
InFile.close();
}

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Binary file operations
In connection with a binary file, the file
mode must contain the ios::binary mode
along with other mode(s)
To read & write a or on to a binary
file, as the case may be blocks of
data are accessed through the use
of C++ read() and write()
respectively.

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Random access file
operations
Every file maintains two internal pointers:
get_pointer and put_pointer
They enable to attain the random access in file
otherwise which is sequential in nature.
In C++ randomness is achieved by manipulating
certain functions

21. Moving within the File
• seekg() / seekp() – moving the reading
(get) / writing (put) marker
– two parameters: offset and anchor
• tellg() / tellp() – getting the position
of the reading (get) / writing (put)
marker

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EOF and File I/O Within Functions
When using a file within a function, the file parameter
MUST BE a reference parameter:
int read_file(ifstream& infile);
As with keyboard input, it is often desirable to process
some unknown amount of data. We use the end-of-file
(EOF) to accomplish this. EOF is automatically included
in text files we create.
Can test for EOF in several ways.
while (in_stream >> num)
OR
in_stream >> num; // priming read
while (! in_stream.eof())
{loop body
in_stream >> num;}

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Summary
• Files in C++ are interpreted as a sequence of
bytes stored on some storage media.
• Bases classes are used to perform I/O
operations.
• The data of a file is stored in either
readable form or in binary code called as
text file or binary file.
• The flow of data from any source to a sink
is called as a stream.

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1. When getting data from a file, there is no need to prompt for input.
2. One program may have multiple input and/or output files, and may
intermix keyboard/ display I/O with file I/O.
3. The file name may be obtained from the user, rather than hard coded in
the program.
4. The layout of a program's output is called the format. A variety of options
are available for controlling the appearance of the output.
5. Flags to control floating point display:
• out_stream.setf(ios::fixed);
• out_stream.setf(ios::showpoint);
• out_stream.precision(2);
6. rd_state() – returns a variable with one or more (check with AND) of the
following options:
• ios::goodbit – OK
• ios::eofbit – marker on EOF
• ios::failbit – illegal action, but alright to continue
• ios:badbit – corrupted file, cannot be used.
7. We can also access the bit we wish to check with eof(), good(), fail(),
bad(),
8. clear() is used to clear the status bits (after they were checked).
More Information on File I/O