1. Chapter 15

2. Objectives You should be able to describe:
I/O File Stream Objects and Methods
Reading and Writing Character-Based Files
Exceptions and File Checking
Random File Access
File Streams as Function Arguments
Common Programming Errors

3. I/O File Stream Objects and Methods
File: Collection of data that is stored together under common name, usually on disk, magnetic tape, or CD
C++ programs stored on disk are examples of files
The stored data in program file is program code that becomes input data to C++ compiler
A C++ program is not usually considered data, and term file, or data file, is typically used to refer only to external files that contain the data used in C++ program

4. I/O File Stream Objects and Methods (continued)
External name: Unique file name for file
External name is how operating system knows file
Contents of directory or folder are listed by external names
Format of external names: Each computer operating system has its own specifications for external file size
Table 15.1 lists specifications for more commonly used operating systems

5. I/O File Stream Objects and Methods (continued)

6. I/O File Stream Objects and Methods (continued)
File naming conventions:
Use descriptive names
Avoid long file names
They take more time to type and can result in typing errors
Manageable length for file name is 12 to 14 characters, with maximum of 25 characters
Choose file names that indicate type of data in file and application for which it is used
Frequently, first eight characters describe data, and an extension describes application

7. I/O File Stream Objects and Methods (continued)
More file naming conventions
Using DOS convention, the following are all valid computer data file names:
prices.dat records info.txt
exper1.dat scores.dat math.mem

8. File Stream Objects
File stream: One-way transmission path that is used to connect file stored on physical device, such as disk or CD, to program
Mode (of file stream): Determines whether path will move data from file into program or from program to file
Input file stream: Receives or reads data from file into program
Output file stream: Sends or writes data to file

9. File Stream Objects (continued)
Direction (mode) of file stream is defined in relation to program and not file:
Data that goes into program are considered input data
Data sent out from program are considered output data
Figure 15.1 illustrates data flow from and to file using input and output streams

10. File Stream Objects (continued)

11. File Stream Objects (continued)
Distinct file stream object must be created for each file used, regardless of file’s type
For program to both read and write to file, both an input and output file stream object are required
Input file stream objects are declared to be of type ifstream
Output file streams are declared to be of type ofstream

12. File Stream Objects (continued)
Two basic types of files: both store data using binary code
Text (character-based) files: Store each character using individual character code (typically ASCII or Unicode)
Advantage: Allows files to be displayed by word processing program or text editor
Binary-based files: Store numbers in binary form and strings in ASCII or Unicode form
Advantage: Provides compactness

13. File Stream Methods
Each file stream object has access to methods defined for its respective ifstream or ofstream class, including:
Opening file: connecting stream object name to external file name
Determining whether a successful connection has been made
Closing file: closing connection
Getting next data item into program from input stream
Putting new data item from program onto output stream
Detecting when end of file has been reached

14. File Stream Methods (continued)
open() method:
Establishes physical connecting link between program and file
Operating system function that is transparent to programmer
Connects file’s external computer name to stream object name used internally by program
Before a file can be opened, it must be declared as either ifstream or ofstream object
File opened for input is said to be in read mode

15. File Stream Methods (continued)
Example: inFile.open("prices.dat");
connects external text file named prices.dat to internal program file stream object named inFile
Accesses file using internal object name inFile
Computer saves file under the external name prices.dat
Calling the open() method uses the standard object notation: objectName.open()

16. File Stream Methods (continued)
fail() method: returns true value if file is unsuccessfully opened, false if open succeeded
Good programming practice is to check that connection is established before using file
In addition to fail() method, C++ provides three other methods, listed in Table 15.2, that can be used to detect file’s status
Program 15.1 illustrates statements required to open file for input including error checking routine to ensure that successful open was obtained

17. File Stream Methods (continued)
Example of use of fail() method:
ifstream inFile; // any object name can be used here
inFile.open("prices.dat"); // open the file
// check that the connection was successfully
// opened
if (inFile.fail()) {
cout << "\nThe file was not successfully opened"
<< "\n Please check that the file currently exists."
<< endl;
exit(1); }

18. File Stream Methods (continued)

19. File Stream Methods (continued)

20. File Stream Methods (continued)

21. File Stream Methods (continued)
Different checking required for output files
If file exists having same name as file to be opened in output mode, existing file is erased and all data lost
To avoid this situation, file is first opened in input mode to see if it exists
If it does, user is given choice of explicitly permitting it to be overwritten (when it is later opened in output mode)
Code used to accomplish this is highlighted in Program 15.2

22. Embedded and Interactive File Names
Embedding a file name in program causes problems
No provision for user to enter desired file name during program execution
Any changes require modification of open() method and recompile
These problems can be solved by assigning file name to string variable as shown in Programs 15.3a and 15.3b
String variable: Variable that can hold string value

23. Embedded and Interactive File Names (continued)

24. Embedded and Interactive File Names (continued)

25. Closing a File
close() method: Breaks connection between file’s external name and file stream object
Object can then be used for another file
Good programming practice is to close files no longer needed
Operating system automatically closes any open files at end of normal program execution
Example: The statement inFile.close(); closes inFile stream’s connection to its current file
close() method takes no argument

26. Reading and Writing Character-Based Files
Operations similar to reading input from keyboard and writing data to display screen
For writing to file, the cout object is replaced by ofstream object name declared in program
Example: If outFile is declared as object of type ofstream, the following output statement is valid:
outFile << descrip << ' ' << price;
The file name directs output stream to file instead of standard display device
Example: Program 15.4

27. Reading and Writing Character-Based Files (continued)
Program 15.4 output:
File named prices.dat is created and saved by computer as text file (the default file type)
prices.dat is sequential file consisting of the following data:
Mats 39.95
Bulbs 3.22
Fuses 1.08
Actual storage of characters in file depends on character codes used by computer
Output file file contains 36 characters (Figure 15.2)

28. Reading and Writing Character-Based Files (continued)

29. Reading from a Text File
Almost identical to reading data from standard keyboard
cin object replaced by ifstream object declared in program
Example: The input statement
inFile >> descrip >> price;
reads next two items in file and stores them in variables descrip and price
File stream name directs input to come from file stream rather than standard input device stream

30. Reading from a Text File (continued)
Program 15.5 illustrates how the prices.dat file created in Program 15.4 can be read
Also illustrates method of detecting end-of-file (EOF) marker using good() function (see Table 15.2)
Other methods that can be used for stream input are listed in Table 15.3
Each method must be preceded by stream object name

31. Reading from a Text File (continued)

32. Standard Device Files C++ supports logical and physical file objects
Logical file object: Stream that connects file of logically related data (data file) to a program
Physical file object: Stream that connects to hardware device such as keyboard or printer
Standard input file: Physical device assigned to program for data entry
Standard output file: Physical device on which output is automatically displayed

33. Other Devices
The keyboard, display, error reporting, and logging streams are automatically connected to the stream objects named cin, cout, cerr, clog
Requires iostream header file
Other devices can be used if the name assigned by system is known
Example: Most personal computers assign name prn to printer connected to computer
Statement outFile.open("prn") connects printer to ofstream object named outFile

34. Exceptions and File Checking
Error detection and exception handling are used in C++ programs that access one or more files
General exception handling code (section 7.3)
try {
// one or more statements, at least one
// of which should throw an exception }
catch(exceptionDataType parameterName)
// one or more statements
Program 15.7 illustrates file opening exception handling

35. Opening Multiple Files
Example: Read the data from character-based file named info.txt, one character at a time, and write this data to file named backup.txt
Essentially, this is a file-copy program
Figure 15.4 illustrates structure of streams needed to produce file copy
Program 15.9 creates backup.txt file as an exact duplicate of info.txt file using procedure described in Figure 15.4

36. Opening Multiple Files (continued)

37. Random File Access File access: Retrieving data from file
File organization: The way data is stored in a file
Sequential organization: Characters in file are stored in sequential manner, one after another
Random Access: Any character in an open file can be read directly without having to read characters ahead of it

38. Random File Access (continued)
File position marker: Long integer that represents an offset from the beginning of each file
Keeps track of where next character is to be read from or written to
Allows for random access of any individual character
Table 15.4 shows functions used to access and change the file position marker
Program illustrates use of seek() and tell() to read and display file in reverse order

39. Random File Access (continued)

40. File Streams as Function Arguments
A file stream object can be used as function argument
Function’s formal parameter must be a reference (see Section 6.3) to appropriate stream, either as ifstream& or ofstream&
Example: Program 15.11
ofstream object named outfile is opened in main()
Stream object is passed to the function inOut()
inOut() is used to write five lines of user-entered text to file

41. Common Programming Errors
Using file’s external name in place of internal file stream object name when accessing file
Opening file for output without first checking that file with given name already exists
Not checking for preexisting file ensures that file will be overwritten
Not understanding that end of a file is detected only after EOF sentinel has either been read or passed over

42. Common Programming Errors (continued)
Attempting to detect end of file using character variables for EOF marker
Any variable used to accept EOF must be declared as an integer variable
Using integer argument with the seekg() and seekp() functions
Offset must be a long integer constant or variable
Any other value passed to these functions can result in unpredictable result

43. Summary
A data file is any collection of data stored in an external storage medium under a common name
A data file is connected to file stream using fstream’s open() method
This function connects file’s external name with internal object name
A file can be opened in input or output mode
An opened output file stream either creates a new data file or erases data in an existing opened file

44. Summary (continued)
All file streams must be declared as objects of either the ifstream or ofstream classes
In addition to any files opened within a function, the standard stream objects cin, cout, and cerr are automatically declared and opened when a program is run

45. Summary (continued)
Data files can be accessed randomly using the seekg(), seekp(), tellg(), and tellp() methods
The g versions of these functions are used to alter and query file position marker for input file streams
The p versions do the same for output file streams
Table 15.5 lists the methods supplied by the fstream class for file manipulation