Csc1100 lecture15 ch09

A First Book of C++A First Book of C++
Chapter 9Chapter 9
I/O Streams and Data FilesI/O Streams and Data Files

∗ In this chapter, you will learn about:
∗ I/O File Stream Objects and Methods
∗ Reading and Writing Text Files
∗ Random File Access
∗ File Streams as Function Arguments
∗ Common Programming Errors
∗ The iostream Class Library
A First Book of C++ 4th Edition 2
Objectives

∗ To store and retrieve data outside a C++ program, you
need two things:
∗ A file
∗ A file stream object
I/O File Stream Objects and Methods

∗ File: collection of data stored together under
common name, usually on disk, USB drive, or CD/DVD
∗ C++ programs stored on disk are examples of files
∗ Stored data in program file is the code that becomes
input data to C++ compiler
∗ A C++ program is not usually considered data file
∗ Data file typically refers only to files containing the
data used in C++ program
Files

∗ External name: unique filename for file
∗ External name is how operating system knows file
∗ Contents of directory or folder are listed by external
names
∗ Each computer operating system has its own
specifications for external filename size
∗ Table 9.1 lists specifications for more commonly used
operating systems
Files (cont'd.)

Files (cont'd.)

∗ Use descriptive names
∗ Avoid long filenames
∗ They take more time to type and can result in typing
errors
∗ Manageable length for filename is 12 to 14 characters,
with maximum of 25 characters
∗ Choose filenames 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
Files (cont'd.)

∗ Using DOS convention, the following are all valid
computer data filenames:
prices.dat records info.txt
exper1.dat scores.dat math.mem
Files (cont'd.)

∗ 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
Files (cont'd.)

∗ File stream: one-way transmission path used to
connect a file to a program
∗ Mode (of file stream): determines whether path will
move data from file into program or from program to
file
∗ Input file stream: used to transfer data from a file to a
program
∗ Output file stream: sends data from a program to a file
File Stream Objects

∗ 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 9.1 illustrates data flow from and to file using
input and output file streams
File Stream Objects (cont'd.)

∗ 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

∗ 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
filename
∗ 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
File Stream Methods

∗ 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
∗ Provided by the ifstream and ofstream classes
∗ File opened for input is said to be in read mode
File Stream Methods (cont'd.)

∗ 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()
user-defined variable

∗ 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 9.2, that can be used to
detect file’s status
∗ Program 9.1 illustrates statements required to open
file for input, including error-checking routine to
ensure that successful open was obtained

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

∗ 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
9.2 (refer textbook)

∗ Embedding a filename in program causes problems
∗ No provision for user to enter desired filename during
program execution
∗ Any changes require modification of open() method
and recompile
∗ These problems can be solved by assigning filename
to string variable, as shown in Programs 9.3a and 9.3b

// ask user to enter filename
// declare inFile of type ifstream (input stream)

∗ 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: inFile.close(); closes inFile
stream’s connection to its current file
∗ close() method takes no argument

∗ 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 filename directs output stream to file instead of standard
display device
∗ Example: Program 9.4
Reading and Writing Text Files

Program 9.4

Program 9.4(cont…)

∗ Program 9.4 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 contains 36 characters (Figure 9.2)
(cont'd.)

(cont'd.)

∗ 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 the keyboard
Reading from a Text File

∗ Program 9.5 illustrates how the prices.dat file
created in Program 9.4 can be read
∗ Also illustrates method of detecting end-of-file (EOF)
marker using good() function (see Table 9.2)
∗ Other methods that can be used for stream input are
listed in Table 9.3
∗ Each method must be preceded by stream object name
Reading from a Text File (cont'd.)

Program 9.5
#include <iostream>
#include <fstream> // file stream
#include <cstdlib> // needed for exit
#include <string>
using namespace std;
int main(){
string filename = "prices.dat"; // put the filename up front
string descrip;
double price;
ifstream inFile;
inFile.open(filename.c_str());
if(inFile.fail()) { // check for successful open
cout << "The file is not successfully opened"
<< "n Please check that the file currently exists."
<< endl;
exit(1);
}

Program 9.5 (cont…)
//read and display file’s contents
while (inFile.good()) // check next character
{
cout << descrip << ' ' << price << endl;
}
inFile.close();
return 0;
}

Reading from a Text File (cont'd.)

∗ 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
Standard Device Files

∗ The keyboard, display, error, and log 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
Other Devices

∗ 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
Random File Access

∗ 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 9.4 shows functions used to access and change the
file position marker
∗ Program 9.7 illustrates use of seekg() and tellg() to
read and display file in reverse order (refer textbook, Pg
418)
Random File Access (cont'd.)

Random File Access (cont'd.)

∗ A file stream object can be used as function argument
∗ Function’s formal parameter must be a reference (see
Section 6.3) to correct stream, either as ifstream& or
ofstream&
∗ Example: Program 9.8
∗ 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
File Streams as Function Arguments

Program 9.8

Program 9.8 (cont…)

∗ Forgetting to open a file before attempting to read from it
or write to it
∗ 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
Common Programming Errors

∗ 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
an unpredictable result
Common Programming Errors
(cont'd.)

∗ 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
Summary

∗ 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
Summary (cont'd.)

∗ 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 9.5 lists the methods supplied by the fstream
class for file manipulation
Summary (cont'd.)

∗ iostream class library accesses files by using
entities called streams
∗ For most systems, the data bytes transferred on a
stream represent ASCII characters or binary numbers
∗ Mechanism for reading a byte stream from a file or
writing a byte stream to a file is hidden when using a
high-level language, such as C++
Chapter Supplement: The iostream
Class Library

File Stream Transfer Mechanism

Components of the iostream Class
Library

∗ Class strstream is derived from the ios class
∗ Uses the strstreambuf class shown in Figure 9.5
∗ Requires the strstream header file
∗ Provides capabilities for writing and reading strings to and
from in-memory defined streams
∗ In-memory streams are typically used to “assemble” a
string from smaller pieces
∗ Until a complete line of characters is ready to be written to
cout or to a file
∗ Attaching a strstream object to a buffer is similar to
attaching an fstream object to an output file
In-Memory Formatting

Csc1100 lecture15 ch09

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