File I/O

Stream Objects

cin – the standard input stream - an object of the istream class,

cout – the standard output stream – an object of the ostream class

These streams are automatically created for you whenyour program executes. To use them you only need to#include <iostream> and the appropriate using directives.

You have been introduced to these stream objects …

To do file I/O, we will use a newset of stream classes

ifstream – objects of this class represent file input

ofstream – objects of this class represent file output

File I/O

When a program takes input from a file, we say thatit reads from the file.

When a program puts data into a file, we say that it writes to the file.

To read or write to a file, we create a stream object, and connect it to the file.

Text Files

Data in a file can either be text or binary.

Everything in a text file appears as readablecharacters. You can look at the file with a texteditor, such as notepad.

Text files are sometimes referred to as Formatted files.

This semester we will only deal with text files.

The fstream classes

We use objects of the ifstream class to read from a file, and objects of the ofstream class to write toa file.

These classes are defined in <fstream>. To use themwe must write

#include <fstream>

ifstream Operations

>> stream extraction operatorget ( ) extract one charactergetline( ) read a line of data into a stringignore( ) skip a character

these ought to look familiar, they are the samefunctions we use on standard In and standard Out.

ifstream fileData (“c:\\thefile.txt”);

char a;string s;int n;

a = fileData.get( );fileData.ignore( ); // pass over newlinegetline(fileData, s);fileData >> n;

Example Code

Declaring A Stream

To use a stream object, it must first be declared

ifstream inStream;ofstream outStream;

Stream variables

A stream object is like any other variable, butthere are some important exceptions:

You cannot assign a value to a stream variable

If you pass a stream variable as a parameter, youmust pass it by reference.

Connecting a Stream to a File

ifstream inputStream;inputStream.open (“theData.txt”);

Widget 123V89001 12.95theData.txt

inputStream

to program

The preferred method for opening a file is to supplythe path when the stream object is created, like this

ifstream inputStream (“theData.txt”);

Paths

ifstream inputStream;inputStream.open (“theData.txt”);

if no path is specified, the file is assumedto be in the same directory as the programthat is executing.

When you code a \ in a pathname, you mustwrite \\. Why?

inputStream.open (“c:\\theData.txt”);

More on Paths andVisual C++ Express

If this is the folderfor your project

You need the file here whentesting your code within C++

Visual Express Edition

You need the file in here whentesting your code from the

Console.

Reading From a Text File

To read data from a text file, use the streamextraction operator, just as you would use it to readfrom the keyboard.

string description; // no spacesstring partNumber; // no spacesdouble price;

ifstream inputStream;inputStream.open (“theData.txt”);…inputStream >> description >> partNumber >> price;…

Reading From a Text File

If the data contains spaces, e.g. a person’s namethen you must use the getline function from <string>

string name;

ifstream inputStream;inputStream.open (“theData.txt”);…getline(inputStream, name);…

Type Safe I/O

In the C programming language, input and outputare done with very different libraries than thoseused in C++ (Although you can still use the C I/Olibraries in C++ if you want to).

One of the advantages to using the new C++ librariesis that C++ I/O is typesafe. The << and >> operators areoverloaded to do the correct thing for all of the standarddata types.

These operators can also be overloaded for user defineddata types.

Widget 123V89001 12.95theData.txt

inputStream

Data is read from the file and converted intothe appropriate data type.

double price;inputStream >> price;

in the file, this ischaracter data.

price

convert fromcharacter to double

12.95

…111000011010…

Writing to a Text File

Use the stream insertion operator to write to a text file,just as if you were writing to the display.

ofstream outputStream;outputStream.open (“theData.txt”);

…

outputStream << price;

Opening an Output file

If the named file does not exist, the file is created.

If the named file already exists, it is opened, andthe contents of the file are discarded, by default.

Formatting the Output

Most of the time, when we write data to a file, it iswith the idea in mind that the data will be read infrom this or some other program.

It is up to the programmer to format the data in theoutput file, so that it can later be read in a meaningfulway.

Example

int a = 5;int b = 15;int c = 239;

ofstream myOutputStream (“mydata.txt”);myOutputStream << a << b << c;

515239

What happens when you try to read this file?

int a = 5;int b = 15;int c = 239;

ofstream myOutputStream (“mydata.txt”);myOutputStream << a << “ “ << b << “ “ << c;

add white space toseparate the fields!

Closing the File

It is good programming practice to close a fileas soon as you are done using it.

myOutputStream.close ( );

Although a file is closed automatically when aprogram ends normally, it is not closed if an erroroccurs and the program terminates abnormally.If the file is not closed, all of the data written tothe file is lost!

Mode Bits

When a file is opened, you can supply bits that furtherdefine the file. These mode bits are defined in the iosclass. The ios class contains a number of importantconstants we use in file I/O.

ios::in open the file for reading, the default for ifstreamios::out open the file for writing, the default for ofstreamios::app open the file for appending. All data is written at the end of the fileios::trunc open the file and discard contents, the default for ofstream

ios::binary open the file for binary content, Note there is no ios::text which is the default

Example

#include <fstream>using std::ofstream;using std::ios;

ofstream myOutputStream (“TheData.txt”, ios::app);

…

Stream States

Objects of all of the stream classes have a state thatexpresses the condition of the stream..

The stream classes provide functions to query thestream’s state.

good( ) Everything’s fineeof( ) An input operation tried to read beyond the end of the filefail( ) An input operation failed to read the expected character, or an output operation failed to generate the desired charactersbad( ) Indicates the loss of integrity of the underlying input or output sequence

Checking that a File Opened

In C++, errors are not reported unless the programmerexplicitly asks. For example, your program could callfor a file to be opened and then read data from the file.If the file does not exist, no error is reported and youthink that everything worked fine!

ifstream myInputStream;myInputStream.open (“someData.txt”);

if (myInputStream.fail( )){ cout << “Could not open file”;}else{ …

check the state of thestream here…

We could also write

if (!myInputStream) {

Because I/O is subject to so many errors,it is good programming practice to check

the state of the stream after every I/O operation!

Writing a loop that reads until end of file

try to open the file

did it open?

print an error

message andquit

no

is the endof file flag

set?

yes

no

is the eofflag set

?

read a piece of data

was theread good

?

no

process the data

yes

continue with the rest of your program

yes

was the eofflag set

?

no

yes

display an errormessage and quit

no

We can read and test the state of the stream usingthe expression

while (myFile >> theData)

theFile.open("c:\\theFile02.txt");

while (theFile >> theData){ cout << "\nSuccessfully read in the value " << theData;}

// dropped out, so read failed, see if it is eof or an errorif (!theFile.eof( ) ){ cout << "\nCould not read an integer - bad data in the file.";}

else{ cout << "\nReached the end of the file.";}

File Names as Input

string fileName;cout << “Enter in a file name: “;cin >> fileName;ifstream myData (fileName);…

Formatting Flags

Formatting flags are defined in the ios class.

Stream classes have a member function,setf( flag ) that sets these formatting flags.

left left align output not setright right align output setdec output as decimal sethex output as hexadecimal not setoct output as octal (base 8) not setshowpoint show decimal point on output not setscientific output in exponential format not setfixed output in fixed format (not scientific) not set

You can combine flags with the | operator

myStream.setf (ios::fixed | ios::showpoint);

A flag remains set until it is unset.

myStream.unsetf (ios::fixed);

Stream Manipulators

Stream manipulators go directly into the stream.They are inserted using the stream insertion operator

dechexoctendlsetw (w)setprecision (n);setiosflags (flags);setfill (fillChar);

myStream << setw (5) << theData;

Side Effects

You don’t want a function to have an unwanted side effect.One example would be setting I/O flags in a function andleaving them that way when you exit the function.

The function flags returns a long data type that contains the settings of all of the I/O flags.

long myFlags = myStream.flags ( );…

myStream.flags (myFlags);

without an argument the function returns the flags.

with an argument, the flags are restored to those set in the parameter.

File Pointers

Streams have pointers associated with them

get pointer points to the place next character will be read from

put pointer points to where the next character will be written

tellg( ) returns current get pointertellp ( ) returns current put pointer

seekg ( ) positions get pointerseekp ( ) positions put pointer

seekp (15); move 15 bytes from start of file

seekp (-10, ios::end);move -10 bytes from end of file

seekg (6, ios::cur); move 6 bytes from current position

Binary I/O

We will not do any binary I/O in this course.

Data is written to the output device exactly as itIs stored in memory.

Binary I/O is done with the functions * read( ) * write( )

The parameters are the address of the data bufferand the number of bytes to read or write.

The address must be cast to a char*

Persistence

We say that a class has persistence when it is possiblefor an object of that class to exist outside of the life of the program.

We accomplish this by having the object save itself(i.e. its data) to a file, and then make it so the objectcan read itself (its data) back in from a file.