CSCI 333Data Structures

I/O with C++

20 October, 2003

C++ Stream I/O hierarchy

ios

istream ostream

iostream

ifstream

multipleinheritance

virtualbase class

virtualbase class

fstreamofstream

ios class

• Formatting– Width of field, base of numbers

• State– Open, closed, end-of-file, failed

• Constants– Open for read/write/append

• Buffering– Using streambuf class

ANSI C++ ios

• To support internationalization– ios is a template of

•basic_ios

– Specialized for character set

• But western programmers don’t worry– Except when looking at the documentation

ios flags for opening files

ios::in Open for input

ios::out Open for output

ios::app Append to file

ios::trunc Truncate file

ios::nocreate File if file doesn’t exist

ios::noreplace Fail if file exists

ios state methods

F.good() No errors

F.fail() Failure, stream usable

F.bad() Failure, stream not usable

F.eof() End-of-file

!F Failed or bad

ios formatting methods

F.setw(int) Sets width,

returns the stream

F.width(int) Sets width

returns current width

F.fill(char) Sets fill character

returns current fill

ios flags for moving in files

ios::beg Move relative to beginning of file

ios::cur Move relative to

current position

ios::end Move relative to

end of file

istream class

• Stream input class

• Read from files– to all sorts of variables

• Seek to arbitrary file position

• Templated in ANSI C++– from basic_istream

Some istream methods

F >> x Extract from stream

F.read(buff, n) Extract into buffer

F.seekg(newPos) Move get pointer

F.seekg(nP,skpDr) Move get pointer,

relative to seek direction

F.tellg() Returns get pointer

Some ostream methods

F << x Insert into stream

F.write(buff, n) Insert from buffer

F.seekp(newPos) Move put pointer

F.seekp(nP,skpDr) Move put pointer,

relative to seek direction

F.tellp() Returns put pointer

The iostream methods

• Multiple inheritence – istream– ostream

• Single ios subclass– Since ios is a virtual subclass

The standard streams

cin Standard input

cout Standard output

cerr Standard error

The fstream methods

• Most inherited from *stream methods fstream F ; fstream F(filename, flags) ;– F.open(filename, flags) ;– F.close() ;

// Opening and closing with ifstream and ofstream#include <fstream>using namespace std ; int main(int argc, char *argv[]) { ifstream RStream ; ofstream WStream ; RStream.open(argv[1]) ; if (RStream.bad()) cerr << "Unable to read " << argv[1] << endl ; RStream.close() ; WStream.open(argv[1]) ; if (WStream.bad()) cerr << "Unable to write " << argv[1] <<

endl ; WStream.close() ;}

// Opening and closing with fstream

#include <fstream>

using namespace std ;

int main(int argc, char *argv[])

{

fstream RWStream ;

RWStream.open(argv[1], ios::in) ;

if (RWStream.bad())

cerr << "Unable to read " << argv[1] << endl ;

RWStream.close() ;

RWStream.open(argv[1], ios::out) ;

if (RWStream.bad())

cerr << "Unable to write " << argv[1] << endl ;

RWStream.close() ;

}

// Silly file updates

#include <fstream>

using namespace std ;

int main(int argc, char *argv[])

{

fstream RWStream ;

RWStream.open(argv[1], ios::in | ios::out | ios::binary) ;

if (RWStream.bad())

cerr << "Unable to read or write" << argv[1] << endl ;

else

{

char buff[7] ;

// Read from characters 7 to 13

RWStream.seekg(7, ios::beg) ;

RWStream.read(buff, 7) ;

// Write to characters 14 to 20

RWStream.seekp(14, ios::beg) ;

RWStream.write(buff, 7) ;

RWStream.close() ;

}

}

// Reverse a file#include <fstream> using namespace std ;

int main(int argc, char *argv[]) { fstream RWStream ;

RWStream.open(argv[1], ios::in | ios::out | ios::binary) ; if (RWStream.bad()) cerr << "Unable to read or write" << argv[1] << endl ; else { char buffFront[1] ; // extremely inefficient char buffRear[1] ; RWStream.seekp(0, ios::end) ; int fileSize = RWStream.tellp() ; for (int i=0; i<fileSize/2; ++i) { RWStream.seekg(i, ios::beg) ; RWStream.read(buffFront, 1) ; RWStream.seekg(fileSize-i-1, ios::beg) ; RWStream.read(buffRear, 1) ; RWStream.seekp(i, ios::beg) ; RWStream.write(buffRear, 1) ; RWStream.seekp(fileSize-i-1, ios::beg) ; RWStream.write(buffFront, 1) ; } } RWStream.close() ;}

// writing numbers in ASCII and binary#include <fstream> using namespace std ;

int main(int argc, char *argv[]) { ofstream WStream ; WStream.open(argv[1], ios::binary) ; if (WStream.bad()) cerr << "Unable to write" << argv[1] << endl ; else { for (int i=333; i<343; ++i) WStream << i ; for (int i=333; i<343; ++i) WStream.write((char *)&i, sizeof(i)) ; } WStream.close() ;}

// formatted I/O with ios methods#include <iostream>using namespace std ;int main(int argc, char *argv[]) { cout.fill('0') ; for (int i=0; i<100; ++i) { cout.width(2) ; cout << i ; cout.width(4) ; cout << i*i << endl ; }}

// formatting with io manipulators

#include <iostream>

#include <iomanip>

using namespace std ;

int main(int argc, char *argv[])

{

cout.fill('0') ;

for (int i=0; i<100; ++i)

cout << setw(2) << i << " "

<< setw(4) << i*i << endl ;

}

// Reading integers or characters#include <iostream>using namespace std ;

int main(int argc, char *argv[]) { int i ; cin >> i ; if (cin.fail()) { cin.clear() ; char c ; cin >> c ; if (cin.fail()) cout << "Failure on I/O" ; else cout << "Read character '" << c << "'" << endl ; } else cout << "Read integer " << i << endl ;}

// Summing integers#include <iostream>using namespace std ; int main(int argc, char *argv[]) { int i ; int sum = 0 ; while(cin >> i && cin.good()) sum += i ; cout << "Total = " << sum << endl ;}

#include <iostream>#include <sstream>using namespace std ; #define INBUFFSIZE 4096int main(int argc, char *argv[]) { int sum = 0 ; char inLine[INBUFFSIZE+1] ; while (cin.getline(inLine, INBUFFSIZE) && cin.gcount()) { int lineSum = 0 ; int i ; istringstream lineStream(inLine) ; while (lineStream>>i && lineStream.good()) lineSum += i ; if (lineStream.eof()) sum += lineSum ; else cerr << "Ignoring bad input line: " << inLine << endl ; } cout << "Sum = " << sum << endl;}