file handling in cpp

FILE HANDLING IN C++

SOBIN JOSESobin.jose@outlook.com

Sobin jose

Sobin jose

Typing speed:21wpm

Disclaimer: This presentation is prepared by trainees ofbaabtra.com as a part of mentoring program. This is notofficial document of baabtra.com – Mentoring Partner

What is a File?

• A file is a collection on information, usually stored on a computer’s disk. Information can be saved to files and then later reused.

Classes for file stream operation ofstream: Stream class to write on filesifstream: Stream class to read from filesfstream: Stream class to both read and write from/to files.

The Process of Using a File

• Using a file in a program is a simple three-step process

– The file must be opened. If the file does not yet exits, opening it means creating it.

– Information is then saved to the file, read from the file, or both.

– When the program is finished using the file, the file must be closed.

Opening a File

• Before data can be written to or read from a file, the file must be opened.

ifstream inputFile;

inputFile.open(“customer.dat”);

// This program demonstrates the declaration of an fstream

// object and the opening of a file.

#include <iostream.h>

#include <fstream.h>

void main(void)

{

fstream dataFile; // Declare file stream object

char fileName[81];

cout << "Enter the name of a file you wish to open\n";

cout << "or create: ";

cin.getline(fileName, 81);

dataFile.open(fileName, ios::out);

cout << "The file " << fileName << " was opened.\n";

}

File Modes

Name Description

ios::in Open file to read

ios::out Open file to write

ios::app All the date you write, is put at the end of the file.

It calls ios::out

ios::ate All the date you write, is put at the end of the file.

It does not call ios::out

ios::trunc Deletes all previous content in the file. (empties

the file)

ios::nocreate If the file does not exist, opening it with the open()

function gets impossible.

ios::noreplace If the file exists, trying to open it with the open()

function, returns an error.

ios::binary Opens the file in binary mode.

#include<fstream>

int main(){

ofstream fout;fout.open("out.txt");char str[300]="Time is a great teacher

but unfortunately it kills all its pupils. Berlioz";

fout<<str;fout.close();return 0;

}

INPUT AND OUTPUT OPERATIONput() and get() functionthe function put() writes a single character to the associated stream. Similarly, the function get() reads a single character form the associated stream.example :file.get(ch);file.put(ch);write() and read() functionwrite() and read() functions write and read blocks of binary data.example:file.read((char *)&obj, sizeof(obj));file.write((char *)&obj, sizeof(obj));

File Pointers And Their Manipulation

•ifstream, like istream, has a pointer known as the get pointer that points to the element to be read in the next input operation. •ofstream, like ostream, has a pointer known as the put pointer that points to the location where the next element has to be written. •Finally, fstream, inherits both, the get and the put pointers, from iostream (which is itself derived from both istreamand ostream).

seekg()moves get pointer(input) to a specified location

seekp()moves put pointer (output) to a specified location

tellg()gives the current position of the get pointer

tellp()gives the current position of the put pointer

These internal stream pointers that point to the reading or writing locations within a

stream can be manipulated using the following member functions:

The other prototype for these functions is: seekg(offset, refposition ); seekp(offset, refposition );

•The parameter offset represents the number of bytes the file pointer is to be moved from the location specified by the parameter refposition. The refposition takes one of the following three constants defined in the ios class.•ios::beg start of the fileios::cur current position of the pointerios::end end of the file•example:file.seekg(-10, ios::cur);

Binary File I/O Examples//Example 1: Using get() and put()#include <iostream>#include <fstream>void main(){

fstream File("test_file",ios::out | ios::in | ios::binary);char ch;ch='o';File.put(ch); //put the content of ch to the fileFile.seekg(ios::beg); //go to the beginning of the fileFile.get(ch); //read one charactercout << ch << endl; //display itFile.close();

}

File I/O Example: Reading#include <iostream>#include <fstream>#include <string>

int main(void){

ifstream openFile(“data.txt"); //open a text file data.txtstring line;

if(openFile.is_open()){ //while(!openFile.eof()){getline(openFile,line);//read a line from data.txt and put it in a string

cout << line;}else{cout<<“File does not exist!”<<endl;exit(1);}

}openFile.close();

return 0;}

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