File Input And Output

Introduction

The fstream header defines three types to support file IO:

1. ifstream, derived from istream, reads from a file.
2. ofstream, derived from ostream, writes to a file.
3. fstream, derived from iostream, reads and writes the same file.

When we want to read or write a file, we must define our own objects, and bind them to the desired files.

File Output

• use of the ofstream class

Example 1

#include <fstream>
#include <iostream>
#include <cstdlib>
int main()
{
    using namespace std;
 
    // ofstream is used for writing files
    // We'll make a file called Sample.dat
    ofstream outf("Sample.dat");
 
    // If we couldn't open the output file stream for writing
    if (!outf)
    {
        // Print an error and exit
        cerr << "Uh oh, Sample.dat could not be opened for writing!" << endl;
        exit(0);
    }
 
    // We'll write two lines into this file
    outf << "This is line 1" << endl;
    outf << "This is line 2" << endl;
 
    return 0;
 
    // When outf goes out of scope, the ofstream
    // destructor will close the file
}

File Input

• Use of ifstream class

Example 2

#include <fstream>
#include <iostream>
#include <string>
#include <cstdlib>
 
int main()
{
    using namespace std;
 
    // ifstream is used for reading files
    // We'll read from a file called Sample.dat
    ifstream inf("Sample.dat");
 
    // If we couldn't open the input file stream for reading
    if (!inf)
    {
        // Print an error and exit
        cerr << "Uh oh, Sample.dat could not be opened for reading!" << endl;
        exit(1);
    }
 
    // While there's still stuff left to read
    while (inf)
    {
        // read stuff from the file into a string and print it
        std::string strInput;
        getline(inf, strInput);
        cout << strInput << endl;
    }
 
    return 0;
 
    // When inf goes out of scope, the ifstream
    // destructor will close the file
}

Files Modes

• File stream constructors take an optinal second parameter that allows the programmer to specify information about how the file should be opened. This is called mode.

Ios file mode	Meaning
app	Opens the file in append mode
ate	Seeks to the end of the file before reading/writing
binary	Opens the file in binary mode (instead of text mode)
in	Opens the file in read mode (default for ifstream)
nocreate	Opens the file only if it already exists
noreplace	Opens the file only if it does not already exist
out	Opens the file in write mode (default for ofstream)
trunc	Erases the file if it already exists

Random File I/O

The file pointer

• Each file stream class contains a file pointer that is used to keep track of the current read/write position within the file.
• When something is read from or written to a file, the reading/writing happens at the file pointer’s current location.