Understanding File Stream Classes in C++

In this lesson, you will learn.

• Introduction
• File Stream Classes
• Example: Reading a File
• Example: Writing to a File

Introduction

To handle large volumes of data, we need some devices like hard disks to store data. A file stores the data in hard disks and contains read and write operations.

Generally, a C++ program involves two task

• Communication between the console unit and the program (Already discussed in the previous lesson using cin and cout objects).
• Data communication between the program and disk files

Transferring the data between the program and disk files can be achieved by file handling concepts in C++.

So, to handle files, the file stream classes are used that supply the data through the input stream and receive it from the program as an output stream.

File Stream Classes

• In C++, file streams are used for input and output operations on files.
• The standard file stream classes are part of <fstream> header.
• The main classes for file stream handling in C++ are ifstream, ofstream, and fstream.

The following Figure shows the file stream class hierarchy.

Example 1: Reading a File

//FileStreamReadingEx1.cpp
#include <iostream>
#include <fstream>
using namespace std;

int main() {
    ifstream inputFile("example.txt");  // Open file for reading

    if (inputFile.is_open()) {
        // Read data from the file
        string line;
        while(getline(inputFile, line)) {
            cout << line <<endl;
        }

        inputFile.close();  // Close the file
    } else {
        cerr << "Error opening file!" <<endl;
    }

    return 0;
}

Output

Hello, File I/O!
This is a new line.

Explanation

ifstream inputFile("example.txt"); This line creates an ifstream object named inputFile and opens the file named “example.txt” for reading.

If the file does not exist or cannot be opened for some reason, you can check the state of the stream using the is_open() member function.

getline() function reads the file line by line and stores it in the line string.

inputFile.close(); Finally, after reading the file, it’s good practice to close the file using the close() member function to free up resources associated with the file stream.

Example 2: Writing to a File

//FileStreamWritingEx1.cpp
#include <iostream>
#include <fstream>
using namespace std;

int main() {
    ofstream outputFile("example.txt");  // Open file for writing

    if (outputFile.is_open()) {
        // Write data to the file
        outputFile<< "Hello, File I/O!" <<endl;
        outputFile<< "This is a new line.";

        outputFile.close();  // Close the file
        cout << "File written\n";
    } else {
        cerr << "Error opening file!" << std::endl;
    }

    return 0;
}

Output

File written 

Note: Open the file “example.txt” and check whether the content is written or not.

Example 3: Writing Data

//FileStreamWritingEx2.cpp
#include <fstream> //for file I/O
#include <iostream>
#include <string>
using namespace std;

int main()
{
    char ch = 'x';
    int j = 77;
    double d = 6.02;
    string str1 = "File Handling"; //strings 
    string str2 = "Using C++"; 
    ofstream outfile("fdata.txt"); //create ofstream object
    
    outfile<< ch //insert (write) data
    << j
    << ' ' //needs space between numbers
    << d
    << str1
    << ' '  //needs spaces between strings
    << str2;
    cout << "File written\n";
    
    return 0;
}

Output

File written

Note: Open the file “fdata.txt” and check whether the content is written or not.

Explanation

ofstream outfile(“fdata.txt”);

An object called outfile to be a member of the ofstream class is created and opens the file “fdata.text” on the disk. If the file doesn’t exist, it is created. If it does exist, it is truncated and the new data replaces the old.

Example 4: Reading Data

//FileStreamReadingEx2.cpp
#include <fstream> //for file I/O
#include <iostream>
#include <string>
using namespace std;

int main()
{
    char ch;
    int j;
    double d;
    string str1;
    string str2;

    ifstream infile("fdata.txt"); //create ifstream object
    //extract (read) data from it
    infile >> ch >> j >> d >> str1 >> str2;
    
    cout << ch << endl //display the data
    << j << endl
    << d << endl
    << str1 << endl
    << str2 << endl;
    
    return 0;
}

Output

x
77
6.02
File
Handling

End of the lesson….enjoy learning

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