C++ File Handling

File handling in C++ allows you to work with files by reading, writing, or modifying their contents. The file handling operations in C++ are performed using the fstream library, which provides facilities for both input and output operations on files.

File Classes

1. ifstream: Used for reading data from a file (input file stream).

2. ofstream: Used for writing data to a file (output file stream).

3. fstream: Used for both reading and writing data to a file.

File Modes

When opening a file, you specify the mode in which you want to open it. These modes are passed as flags to the open() function.

ios::in Open a file for reading.

ios::out Open a file for writing.

ios::app Append to the file (writing at the end).

ios::trunc Truncate the file to zero length if it exists.

ios::binary Open the file in binary mode.

1. Reading a File

To reads a file line by line using ifstream and prints its content to the console.

Example:

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

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

    // Check if the file was successfully opened
    if (!myFile) {
        cerr << "Error opening the file for reading!" << endl;
        return 1;
    }

    string line;
    while (getline(myFile, line)) {
        cout << line << endl;  // Print each line from the file
    }

    myFile.close();  // Close the file after reading
    return 0;
}

Explanation:

1. The program opens example.txt for reading.
2. It checks if the file was successfully opened.
3. It reads the file line by line using getline() and prints the content.
4. Finally, the file is closed.

2. Writing to a File

It writes text to a file using ofstream. It creates a new file and writes data to it.

#include <iostream>
#include <fstream>
using namespace std;

int main() {
    // Open the file for writing
    ofstream myFile("output.txt");

    // Check if the file was successfully opened
    if (!myFile) {
        cerr << "Error opening the file for writing!" << endl;
        return 1;
    }

    // Write data to the file
    myFile << "Hello, Friends!" << endl;
    myFile << "This is a example of writing a file." << endl;
    myFile << "End the line" << endl;

    myFile.close();  // Close the file after writing
    cout << "Data written to output.txt successfully." << endl;
    return 0;
}

Explanation:

1. The program opens output.txt for writing.
2. It checks if the file was successfully opened.
3. It writes a few lines of text to the file using the << operator.
4. Finally, the file is closed.

Note: If output.txt already exists, it will be truncated (overwritten) unless you open it in append mode.

3. Appending to a File

It appends new data to an existing file using ofstream with the ios::app mode.

Example:

#include <iostream>
#include <fstream>
using namespace std;

int main() {
    // Open the file in append mode
    ofstream myFile("output.txt", ios::app);

    // Check if the file was successfully opened
    if (!myFile) {
        cerr << "Error opening the file for appending!" << endl;
        return 1;
    }

    // Append new data to the file
    myFile << "This line is appended to the file." << endl;
    myFile << "Appending more data to the file." << endl;

    myFile.close();  // Close the file after appending
    cout << "Data appended to output.txt successfully." << endl;
    return 0;
}

Explanation:

1. The program opens output.txt in append mode using ios::app.
2. It checks if the file was successfully opened.
3. It appends two new lines of text to the existing file.
4. Finally, the file is closed.

Note: If output.txt doesn't exist, it will be created.