File Handling in C++

Why File Handling is Important?

• It allows you to store data permanently.
• Makes data easily accessible for further processing.
• Supports a wide range of applications, such as logging, configuration management, and data analysis.

Basics of File Handling in C++

Header File Required:

#include <fstream>

File Stream Classes:

• ifstream (Input File Stream): For reading files.
• ofstream (Output File Stream): For writing to files.
• fstream (File Stream): For both reading and writing.

File Modes in C++

• ios::in: Open file for reading.
• ios::out: Open file for writing.
• ios::app: Append data to the file.
• ios::trunc: Truncate (clear) file contents.
• ios::binary: Open file in binary mode.

Opening and Closing Files

Opening a File:

ofstream file;
file.open("example.txt");

Closing a File:

file.close();

Example:

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

int main() {
    ofstream file;
    file.open("example.txt"); // Opening file for writing
    file << "Hello, C++ File Handling!" << endl; // Writing to file
    file.close(); // Closing file

    cout << "File created and text written." << endl;
    return 0;
}

Output:

File created and text written.

Content of "example.txt":

Hello, C++ File Handling!

Writing to a File (ofstream)

Writing Text to a File:

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

int main() {
    ofstream file("example.txt");
    if (file.is_open()) {
        file << "This is line 1.\n";
        file << "This is line 2.\n";
        file << "This is line 3.\n";
        file.close();
        cout << "Text written to file." << endl;
    } else {
        cout << "Failed to open file." << endl;
    }
    return 0;
}

Output:

Text written to file.

Content of "example.txt":

This is line 1.
This is line 2.
This is line 3.

Reading from a File (ifstream)

Reading Line by Line:

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

int main() {
    ifstream file("example.txt");
    string line;

    if (file.is_open()) {
        while (getline(file, line)) {
            cout << line << endl;
        }
        file.close();
    } else {
        cout << "Failed to open file." << endl;
    }
    return 0;
}

Output (Reads the file content):

This is line 1.
This is line 2.
This is line 3.

Reading Entire File at Once

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

int main() {
    ifstream file("example.txt");
    string content;

    if (file.is_open()) {
        content.assign((istreambuf_iterator<char>(file)),
                        (istreambuf_iterator<char>()));
        cout << "File Content:\n" << content << endl;
        file.close();
    } else {
        cout << "Failed to open file." << endl;
    }
    return 0;
}

Output:

File Content:
This is line 1.
This is line 2.
This is line 3.

Appending Data to a File (ios::app)

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

int main() {
    ofstream file("example.txt", ios::app); // Append mode
    file << "This is an appended line.\n";
    file.close();
    cout << "Data appended to file." << endl;
    return 0;
}

Content of "example.txt" after appending:

This is line 1.
This is line 2.
This is line 3.
This is an appended line.

Random Access (Reading/Writing Specific Positions)

• You can use seekg() (seek get) and seekp() (seek put) to move the cursor.
• tellg() and tellp() are used to get the current position.

Example:

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

int main() {
    fstream file("example.txt", ios::in | ios::out);

    // Writing at specific position
    file.seekp(15);  // Move to 15th character position
    file << "Inserted Text";

    // Reading from specific position
    file.seekg(0);   // Move to beginning
    string content;
    while (getline(file, content)) {
        cout << content << endl;
    }

    file.close();
    return 0;
}

Output:

This is line 1.
Inserterted Textine 2.
This is line 3.
This is an appended line.

Error Handling in File Operations

You should always check if the file was successfully opened:

ifstream file("nonexistent.txt");
if (!file) {
    cerr << "Error: File could not be opened." << endl;
}

Error Flags:

• file.fail() - Returns true if any file operation fails.
• file.eof() - Returns true if end of file is reached.
• file.bad() - Returns true if a serious error occurs.

Working with Binary Files (ios::binary)

Binary files store data in a raw format (not human-readable).

Writing Binary Data:

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

int main() {
    ofstream file("data.bin", ios::binary);
    int number = 12345;
    file.write(reinterpret_cast<char*>(&number), sizeof(number));
    file.close();
    cout << "Binary data written." << endl;
    return 0;
}

Reading Binary Data:

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

int main() {
    ifstream file("data.bin", ios::binary);
    int number;
    file.read(reinterpret_cast<char*>(&number), sizeof(number));
    file.close();
    cout << "Binary data read: " << number << endl;
    return 0;
}

Output:

Binary data read: 12345

Best Practices for File Handling

1. Always check if the file is successfully opened.
2. Use close() to free file resources.
3. Use ios::app to avoid overwriting existing data.
4. For binary files, use ios::binary mode.
5. Use fstream for read-write access.

Summary

In this tutorial, you learned:

• How to open, close, read, and write files in C++.
• Different file modes (read, write, append, binary).
• How to work with text and binary files.
• Advanced concepts like random access and error handling.