File Handling in C++

# CHAPTER 23

File Handling in C++ #

1. Introduction #

2. Learning Objectives #

• Include the <fstream> library.

• Write to a file using ofstream.

• Read from a file using ifstream.

• Append data to an existing file.

• Check if a file opened successfully.

3. The fstream Library #

• ofstream (Output File Stream): Creates and writes to files.

• ifstream (Input File Stream): Reads from files.

• fstream (File Stream): Can do both read and write.

4. Writing to a File (ofstream) #

#include <iostream>
#include <fstream> // Required for file handling
using namespace std;

int main() {
    // 1. Create and open a text file
    ofstream outFile("player_stats.txt");

    // 2. Check if it opened successfully
    if (outFile.is_open()) {
        // 3. Write data to the file using <<
        outFile << "Name: Arthur" << endl;
        outFile << "Score: 1500" << endl;
        
        // 4. CLOSE THE FILE!
        outFile.close();
        cout << "Data saved successfully!" << endl;
    } else {
        cout << "Error: Could not create file." << endl;
    }
    
    return 0;
}

5. Appending to a File #

// Open file in append mode
ofstream outFile("player_stats.txt", ios::app); 
outFile << "Level: 5" << endl;
outFile.close();

6. Reading from a File (ifstream) #

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

int main() {
    string lineText;
    
    // 1. Open the file to read
    ifstream inFile("player_stats.txt");

    if (inFile.is_open()) {
        // 2. Read line by line until EOF
        while (getline(inFile, lineText)) {
            cout << lineText << endl; // Print the line to console
        }
        
        // 3. Close the file
        inFile.close();
    } else {
        cout << "Error: File not found!" << endl;
    }
    
    return 0;
}

7. File Modes #

• ios::out : Open for writing (default for ofstream).

• ios::in : Open for reading (default for ifstream).

• ios::app : Append to the end of the file.

• ios::trunc : Truncate (clear) the file before writing (default for out).

• ios::binary : Open in binary mode (instead of text mode).

8. Mini Project: Notes Manager #

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

int main() {
    ofstream file("notes.txt", ios::app); // Append mode
    string note;
    
    if (file.is_open()) {
        cout << "Enter a note (or type &#039;EXIT&#039; to quit):" << endl;
        
        while (true) {
            getline(cin, note);
            if (note == "EXIT") break;
            
            file << note << endl; // Save note to file
        }
        file.close();
        cout << "Notes saved!" << endl;
    }
    return 0;
}

9. Memory-Level Explanation #

10. Common Mistakes #

• Forgetting to close() the file: If the program crashes before the file is closed, the buffer might not flush, and your data is lost.

• Reading a non-existent file: If you don't check isopen(), using ifstream on a missing file will fail silently, leading to confusing bugs.

13. Interview Questions #

• Q: Explain the difference between opening a file in Text Mode vs. Binary Mode (ios::binary).

• Q: What is Serialization?

• Q: Explain what a Buffer is in the context of file I/O and why it is used.

14. Summary #

15. Next Chapter Recommendation #