File I/O Streams in C++ (`std::ifstream`, `std::ofstream`, `<fstream>`)

🟡 Intermediate

📖 Definition

File Input and Output (I/O) in C++ is managed through the <fstream> header library using three primary stream classes:

std::ofstream (Output File Stream): Used for creating and writing data to disk files.
std::ifstream (Input File Stream): Used for reading data from disk files.
std::fstream (File Stream): Supports both reading and writing to files simultaneously.

🇮🇳 Hindi Explanation

C++ mein disk par text files create, write, aur read karne ke liye <fstream> library ka use hota hai.

std::ofstream: File banana aur usme data write karna (file << "text";).
std::ifstream: Disk par pehle se maujood file ko kholna aur lines read karna (std::getline(file, line)).
File open karne ke baad use .close() karna zaroori hota hai (halanki destructor use automatically close kar deta hai).

🚩 Marathi Explanation

Disk varil text files tayar karne, lihine (write), aani vachane (read) yasathi <fstream> cha vapar hota.

std::ofstream: File lihinyasathi (Output).
std::ifstream: File vachnyasathi (Input).
File vaaparun zalyavar .close() करणे changle aste.

📊 File Open Modes Summary

Open Mode	Flag	Description
Write	`std::ios::out`	Opens file for writing (truncates existing file by default)
Append	`std::ios::app`	Appends new data to the end of existing file
Read	`std::ios::in`	Opens existing file for reading
Binary	`std::ios::binary`	Reads/writes binary data instead of text lines

💡 Practical Example

#include <iostream>
#include <fstream> // For std::ofstream and std::ifstream
#include <string>

int main() {
    std::string filename = "user_logs.txt";

    // 1. WRITING TO FILE using std::ofstream
    std::cout << "--- 1. WRITING TO FILE ---" << std::endl;
    std::ofstream outFile(filename, std::ios::out);

    if (outFile.is_open()) {
        outFile << "Log 101: System initialized successfully." << std::endl;
        outFile << "Log 102: User 'Rahul' logged in." << std::endl;
        outFile << "Log 103: Database backup completed." << std::endl;
        outFile.close(); // Explicitly closing stream
        std::cout << "Data successfully saved to " << filename << std::endl;
    } else {
        std::cerr << "Failed to create/open " << filename << std::endl;
    }

    // 2. APPENDING TO FILE using std::ios::app
    std::ofstream appendFile(filename, std::ios::app);
    if (appendFile.is_open()) {
        appendFile << "Log 104: Session closed." << std::endl;
        appendFile.close();
    }

    // 3. READING FROM FILE using std::ifstream
    std::cout << "\n--- 2. READING FROM FILE ---" << std::endl;
    std::ifstream inFile(filename);

    if (inFile.is_open()) {
        std::string line;
        // Read file line by line until EOF (End Of File)
        while (std::getline(inFile, line)) {
            std::cout << "[READ LINE] " << line << std::endl;
        }
        inFile.close();
    } else {
        std::cerr << "Could not open " << filename << " for reading!" << std::endl;
    }

    return 0;
}

🔍 Code Breakdown

#include <fstream>: Provides ofstream and ifstream classes.
outFile.is_open(): Checks if the file handle was successfully acquired before performing I/O.
std::getline(inFile, line): Continuously reads text line by line until reaching End-Of-File (EOF).
std::ios::app: Specifies append mode so new log entries don’t overwrite previous logs.

👀 Output

--- 1. WRITING TO FILE ---
Data successfully saved to user_logs.txt

--- 2. READING FROM FILE ---
[READ LINE] Log 101: System initialized successfully.
[READ LINE] Log 102: User 'Rahul' logged in.
[READ LINE] Log 103: Database backup completed.
[READ LINE] Log 104: Session closed.

⚠️ Common Mistakes

Forgetting .is_open() Check: Attempting to read a file that doesn’t exist without checking .is_open() fails silently without reading anything.
Default Overwriting in ofstream: Opening a file with std::ofstream outFile("data.txt") truncates/deletes existing contents! Pass std::ios::app if you want to append without deleting.

🛡️ Best Practices

Always check if the file opened successfully (if (file.is_open())).
Leverage C++ RAII: destructors automatically flush and close file streams when file object variables go out of scope.

🧪 Try It Yourself

Write a C++ script that inputs a user’s favorite quote and saves it into quote.txt.
Read quote.txt and print the total number of characters in the file.

🎯 Mini Challenge

Write a Student Grade Exporter program that reads a list of student names and scores from students.txt, calculates their grade, and writes the results to grades_report.txt.

← C++ Home

← Previous: Exception Handling

Next: Namespaces & Modern C++ →

File I/O Streams in C++ (std::ifstream, std::ofstream, <fstream>)