0Pricing
C++ Academy · Lesson

Writing Files with ofstream

Write output files.

Writing Files with ofstream is a free C++ Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C++ Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What Is ofstream?

std::ofstream (output file stream) writes data to files. It works like std::cout but its destination is a file.

#include <iostream>
#include <fstream>

int main() {
    std::ofstream out("hello.txt");
    out << "Hello, file!\n";
    std::cout << "wrote file\n";
    return 0;
}

Writing Text

Use the << operator to write strings and numbers, exactly like printing to the console.

#include <iostream>
#include <fstream>

int main() {
    std::ofstream out("report.txt");
    out << "Score: " << 95 << '\n';
    out << "Grade: A\n";
    std::cout << "done\n";
    return 0;
}

Truncation by Default

Opening an existing file with ofstream erases its previous contents. Each run starts fresh.

#include <iostream>
#include <fstream>

int main() {
    std::ofstream("t.txt") << "old content\n";
    std::ofstream out("t.txt");
    out << "new content\n";
    std::ifstream in("t.txt");
    std::string line; std::getline(in, line);
    std::cout << line << '\n';
    return 0;
}

Appending with app Mode

Pass std::ios::app to keep existing content and add new data at the end.

#include <iostream>
#include <fstream>

int main() {
    std::ofstream("log.txt") << "line1\n";
    std::ofstream out("log.txt", std::ios::app);
    out << "line2\n";
    out.close();
    std::ifstream in("log.txt");
    std::string l; int n = 0;
    while (std::getline(in, l)) ++n;
    std::cout << n << " lines\n";
    return 0;
}

Checking the Write Succeeded

Test the stream after opening. If it is in a bad state, writing will silently fail.

#include <iostream>
#include <fstream>

int main() {
    std::ofstream out("ok.txt");
    if (!out) {
        std::cout << "failed to open\n";
        return 1;
    }
    out << "safe write\n";
    std::cout << "write ok\n";
    return 0;
}

Writing Numbers and Formatting

I/O manipulators like std::fixed and std::setprecision apply to file streams too.

#include <iostream>
#include <fstream>
#include <iomanip>

int main() {
    std::ofstream out("pi.txt");
    out << std::fixed << std::setprecision(3) << 3.14159 << '\n';
    out.close();
    std::ifstream in("pi.txt");
    std::string s; in >> s;
    std::cout << s << '\n';
    return 0;
}

Writing a Loop of Data

Write many lines by looping. Each iteration sends a record to the file.

#include <iostream>
#include <fstream>

int main() {
    std::ofstream out("squares.txt");
    for (int i = 1; i <= 5; ++i) out << i << ' ' << i * i << '\n';
    out.close();
    std::cout << "wrote 5 rows\n";
    return 0;
}

Flushing the Buffer

Output is buffered. std::flush or std::endl forces the buffer to disk immediately; closing also flushes.

#include <iostream>
#include <fstream>

int main() {
    std::ofstream out("flush.txt");
    out << "important" << std::flush;
    std::cout << "flushed to disk\n";
    return 0;
}

endl vs newline

'\n' just adds a newline. std::endl adds a newline and flushes, which is slower in tight loops.

#include <iostream>
#include <fstream>

int main() {
    std::ofstream out("nl.txt");
    out << "fast\n";          // newline only
    out << "slow" << std::endl; // newline + flush
    std::cout << "both written\n";
    return 0;
}

Round-Trip Write then Read

A common pattern: write data, close, then reopen to read it back.

#include <iostream>
#include <fstream>

int main() {
    {
        std::ofstream out("rt.txt");
        out << 7 << ' ' << 8 << '\n';
    }
    std::ifstream in("rt.txt");
    int a, b; in >> a >> b;
    std::cout << a + b << '\n';
    return 0;
}

Choosing Open Modes

Combine flags with |. Common modes: ios::out (default), ios::app (append), ios::trunc (erase).

#include <iostream>
#include <fstream>

int main() {
    std::ofstream out("modes.txt", std::ios::out | std::ios::trunc);
    out << "fresh\n";
    std::cout << "opened with explicit modes\n";
    return 0;
}

Quick Check

Test your understanding of ofstream modes.

Recap

You learned that std::ofstream:

  • writes with <<, truncating by default
  • appends with std::ios::app
  • buffers output; std::flush/std::endl/closing send it to disk

Next, you'll handle raw bytes with binary file I/O.

Frequently asked questions

Is the “Writing Files with ofstream” lesson free?

Yes — the full text of “Writing Files with ofstream” is free to read here on the web, and the C++ Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C++ Academy course, upgrade to CoddyKit PRO.

What will I learn in “Writing Files with ofstream”?

Write output files. You practise C++ Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C++ Academy?

No prior experience is required. C++ Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Writing Files with ofstream” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C++ Academy lesson?

Yes. Every C++ Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Reading Files with ifstream
  2. Writing Files with ofstream
  3. Binary File I/O
  4. Error Handling and State
← Back to C++ Academy