std::ofstream

Learn to write files with std::ofstream.

Writing with std::ofstream

std::ofstream opens a file for writing. You send data to the file with the << operator. If the file does not exist it is created; if it exists its content is erased and overwritten.

Example: std::ofstream out("data.txt"); out << "Hi"; out.close();. These three lines create data.txt, write Hi into it and close it. After writing you can read the file back with std::ifstream to verify the content. Calling close() when done is a good habit; it flushes the buffer to disk.

Writing multiple values

You can write several values in a single << chain. out << "Name: " << name << " " << age; writes a string, a variable and a number in sequence.

To separate lines use \n or std::endl. Example: out << "First" << std::endl << "Second"; writes two lines. Numbers need no extra work: out << 42; writes 42 directly. Data written to a file can later be read back with >>; the format is preserved.

Exercise Write to a file and read it back

The starting code reads the file and prints it. Open data.txt with std::ofstream, write "Hi" into it and close it. The reading part is ready; it will read from the file you wrote.

Show example solution

#include <iostream>
#include <fstream>
#include <string>

int main() {
    std::ofstream out("data.txt");
    out << "Hi";
    out.close();

    std::ifstream in("data.txt");
    std::string word;
    in >> word;
    std::cout << word;
    return 0;
}

Test yourself

Write to the file
```
out << ____;
```
Show answer
```
out << "Hi";
```
What happens if the file opened with std::ofstream already exists?
```
std::ofstream out("data.txt");
```
- Content is appended
- An error is thrown
- Content is erased and overwritten
- It opens read-only
Show answer

Content is erased and overwritten — ofstream erases and overwrites by default.
Match each piece to its role

std::ofstream

File writing
<<

Data writing
out.close()

Closing the file
std::endl

Line ending

Next lesson: std::fstream

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