C++ Files

File I/O with fstream

C++ uses the <fstream> header for file operations. Three classes:

Writing to a File

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

int main() {
    ofstream file("students.txt");   // creates or overwrites
    if (!file) {
        cerr << "Cannot open file!" << endl;
        return 1;
    }
    file << "Alice, 95" << endl;
    file << "Bob,   82" << endl;
    file << "Carol, 78" << endl;
    file.close();
    cout << "File written successfully." << endl;
    return 0;
}

Reading from a File

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

int main() {
    ifstream file("students.txt");
    string line;
    while (getline(file, line)) {
        cout << line << endl;
    }
    file.close();
    return 0;
}

Output:

Alice, 95
Bob,   82
Carol, 78

Append Mode

ofstream file("log.txt", ios::app);   // append without erasing
file << "New log entry" << endl;
file.close();

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

int main() {
    // Write
    ofstream out("numbers.txt");
    for (int i = 1; i <= 5; i++) out << i << endl;
    out.close();

    // Read and sum
    ifstream in("numbers.txt");
    int n, sum = 0;
    while (in >> n) sum += n;
    in.close();
    cout << "Sum: " << sum << endl;   // 15
    return 0;
}

Summary

• #include <fstream> for file I/O.
• ofstream writes; ifstream reads; fstream does both.
• Always check if the file opened and close it when done.
• Use ios::app to append without erasing existing content.