C++ File Handling — ofstream, ifstream, fstream | Tutorials Logic

File Streams in C++

C++ uses stream classes from <fstream> for file I/O:

Class	Purpose
`ofstream`	Write to file (output file stream)
`ifstream`	Read from file (input file stream)
`fstream`	Read and write (both directions)

Mode Flag	Description
`ios::in`	Open for reading
`ios::out`	Open for writing (truncates existing)
`ios::app`	Append to end of file
`ios::binary`	Binary mode
`ios::trunc`	Truncate file to zero length

Write and Read Text Files

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

int main() {
    // Write to file
    ofstream outFile("data.txt");

    if (!outFile.is_open()) {
        cerr << "Failed to open file!" << endl;
        return 1;
    }

    outFile << "Line 1: Hello, C++ File I/O!" << endl;
    outFile << "Line 2: Writing to a file." << endl;
    outFile << "Line 3: Done." << endl;

    outFile.close();
    cout << "File written successfully." << endl;

    // Append to file
    ofstream appendFile("data.txt", ios::app);
    appendFile << "Line 4: Appended line." << endl;
    appendFile.close();

    return 0;
}

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

int main() {
    ifstream inFile("data.txt");

    if (!inFile.is_open()) {
        cerr << "Cannot open file!" << endl;
        return 1;
    }

    // Read line by line
    string line;
    int lineNum = 1;
    while (getline(inFile, line)) {
        cout << lineNum++ << ": " << line << endl;
    }

    inFile.close();

    // Read word by word
    ifstream wordFile("data.txt");
    string word;
    int wordCount = 0;
    while (wordFile >> word) wordCount++;
    cout << "Word count: " << wordCount << endl;

    return 0;
}

Binary File I/O and fstream

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

struct Student {
    char name[50];
    int  age;
    double gpa;
};

int main() {
    // Write binary
    Student s1 = {"Alice", 20, 3.8};
    Student s2 = {"Bob",   22, 3.5};

    ofstream out("students.bin", ios::binary);
    out.write(reinterpret_cast<char*>(&s1), sizeof(Student));
    out.write(reinterpret_cast<char*>(&s2), sizeof(Student));
    out.close();

    // Read binary
    ifstream in("students.bin", ios::binary);
    Student s;
    while (in.read(reinterpret_cast<char*>(&s), sizeof(Student))) {
        cout << s.name << " | Age: " << s.age << " | GPA: " << s.gpa << endl;
    }
    in.close();

    // fstream - read and write
    fstream rw("data.txt", ios::in | ios::out);
    rw.seekg(0, ios::end);  // seek to end
    long size = rw.tellg(); // get file size
    cout << "File size: " << size << " bytes" << endl;
    rw.close();

    return 0;
}