File Handling & Command Line

Stream state and error handling

• Failed open, bad path, or bad extraction sets failbit — check after operations.
• After a failed >>, clear with clear() and recover (e.g. ignore).
• RAII: stream destructor closes the file — still call close() when you want an early flush.

Copy file & CSV-style parsing

• Copy: read lines/chars from one stream and write to another.
• CSV: getline each line, then parse with stringstream splitting on commas.
• **Real-life:** convert marks.csv into totals per student.

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

int main() {
    ifstream in("data.txt");
    string line;
    while (getline(in, line)) {
        stringstream ss(line);
        string name, marks;
        if (getline(ss, name, ',') && getline(ss, marks)) {
            cout << name << " => " << marks << "\n";
        }
    }
    return 0;
}

Binary write/read of a POD struct (careful)

• Possible for simple POD structs; **not** safe for std::string / pointers inside.
• Prefer portable text/CSV or a defined binary format for real apps.

#include <fstream>
using namespace std;

struct Rec { int id; double value; };

int main() {
    Rec r{1, 99.5};
    ofstream out("rec.bin", ios::binary);
    out.write(reinterpret_cast<char*>(&r), sizeof(r));
    out.close();

    Rec loaded{};
    ifstream in("rec.bin", ios::binary);
    in.read(reinterpret_cast<char*>(&loaded), sizeof(loaded));
    return 0;
}

Real-life file handling models

• School: export attendance CSV daily.
• Shop: append each sale to sales.log.
• App config: read settings.txt key=value lines at startup.
• CLI tool: converter in.txt out.txt via argv.
• Backup: copy user notes from one path to another.

Write text with ofstream

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

int main() {
    ofstream out("hello.txt");
    if (!out) return 1;
    out << "Hello File Handling\n";
    out.close();
    cout << "Written\n";
    return 0;
}

Read lines with getline

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

int main() {
    ifstream in("hello.txt");
    string line;
    while (getline(in, line)) cout << line << "\n";
    return 0;
}

Append to a log

#include <fstream>
using namespace std;

int main() {
    ofstream log("app.log", ios::app);
    log << "Event at end of file\n";
    return 0;
}

Count lines, words, chars

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

int main() {
    ifstream in("hello.txt");
    string line;
    int lines = 0, words = 0, chars = 0;
    while (getline(in, line)) {
        lines++;
        chars += static_cast<int>(line.size()) + 1;
        stringstream ss(line);
        string w;
        while (ss >> w) words++;
    }
    cout << lines << " " << words << " " << chars << "\n";
    return 0;
}

Copy file

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

int main() {
    ifstream in("hello.txt");
    ofstream out("hello_copy.txt");
    string line;
    while (getline(in, line)) out << line << "\n";
    return 0;
}

Read numbers with >>

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

int main() {
    ifstream in("nums.txt"); // e.g. 10 20 30
    int x, sum = 0;
    while (in >> x) sum += x;
    cout << "Sum = " << sum << "\n";
    return 0;
}

argc/argv echo

#include <iostream>
using namespace std;

int main(int argc, char* argv[]) {
    for (int i = 0; i < argc; i++)
        cout << argv[i] << "\n";
    return 0;
}