Step 5 of 6

Writing files with ofstream

std::ofstream writes a file with <<. By default opening it truncates: the old contents are thrown away, like "w" in C's fopen. Pass std::ios::app to append to the end instead.

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

bool log_line(const std::string& path, const std::string& msg) {
    std::ofstream out(path, std::ios::app);
    out << msg << "\n";
    return static_cast<bool>(out);           // false if the open or the write failed
}

int main() {
    std::ofstream("log.txt") << "start\n";   // truncate and write one line
    log_line("log.txt", "round 1");
    log_line("log.txt", "round 2");

    std::ifstream in("log.txt");
    std::string line;
    while (std::getline(in, line)) std::cout << line << "\n";

    std::cout << log_line("no_such_dir/log.txt", "x") << "\n";
}

start
round 1
round 2
0

How it works

• Every log_line call opens the file in append mode, adds one line and closes it again when out is destroyed.
• The last call fails because the folder doesn't exist, and the function reports that by returning false (printed as 0).
• static_cast<bool>(out) asks the stream "did everything so far succeed?". A stream remembers failures, so checking once at the end covers the open and every write.

Why check after writing?

Opening can succeed and a later write can still fail: the disk fills up, a USB stick is removed, a quota is exceeded. If the data matters, check the stream after writing, and report failure to the caller rather than pretending all is well.

Common mistakes

• Forgetting std::ios::app and wiping a log every time the program starts.
• Writing a file and reading it back in the same scope while the ofstream is still open. Its data may still be in a buffer. Let it go out of scope (or call flush()) first.

Your turn: write bool save_scores(const std::string& path, const std::map<std::string, int>& scores) that writes one name=score line per entry (the map's order), replacing any existing file. Return whether everything succeeded.

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