Question

…as someone may remember, I’m still stuck on C++ strings. Ok, I can write a string to a file using a fstream as follows

outStream.write((char *) s.c_str(), s.size());

When I want to read that string, I can do

inStream.read((char *) s.c_str(), s.size());

Everything works as expected. The problem is: if I change the length of my string after writing it to a file and before reading it again, printing that string won’t bring me back my original string but a shorter/longer one. So: if I have to store many strings on a file, how can I know their size when reading it back?

Thanks a lot!

Answer 1

You shouldn’t be using the unformatted I/O functions (read() and write()) if you just want to write ordinary human-readable string data. Generally you only use those functions when you need to read and write compact binary data, which for a beginner is probably unnecessary. You can write ordinary lines of text instead:

std::string text = "This is some test data.";
{
    std::ofstream file("data.txt");
    file << text << '\n';
}

Then read them back with getline():

{
    std::ifstream file("data.txt");
    std::string line;
    std::getline(file, line);
    // line == text
}

You can also use the regular formatting operator >> to read, but when applied to string, it reads tokens (nonwhitespace characters separated by whitespace), not whole lines:

{
    std::ifstream file("data.txt");
    std::vector<std::string> words;
    std::string word;
    while (file >> word) {
        words.push_back(word);
    }
    // words == {"This", "is", "some", "test", "data."}
}

All of the formatted I/O functions automatically handle memory management for you, so there is no need to worry about the length of your strings.