使用ofstream将数据写入文件后更新ifstream对象

Question

While testing my code, I faced an issue, that ifstream doesn't get updated when more data is written to its file. So here is a sample code that demonstrates the problem:

    ifstream is(filename);
    string line;
    while (getline(is, line))
        cout << "line: " << line << endl;

    ofstream os(filename, ofstream::out | ofstream::app);
    string additional("additional");
    os << additional;
    os.flush();

    while (getline(is, line))
        cout << "line additional: " << line << endl;

No additional lines were written to stdout , though they are written to the file. I'm not using fstream instead of a couple of if/ofstream because I need it like this for testing purposes.

How to make ifstream "see" the changes in the file?

UPDATE : I cleared the bits using clear method. It works OK on my Ubuntu machine with gcc. But it doesn't work on my Mac OSX with llvm. Do you know how to do it platform independently?

Answer 1

You need to call std::ios::clear on the input stream after the first read. When you read the whole file, it sets the failbit in the stream and will refuse to keep reading, even if the file actually changed in the meantime.

ifstream is(filename);
string line;
while (getline(is, line))
    cout << "line: " << line << endl;

ofstream os(filename, ofstream::out | ofstream::app);
string additional("additional");
os << additional;
os.flush();

is.clear(); //< Now we can read again
while (getline(is, line))
    cout << "line additional: " << line << endl;

Answer 2

When you hit the end of the file while reading is the first time, it sets the eofbit in the stream's internal error state.

You need to clear it before you can continuing reading, by calling the is.clear() function, which resets the internal error state.

Answer 3

I think the reason is that you have opened output stream file using "ofstream::out"

ofstream os(filename, ofstream::out | ofstream::app);

now, the "ofstream::out" truncates the file as soon as it opens it. That is, all the previous contents of your file will be deleted once it is opened. Please try using this instead:

ofstream os(filename, ios::app);