Question

Why does ifstream set the failbit to 1 after reading the last line of the specified file? How do I know if the specified file has been read correctly?

bool read_csv_file(const char* filename, vector<string>& lines, bool adding = false)
{
    if( !adding ) lines.clear();

    ifstream csvfile;
    csvfile.open(filename);

    if( csvfile.is_open() )
    {
        string line;
        while( csvfile.good() && getline(csvfile,line) )
        {
            lines.push_back(line);
            cout << "fail: " << csvfile.fail() << endl;
        }
        cout << "fail: " << csvfile.fail() << endl;
        csvfile.close();
        return (!csvfile.fail());
    }

    return false;
}

Answer 1

The only reason failbit could be set after reading the last
line (or any line) is if there were an error in the library, and
I don’t really believe it. If failbit is set, it means that
you didn’t read anything. In your case, it will never be set
when you’re in the loop; if it were set, getline would have
evaluated to false, and you wouldn’t have entered the loop.
And of course, the loop terminates precisely because getline
fails (or would fail—normally, you would not test for
good before doing input, and if you do, consider that the
failbit was set, regardless, if the test fails).

The usual pattern for this sort of thing is:

while ( someInput ) {
    //  ...
}
if ( csvfile.bad() ) {
    //  Serious error (disk read error, etc.)...
} else if ( ! csvfile.eof() ) {
    //  Formatting error...
} else {
    //  Normal end of file...
}

When someInput is std::getline(), however, you will never
fail because of a formatting error, so the else if above will
never be true (and a lot of code treats hard disk errors as if
they were an end of file, and so ignores the if part as well).