Question

It’s been a while since I’ve worked with File I/O in C++ (and just C++ in general) but I recently decided to use it to make a small console project for a friend.
My issue is that I’m having some issues with a string array and File I/O (I’m not sure which is causing the problem). My code is as follows (ReadPSWDS is an ifstream):

                int i = 0;
            string str[200];

            ReadPSWDS.clear();
            ReadPSWDS.open("myPasswords.DoNotOpen");

            if(ReadPSWDS.is_open())
            {
                while(!ReadPSWDS.eof())
                {
                    getline(ReadPSWDS, str[i]); //Store the line
                    if(str[i].length()<1 || str[i] == "")
                    {
                        //Ignore the line if it's nothing
                    }
                    else
                    {
                        i++; //Move onto the next 'cell' in the array
                    }
                }
            }

            ReadPSWDS.close();

My issue is that on testing this out, the string array would appear to be empty (and on writing all those lines to a file, the file is empty as expected).
Why is the string array empty and not filled with the appropriate lines of the text file?

Regards,

Joe

Answer 1

The loop you’ve written is clearly wrong: you’re testing eof() before
failure, and you’re not testing for failure after the getline. C++
I/O isn’t predictive. (I can’t be, since whether you’re at eof() will
depend on what you try to read.) The correct pattern would be:

while ( i < size(str) && getline( readSWDS, str[i] ) ) {
    if ( !str[i].empty() ) {
        ++ i;
    }

Note that I’ve added a test for i. As written, if your file contains
more than 200 lines, you’re in deep trouble.

I’m not sure that this is your problem, however; the loop as you’ve
written it will normally only cause problems on the last line.
(Typically, if the last line ends with a '\n', and is not empty, it
will appear twice in your array.) Unless, of course, your file does
contain more than 200 lines.

I might add that an even more typical idiom would be to make str an
std::vector<std::string>, and write the loop:

std::string line;
while ( std::getline( readSWDS, line ) ) {
    if ( !line.empty() ) {
        str.push_back(line);
    }
}

This avoids having to define a fixed maximum number of lines.