Unable to understand this C++ program return value

Question

I came across the following program in TCPPPL by Stroustrup:

int main()
{
    string from, to;
    cin >> from >> to;  // get source and target file names

    ifstream is {from};  // input stream for file "from"
    istream_iterator<string> ii {is};  // input iterator for stream
    istream_iterator<string> eos {};  // input sentinel

    ofstream os{to};    // output stream for file "to"
    ostream_iterator<string> oo {os,"\n"};   // output iterator for stream

    vector<string> b {ii,eos};  // b is a vector initialized from input [ii:eos)
    sort(b.begin(),b.end());   // sor t the buffer

    unique_copy(b.begin(),b.end(),oo);// copy buffer to output, discard //replicated values
    return !is.eof() || !os; // return error state (§2.2.1, §38.3)
}

My questions is what is the last line, i.e. return !is.eof() ||!os; doing. I know that if main returns non-zero value then it means an error but what is being returned here?

Answer 1

!is.eof()

returns whether you aren't at the end of the file you are reading (so true for not being at the end and false for being at the end) and

!os

returns whether or not your output file is not writable. In other words, this program returns:

1. true for when you haven't reached the end of your input file (regardless of 2.)
2. true for when the output file is not writable (regardless of 1.)
3. false for when you are at the end of the file and the output file is writable.