How do I iterate over cin line by line in C++?

Question

I want to iterate over std::cin, line by line, addressing each line as a std::string. Which is better:

string line;
while (getline         


        

Answer 1

Since UncleBen brought up his LineInputIterator, I thought I'd add a couple more alternative methods. First up, a really simple class that acts as a string proxy:

class line {
    std::string data;
public:
    friend std::istream &operator>>(std::istream &is, line &l) {
        std::getline(is, l.data);
        return is;
    }
    operator std::string() const { return data; }    
};

With this, you'd still read using a normal istream_iterator. For example, to read all the lines in a file into a vector of strings, you could use something like:

std::vector lines;

std::copy(std::istream_iterator(std::cin), 
          std::istream_iterator(),
          std::back_inserter(lines));

The crucial point is that when you're reading something, you specify a line -- but otherwise, you just have strings.

Another possibility uses a part of the standard library most people barely even know exists, not to mention being of much real use. When you read a string using operator>>, the stream returns a string of characters up to whatever that stream's locale says is a white space character. Especially if you're doing a lot of work that's all line-oriented, it can be convenient to create a locale with a ctype facet that only classifies new-line as white-space:

struct line_reader: std::ctype {
    line_reader(): std::ctype(get_table()) {}
    static std::ctype_base::mask const* get_table() {
        static std::vector 
            rc(table_size, std::ctype_base::mask());

        rc['\n'] = std::ctype_base::space;
        return &rc[0];
    }
};  

To use this, you imbue the stream you're going to read from with a locale using that facet, then just read strings normally, and operator>> for a string always reads a whole line. For example, if we wanted to read in lines, and write out unique lines in sorted order, we could use code like this:

int main() {
    std::set lines;

    // Tell the stream to use our facet, so only '\n' is treated as a space.
    std::cin.imbue(std::locale(std::locale(), new line_reader()));

    std::copy(std::istream_iterator(std::cin), 
        std::istream_iterator(), 
        std::inserter(lines, lines.end()));

    std::copy(lines.begin(), lines.end(), 
        std::ostream_iterator(std::cout, "\n"));
    return 0;
}

Keep in mind that this affects all input from the stream. Using this pretty much rules out mixing line-oriented input with other input (e.g. reading a number from the stream using stream>>my_integer would normally fail).