C++ File I/O Reading a file into a container


Example

In the example below we use std::string and operator>> to read items from the file.

    std::ifstream file("file3.txt");

    std::vector<std::string>  v;

    std::string s;
    while(file >> s) // keep reading until we run out
    {
        v.push_back(s);
    }

In the above example we are simply iterating through the file reading one "item" at a time using operator>>. This same affect can be achieved using the std::istream_iterator which is an input iterator that reads one "item" at a time from the stream. Also most containers can be constructed using two iterators so we can simplify the above code to:

    std::ifstream file("file3.txt");

    std::vector<std::string>  v(std::istream_iterator<std::string>{file},
                                std::istream_iterator<std::string>{});

We can extend this to read any object types we like by simply specifying the object we want to read as the template parameter to the std::istream_iterator. Thus we can simply extend the above to read lines (rather than words) like this:

// Unfortunately there is  no built in type that reads line using >>
// So here we build a simple helper class to do it. That will convert
// back to a string when used in string context.
struct Line
{
    // Store data here
    std::string data;
    // Convert object to string
    operator std::string const&() const {return data;}
    // Read a line from a stream.
    friend std::istream& operator>>(std::istream& stream, Line& line)
    {
        return std::getline(stream, line.data);
    }
};


    std::ifstream file("file3.txt");

    // Read the lines of a file into a container.
    std::vector<std::string>  v(std::istream_iterator<Line>{file},
                                std::istream_iterator<Line>{});