Join vector of strings to std::ostream (like boost::join)

Question

I have a vector of strings and I want output it to stream (file stream, actually). And I want to have a delimiter between vector elements. There is a way to use standard ostream_iterator

std::vector <std::string> strs;
std::ostream_iterator<std::string> out_file_iterator ( out_file, delim );
std::copy ( strs.begin(), strs.end(), out_file_iterator );

I didn't like this way because there is a delim text after each element, but I don't need to have a delim after last element. I'd like to use something like boost::join. However boost::join returns string and my vector too large to output it to string.

What is most elegant way to achieve my goal?

Answer 1

For a general solution (untested):

template<class T>
class ostream_join_iterator {
public:

    // Construct like an ostream_iterator.
    ostream_join_iterator(std::ostream& stream,
        const std::string& delimiter = "")
        : stream(stream), delimiter(delimiter), first(true) {}

    // Behave like an output iterator.
    ostream_join_iterator& operator++()    { return *this; }
    ostream_join_iterator& operator++(int) { return *this; }
    ostream_join_iterator& operator*()     { return *this; }

    // Output a delimiter before all but the first element.
    template<class T>
    ostream_join_iterator& operator=(const T& value) {
        if (!first) {
            stream << delimiter;
        } else {
            first = false;
        }
        stream << value;
        return *this;
    }

private:

    std::ostream& stream;
    const std::string delimiter;
    bool first;

};

You can use it like a regular std::ostream_iterator:

std::copy(strings.begin(), strings.end(),
    ostream_join_iterator<std::string>(file, delimiter));