C++ cout auto separator

Well, I got beaten to it. I’ll post this anyway.

Edit : well, after reading Nim’s answer, mine does achieve the exact syntax OP wished for.

#include <iostream>
#include <algorithm>

struct with_separator {
    with_separator(std::string sep)
    : sep(std::move(sep)) {}

    std::string sep;
};

struct separated_stream {
    separated_stream(std::ostream &stream, std::string sep)
    : _stream(stream), _sep(std::move(sep)), _first(true) {}

    template <class Rhs>
    separated_stream &operator << (Rhs &&rhs) {
        if(_first)
            _first = false;
        else
            _stream << _sep;

        _stream << std::forward<Rhs>(rhs);
        return *this;
    }

    separated_stream &operator << (std::ostream &(*manip)(std::ostream&)) {
        manip(_stream);
        return *this;
    }

    private:
    std::ostream &_stream;
    std::string _sep;
    bool _first;
};

separated_stream operator << (std::ostream &stream, with_separator wsep) {
    return separated_stream(stream, std::move(wsep.sep));
}

int main()
{
    std::cout << with_separator(", ") << 1 << 2 << 3 << std::endl;
}

Output :

1, 2, 3