std::string substring reference without copying

RefString.cpp

struct RefString
{
    RefString(const string & s, int i, int l) : s(s), i(i), l(l) {}

    const char & operator [] (int x) const {
        return s[i+x];
    }
    
    size_t length() const {
        return l;
    }
    
    bool operator < (const RefString & s2) const {
        return s.compare(i, l, s2.s, s2.i, s2.l) < 0;
    }
    
private:
    const string & s;
    int i;
    int l;
};

ostream & operator << (ostream &stream, const RefString & ms) {
    for (int i = 0; i < ms.length(); i++)
        stream << ms[i];
    return stream;
}