class Strings
{
    std::string d_emptyString;

    std::string *d_str;

    ...

    private:
        std::string &stringAt(size_t const idx) const;
}

// ---------------------------------------------------- //

std::string &Strings::stringAt(size_t const idx) const
{
    if(idx < d_size)
        return d_str[idx];
    else
        return d_emptyString;
}