class ImmutableString
{
public:
    ImmutableString substring(std::size_t pos, std::size_t len)
    {
        return ImmutableString(
            std::shared_ptr<char[]>(m_data.get() + pos, m_data),
            len
        );
    }

private:
    std::shared_ptr<char[]> m_data;
    std::size_t m_length;
};