处理可能的空指针的类的增强序列化

我想序列化下面的类，包装一个可以处理null m_element的指针，就像你在调用默认构造函数时看到的那样。这个跟着这个问题。

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template <typename T>
struct Ptr { // Ptr could use init constructor here but this is not the point
    Ptr() { m_elem = 0; }
    Ptr(const T* elem) {
        if (elem)
            m_elem = new T(*elem);
        else
            m_elem = 0;
    }
    Ptr(const T& elem)
    {
        m_elem = new T(elem);
    }
    Ptr(const Ptr& elem)
    {
        if (elem.m_elem)
            m_elem = new T(*(elem.m_elem));
        else
            m_elem = 0;
    }
    virtual ~Ptr() { delete m_elem; m_elem = 0; };
    const T& operator*() const { return *m_elem; };
    T& operator*() { return *m_elem; };
    const T* operator->() const { return m_elem; };
    T* operator->() { return m_elem; };
    T* m_elem;
};
namespace boost { namespace serialization {
    // Not sure about the approach to manage null m_elem here
    template<class Archive, class T>
    void save(Archive & ar, const Ptr<T> &ptr, const unsigned int version)
    {
        T elem = 0;
        if (ptr.m_elem != 0)
            ar& boost::serialization::make_nvp("data", *ptr.m_elem);
        else
            ar& boost::serialization::make_nvp("data", elem);
    }
    // How to implement load ?
    template<class Archive, class T>
    void load(Archive & ar, Ptr<T> &ptr, const unsigned int version)
    {
        ar& boost::serialization::make_nvp("data", *ptr.m_elem);
    }
    template<class Archive, class T>
    void serialize(Archive & ar, Ptr<T> &ptr, const unsigned int version)
    {
        boost::serialization::split_free(ar, ptr, version);
    }
}} // end namespace

int main()
{
    {
        Ptr<A> p;
        std::ostringstream oss;
        boost::archive::xml_oarchive oa(oss);
        oa << BOOST_SERIALIZATION_NVP(p);
        std::cout << oss.str() << std::endl;
        // segfault
        Ptr<double> po;
        std::istringstream iss;
        iss.str(oss.str());
        boost::archive::xml_iarchive ia(iss);
        ia >> BOOST_SERIALIZATION_NVP(po);
    }
    {
        Ptr<double> p(new double(2.0));
        std::cout << *(p.m_elem) << std::endl;
        std::ostringstream oss;
        boost::archive::xml_oarchive oa(oss);
        oa << BOOST_SERIALIZATION_NVP(p);
        std::cout << oss.str() << std::endl;
        // segfault
        Ptr<double> po;
        std::istringstream iss;
        iss.str(oss.str());
        boost::archive::xml_iarchive ia(iss);
        ia >> BOOST_SERIALIZATION_NVP(po);
    }
}

序列化似乎工作，但反序列化给出了一个段错误。我正在使用c++ 0x。

• 如果可能的话，我如何提供安全的保存和加载函数来序列化Ptr而不改变Ptr ?
• 如果我需要修改Ptr，你有什么建议?

编辑:感谢Jarod42评论，我提出了以下保存/加载函数使用布尔值检测空指针或不。现在，当m_elem为空时，我不再有段错误，但当它不为空时，我有一个。

template<class Archive, class T>
void save(Archive & ar, const Ptr<T> &ptr, const unsigned int version)
{
    bool is_null;
    if (ptr.m_elem != 0) {
        is_null = false;
        ar& boost::serialization::make_nvp("is_null", is_null);
        ar& boost::serialization::make_nvp("data", *ptr.m_elem);
    }
    else
    {
        is_null = true;
        ar& boost::serialization::make_nvp("is_null", is_null);
    }
}
template<class Archive, class T>
void load(Archive & ar, Ptr<T> &ptr, const unsigned int version)
{
    bool is_null;
    ar& boost::serialization::make_nvp("is_null", is_null);
    if (is_null == true) {
        ptr.m_elem = 0;   
    }
    else
    {
        ar& boost::serialization::make_nvp("data", *ptr.m_elem);
    }
}