Common confusions with serializing polymorphic types

Question

I have seen many questions, tutorials, and documentation involving serializing derived classes, and I haven\'t been able to reach a consensus on several issues, including (a

Answer 1

Following @sehe's advice, here are some example uses:

Serialize derived class object, not forwarding to parent

#include 
#include 

#include 

class AbstractPoint
{
public:
    virtual ~AbstractPoint(){}
    virtual void DoSomething() = 0;
};

class Point : public AbstractPoint
{
public:
    Point() = default;
    Point(const double data) : mData(data) {}

    void DoSomething(){}

    template
    void serialize(TArchive& archive, const unsigned int version)
    {
        archive & mData;
    }

    double mData;
};

int main()
{
    Point point(7.4);

    std::ofstream outputStream("test.txt");
    boost::archive::text_oarchive outputArchive(outputStream);
    outputArchive << point;
    outputStream.close();

    Point pointRead;
    std::ifstream inputStream("test.txt");
    boost::archive::text_iarchive inputArchive(inputStream);
    inputArchive >> pointRead;

    std::cout << pointRead.mData << std::endl;
    return 0;
}

Serialize derived class object, including (automatic) forwarding to parent:

#include 
#include 

#include 

class AbstractPoint
{
public:
    virtual ~AbstractPoint(){}
    virtual void DoSomething() = 0;

    double mParentData = 3.1;

    template
    void serialize(TArchive& archive, const unsigned int version)
    {
        archive & mParentData;
    }
};

class Point : public AbstractPoint
{
public:
    Point() = default;
    Point(const double data) : mData(data) {}

    void DoSomething(){}

    template
    void serialize(TArchive& archive, const unsigned int version)
    {
        // this is not required, the parent serialize() seems to be called automatically
        // archive & boost::serialization::base_object(*this);

        archive & mData;
    }

    double mData;
};

int main()
{
    Point point(7.4);

    std::ofstream outputStream("test.txt");
    boost::archive::text_oarchive outputArchive(outputStream);
    outputArchive << point;
    outputStream.close();

    Point pointRead;
    std::ifstream inputStream("test.txt");
    boost::archive::text_iarchive inputArchive(inputStream);
    inputArchive >> pointRead;

    std::cout << pointRead.mParentData << std::endl;
    std::cout << pointRead.mData << std::endl;
    return 0;
}

Serialize derived class pointer, not forwarding to parent (note nothing changes from the object case)

#include 
#include 
#include 

#include 

class AbstractPoint
{
public:
    virtual ~AbstractPoint(){}
    virtual void DoSomething() = 0;
};

class Point : public AbstractPoint
{
public:
    Point() = default;
    Point(const double data) : mData(data) {}

    void DoSomething(){}

    template
    void serialize(TArchive& archive, const unsigned int version)
    {
        archive & mData;
    }

    double mData;
};

int main()
{
    std::shared_ptr point(new Point(7.4));

    std::ofstream outputStream("test.txt");
    boost::archive::text_oarchive outputArchive(outputStream);
    outputArchive << point;
    outputStream.close();

    std::shared_ptr pointRead;
    std::ifstream inputStream("test.txt");
    boost::archive::text_iarchive inputArchive(inputStream);
    inputArchive >> pointRead;

    std::cout << pointRead->mData << std::endl;
    return 0;
}

Serialize derived class pointer, forwarding to parent (note nothing changes from the object case)

#include 
#include 
#include 

#include 

class AbstractPoint
{
public:
    virtual ~AbstractPoint(){}
    virtual void DoSomething() = 0;

    template
    void serialize(TArchive& archive, const unsigned int version)
    {
        archive & mParentData;
    }

    double mParentData = 3.1;
};

class Point : public AbstractPoint
{
public:
    Point() = default;
    Point(const double data) : mData(data) {}

    void DoSomething(){}

    template
    void serialize(TArchive& archive, const unsigned int version)
    {
        archive & mData;
    }

    double mData;
};

int main()
{
    std::shared_ptr point(new Point(7.4));

    std::ofstream outputStream("test.txt");
    boost::archive::text_oarchive outputArchive(outputStream);
    outputArchive << point;
    outputStream.close();

    std::shared_ptr pointRead;
    std::ifstream inputStream("test.txt");
    boost::archive::text_iarchive inputArchive(inputStream);
    inputArchive >> pointRead;

    std::cout << pointRead->mParentData << std::endl;
    std::cout << pointRead->mData << std::endl;
    return 0;
}

Serialize base class pointer (We now have to register the type of the derived class with the archives, as well as use boost::serialization::base_object)

#include 
#include 
#include 
#include 

#include 

class AbstractPoint
{
public:
    virtual ~AbstractPoint(){}
    virtual void DoSomething() = 0;

    // This is required if we want to serialize an AbstractPoint pointer
    template
    void serialize(TArchive& archive, const unsigned int version)
    {
        // do nothing
    }
};

class Point : public AbstractPoint
{
public:
    Point() = default;
    Point(const double data) : mData(data) {}

    void DoSomething(){}

    template
    void serialize(TArchive& archive, const unsigned int version)
    {
        // Without this, we get unregistered void cast
        archive & boost::serialization::base_object(*this);

        archive & mData;
    }

    double mData;
};

int main()
{
    std::shared_ptr point(new Point(7.4));

    std::ofstream outputStream("test.txt");
    boost::archive::text_oarchive outputArchive(outputStream);
    outputArchive.register_type();
    outputArchive << point;
    outputStream.close();

    std::shared_ptr pointRead;
    std::ifstream inputStream("test.txt");
    boost::archive::text_iarchive inputArchive(inputStream);
    inputArchive.register_type();
    inputArchive >> pointRead;

    std::shared_ptr castedPoint = std::dynamic_pointer_cast(pointRead);
    std::cout << castedPoint->mData << std::endl;
    return 0;
}