If an object is serialized (saved) from a pointer, the loading part automatically creates (instantiates) a new object. However, there are situations when the object is already created and all we need is to restore its state.

The following code demonstrates one possible - although not elegant - solution using a temporary object:

#include <fstream>
#include <iostream>

#include <boost/archive/xml_iarchive.hpp>
#include <boost/archive/xml_oarchive.hpp>
#include <boost/serialization/string.hpp>
#include <boost/serialization/export.hpp>

class A
{
public:
    A()
    : m_val(100)
    {
    }
    
    int m_val;

private:
    friend class boost::serialization::access;
    template <typename Archive>
    void serialize(Archive &ar, const unsigned int version)
    {
        ar & BOOST_SERIALIZATION_NVP(m_val);
    }
};

int main(int argc, char* argv[])
{
    A *a = new A();
    
    // Save the object
    {
        std::ofstream os("d:/test.xml");
        boost::archive::xml_oarchive oa(os);
        // Save from pointer
        oa & BOOST_SERIALIZATION_NVP(a);
    }
    
    // Change the object state
    a->m_val = 200;

    // Restore the state
    {
        std::ifstream is("d:/test.xml");
        boost::archive::xml_iarchive ia(is);
        
        A* aTemp;
        ia & BOOST_SERIALIZATION_NVP(aTemp);
        a->m_val = aTemp->m_val; // Copy object state
        assert(a->m_val == 100);
        delete aTemp;
    }
}