How to specify an nvp wrapper for boost::serialization?

I am trying to learn how to use boost::serialization for recording an object. For example, I am using the following codes to serialize an object:

struct Abc
{
    int a;
    float b;
    double c;
};

namespace boost
{
    namespace serialization
    {
        template <typename Archive>
        void serialize(Archive& ar, Abc &obj,const unsigned int version)
        {
            ar & BOOST_SERIALIZATION_NVP(obj.a);
            ar & BOOST_SERIALIZATION_NVP(obj.b);
            ar & BOOST_SERIALIZATION_NVP(obj.c);
        };

    }
};

    Abc kkk;

    std::ofstream ofs(tmpFile);

    boost::archive::xml_oarchive ar(ofs);

    ar & kkk;
    ofs.close();

However, when I compile the codes, I have the following compilation error:

Error   1   error C2664: 'boost::mpl::assertion_failed' : cannot convert parameter 1 from 'boost::mpl::failed ************boost::serialization::is_wrapper<T>::* ***********' to 'boost::mpl::assert<false>::type'   \boost_1_50_0\boost\archive\basic_xml_oarchive.hpp 92

The error message leads to the source code of boost:

 template<class T>
    void save_override(T & t, BOOST_PFTO int)
    {
        // If your program fails to compile here, its most likely due to
        // not specifying an nvp wrapper around the variable to
        // be serialized.
        BOOST_MPL_ASSERT((serialization::is_wrapper< T >));
        this->detail_common_oarchive::save_override(t, 0);
    }

Any ideas? Thanks.

You need to make your myAbc instance of Abc in your code at the bottom an NVP, too:

ar & BOOST_SERIALIZATION_NVP(myAbc);

Everything that's serialized to an XML archive must be constructed this way, not just the subelements.