In general, templates (like serialize) should not be defined in cpp files. The compiler must have the definition available when you call the function so that it can specialize it. If the definition is in a different cpp file than the call site, then the compiler can not specialize the function. It will simply try to call it and hope it got specialized elsewhere; linker errors result.

Your best option whould be to define serialize in the header file, like [gs]etSequenceNumber.

From: boost-users-bounces@lists.boost.org [mailto:boost-users-bounces@lists.boost.org] On Behalf Of Oliver Kania
Sent: Wednesday, August 08, 2007 9:48 AM
To: boost-users
Subject: [Boost-users] Subject : [Serialization] linker problem,serialization method cannot be found.

Hello and thanks for the help I received so far.
here is my current problem:

The class I try to serialize is:

class LIBPALO_NG_CLASS_EXPORT CubeCache : public CUBE_INFO {
        public:
            friend class boost::serialization::access;
            CubeCache();
            ~CubeCache();

            void setSequenceNumber( unsigned int sequenceNumber ) {
                m_SequenceNumber = sequenceNumber;
            }

            unsigned int getSequenceNumber() const {
                return m_SequenceNumber;
            }

        private:
            unsigned int m_SequenceNumber;
            template<class Archive>
            void serialize( Archive &ar, const unsigned int version );

        };

the cpp file looks as follows:

#include "CubeCache.h"
#include <boost/serialization/base_object.hpp>
namespace jedox {
    namespace palo                /* palo */
    {
        template<class Archive>
        void CubeCache::serialize( Archive &ar, const unsigned int version ) {
            // save/load base class information
            ar & boost::serialization::base_object<CUBE_INFO>( *this );
            ar & m_SequenceNumber;
        }

        CubeCache::CubeCache(){;}
        CubeCache::~CubeCache(){;}
    }
}                                /* jedox */

The CUBE_INFO structure serializes just fine. There are no linkage problems with the following code:

int main( int argc, char* argv[] ) {
    CUBE_iNFO* const c = new CUBE_INFO();
    std::ofstream ofs("filename");
    // save data to archive
    {
        boost::archive::text_oarchive oa( ofs );
        // write class instance to archive
        oa << ( c );
        // archive and stream closed when destructors are called
    }
    return 0;
}

However, when I use CubeCache instead of CUBE_INFO in the above code, i get a linkage error:
======================================================================
libpalo_ng.obj : error LNK2019: unresolved external symbol "private: void __thiscall jedox::palo::CubeCache::serialize<class boost::archive::text_oarchive>(class boost::archive::text_oarchive &,unsigned int)" ...
======================================================================

There are no linkage problems with the library when i do not use boost::serialization. I guess this happens
when the linker tries to link with the serialization library. LIBPALO_NG_CLASS_EXPORT is defined as:
#   define LIBPALO_NG_CLASS_EXPORT __declspec(dllexport)
Regards, Oliver