Error LNK2005 (already defined in object) using boost serialization with derived classes

Question

I am using boost serialization to save and load data. My goal is to store a vector containing objects. Their type is a derived class.

The linker tells me that there is some stuff that is already defined in the obj-file of another class (which has nothing to do with my serialization).

Can someone tell me what is wrong with my code? Am I missing something?
Is my code for serializing derived classes correct?

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This is the output:

Error 1 error LNK2005: "public: static struct boost::archive::detail::extra_detail::guid_initializer const & const boost::archive::detail::extra_detail::init_guid::g" (?g@?$init_guid@VCYesNoElement@@@extra_detail@detail@archive@boost@@2ABU?$guid_initializer@VCYesNoElement@@@2345@B) already defined in ElementFactory.obj

Error 2 error LNK2005: "public: static struct boost::archive::detail::extra_detail::guid_initializer const & const boost::archive::detail::extra_detail::init_guid::g" (?g@?$init_guid@VCYesNoElement@@@extra_detail@detail@archive@boost@@2ABU?$guid_initializer@VCYesNoElement@@@2345@B) already defined in ElementFactory.obj

Error 3 error LNK2005: "public: static struct boost::archive::detail::extra_detail::guid_initializer const & const boost::archive::detail::extra_detail::init_guid::g" (?g@?$init_guid@VCYesNoElement@@@extra_detail@detail@archive@boost@@2ABU?$guid_initializer@VCYesNoElement@@@2345@B) already defined in ElementFactory.obj

Error 4 error LNK2005: "public: static struct boost::archive::detail::extra_detail::guid_initializer const & const boost::archive::detail::extra_detail::init_guid::g" (?g@?$init_guid@VCYesNoElement@@@extra_detail@detail@archive@boost@@2ABU?$guid_initializer@VCYesNoElement@@@2345@B) already defined in ElementFactory.obj

Error 5 error LNK2005: "public: static struct boost::archive::detail::extra_detail::guid_initializer const & const boost::archive::detail::extra_detail::init_guid::g" (?g@?$init_guid@VCYesNoElement@@@extra_detail@detail@archive@boost@@2ABU?$guid_initializer@VCYesNoElement@@@2345@B) already defined in ElementFactory.obj

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This is the base class (IElement):

#ifndef __ELEMENT_H__
#define __ELEMENT_H__

#include <string>
#include <vector>
#include <cassert>

#include <boost\serialization\base_object.hpp>
#include <boost\serialization\export.hpp>
#include <boost\archive\binary_iarchive.hpp>
#include <boost\archive\binary_oarchive.hpp>

typedef enum eElementType
{
    eMultipleChoice,
    eYesNo,
    eKeyword,
    eText,
    eCloze
} eType;

class IElement
{

public:
    virtual ~IElement();

    void SetFrontText( const std::string& question );
    std::string GetFrontText();
    virtual eType GetType() = 0;

    int GetStatus();
    void StatusUp();
    void StatusDown();
    void Reset();

    template<class Archive>
    void serialize(Archive & ar, const unsigned int version)
    {
        ar & boost::serialization::make_nvp( "Status", m_status );
        ar & boost::serialization::make_nvp( "FrontText", m_frontText );
    }

protected:
    bool VectorContainsString( std::vector<std::string>* _vec, const std::string& _str );
    bool RemoveStringFromVector( std::vector<std::string>* _vec, const std::string& _str );

    std::string m_frontText;

    int m_status;
};
BOOST_SERIALIZATION_ASSUME_ABSTRACT( IElement )

#endif // __ELEMENT_H__

---

This is the derived class (CYesNoElement):

#ifndef __YES_NO_ELEMENT_H__
#define __YES_NO_ELEMENT_H__

#include "Element.h"

class CYesNoElement : public IElement
{
public:
    CYesNoElement();
    virtual ~CYesNoElement();

    bool GetSolution();
    void SetSolution( bool solution );

    bool check( bool given_answer );

    // IElement
    virtual eType GetType();

    template<class Archive>
    void serialize(Archive & ar, unsigned int file_version)
    {
        BOOST_SERIALIZATION_BASE_OBJECT_NVP( IElement );
        ar & boost::serialization::make_nvp( "Solution", m_solution );
    }

private:
    bool m_solution;
};
BOOST_CLASS_EXPORT(CYesNoElement)

#endif // __YES_NO_ELEMENT_H__

---

This is the class CElementFactory where the linker errors are hinting at:

#ifndef __ELEMENTFACTORY_H__
#define __ELEMENTFACTORY_H__

#include "ClozeElement.h"
#include "KeywordElement.h"
#include "MultipleChoiceElement.h"
#include "TextElement.h"
#include "YesNoElement.h"

class CElementFactory
{
public:
    CElementFactory(void);
    virtual ~CElementFactory(void);

    IElement* CreateElement( eType type );

private:
    CClozeElement* CreateClozeElement();
    CKeywordElement* CreateKeywordElement();
    CMultipleChoiceElement* CreateMultipleChoiceElement();
    CTextElement* CreateTextElement();
    CYesNoElement* CreateYesNoElement();
};

#endif // __ELEMENTFACTORY_H__

Answer 1

Put BOOST_CLASS_EXPORT(CYesNoElement) in the .cpp.