Question

I’m trying to serialize a pointer to a polymorphic class Shape. So I need to use the BOOST_CLASS_EXPORT macro to define a GUID for each subclass. The problem: where to put it?

Let me show a minimal test case first:

shapes.hpp

#include <boost/serialization/access.hpp>
#include <boost/serialization/base_object.hpp>
#include <boost/serialization/export.hpp>

class Shape {
    friend class boost::serialization::access;

    template<typename Archive>
    void serialize(Archive &ar, unsigned int const version) {
        // nothing to do
    }

    public:
        virtual ~Shape() { }
};

class Rect : public Shape {
    friend class boost::serialization::access;

    template<typename Archive>
    void serialize(Archive &ar, unsigned int const version) {
        ar & boost::serialization::base_object<Shape>(*this);
    }

    public:
        virtual ~Rect() { }
};

#ifdef EXPORT_IN_HEADER
    BOOST_CLASS_EXPORT(Rect)
#endif

export.cpp

#include <boost/serialization/export.hpp>
#include "shapes.hpp"

#ifdef EXPORT_IN_OBJECT
    BOOST_CLASS_EXPORT(Rect)
#endif

main.cpp

#include <iostream>
#include <boost/archive/text_oarchive.hpp>
#include <boost/serialization/export.hpp>
#include "shapes.hpp"

#ifdef EXPORT_IN_MAIN
    BOOST_CLASS_EXPORT(Rect)
#endif

int main() {
    Shape *shape = new Rect();
    boost::archive::text_oarchive ar(std::cout);
    ar << shape;
}

On gcc, I compile these with

g++ -omain main.cpp export.cpp -Wl,-Bstatic -lboost_serialization-mt -Wl,-Bdynamic -DEXPORT_IN_XXX

Here, export.cpp may look a bit silly. In my actual situation, it contains an enclosing class that uses the PIMPL idiom, and tries to serialize its (polymorphic) Shape implementation. The important point is: the BOOST_CLASS_EXPORT could be in a different object file than the code that invokes the serialization.

So here’s the problem: where to use BOOST_CLASS_EXPORT? I have three options, which can be enabled using the EXPORT_IN_XXX macros.

1. EXPORT_IN_MAIN works, but is not what I want. The code invoking the serialization should not need to know about the implementation details of the PIMPL class.

2. EXPORT_IN_OBJECT compiles, but does not work: it results in a boost::archive::archive_exception with the message unregistered void cast. According to the documentation, this should be solved by serializing base classes using boost::serialization::base_object, like I did, but it doesn’t help.

3. EXPORT_IN_HEADER does not even compile. The macro BOOST_CLASS_EXPORT expands to a template specialization (which we’d like to be in the header file), but also to the definitiof of a static member therein. So I get a linker error about a multiple definition of 'boost::archive::detail::init_guid<Rect>::guid_initializer'.

If it matters, I’m using g++ 4.4.3 and Boost 1.40.

Answer 1

I ended up putting all the serialization code in a header s11n.h that is included from the CPP file that invokes the serialization. Essentially, the EXPORT_IN_MAIN scenario I sketched above, but with the BOOST_CLASS_EXPORT macro invocations in a different file.

This only works as long as only one compilation unit includes s11n.h, of course, so although it works for now, it’s no real solution…