Where to put BOOST_CLASS_EXPORT for boost::serialization?

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

Exporting Class Serialization of the Boost.Serialization docs (1.44.0) states the following:

---

BOOST_CLASS_EXPORT in the same source module that includes any of the archive class headers will instantiate code [...]

Note that the implemenation of this functionality requires that the BOOST_CLASS_EXPORT macro appear after and the inclusion of any archive class headers for which code is to be instantiated. So, code that uses BOOST_CLASS_EXPORT will look like the following:

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_oarchive.hpp>
... // other archives

#include "a.hpp" // header declaration for class a
BOOST_CLASS_EXPORT(a)
... // other class headers and exports

[...] Including BOOST_CLASS_EXPORT in the "a.hpp" header itself as one would do with other serialization traits will make it difficult or impossible to follow the rule above regarding inclusion of archive headers before BOOST_CLASS_EXPORT is invoked. This can best be addressed by using BOOST_CLASS_EXPORT_KEY in the header declarations and BOOST_CLASS_EXPORT_IMPLEMENT in the class definition file.

---

Answer 2

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...