c++ - How to create a interface for serialization in Boost Serialization?

Question

I am newbie and I want to implement an interface to force users to implement the serialize method. This method is template and I can not define as virtual.

I would like the user only has to implement a function like this:

template<class Archive>
void serialize(Archive & ar, const unsigned int version)
{
  ...
};

I try to create this interface:

  class interface_serializing
  {
  public:
    virtual ~interface_serializing(void)
    { }

    friend class boost::serialization::access;

    virtual void serialize(boost::archive::polymorphic_iarchive & ar, 
                   const unsigned int version) = 0;
    virtual void serialize(boost::archive::polymorphic_oarchive & ar, 
                   const unsigned int version)  = 0;
  };

But it forces the user to implement these two methods and is not what I want.

Is there any way to do what I want?

Thank you

Answer 1

There's no builtin way, but you can, of course, always abstract any interface you wish, with the right set of trade-offs.

Here's one imagined solution that uses a custom PolyArchive that could be a reference to either the polymorphic_oarchive or polymorphic_iarchive:

#include <boost/serialization/serialization.hpp>
#include <boost/archive/polymorphic_oarchive.hpp>
#include <boost/archive/polymorphic_iarchive.hpp>
#include <boost/archive/polymorphic_text_oarchive.hpp>
#include <boost/archive/polymorphic_text_iarchive.hpp>
#include <boost/variant.hpp>
#include <sstream>

using PolyArchive = boost::variant<
            boost::archive::polymorphic_oarchive&,
            boost::archive::polymorphic_iarchive&
        >;

struct /*abstract*/ ISerializable {
    virtual void serialize(PolyArchive, unsigned) = 0;
};

struct MyClass : ISerializable {
    std::string data_member = "Morgana"; // something to serialize

    // the one method we need to implement
    virtual void serialize(PolyArchive ar, unsigned) override;
};

Now, let's do the implementation a bit with a bit of C++14 fairy-dust:

void MyClass::serialize(PolyArchive ar, unsigned) {
    boost::apply_visitor(make_visitor([=](auto& ar) { 
            ar & data_member;
        }), ar);
}

The astute reader will spot that the user still supplies the template method, but hides it inside a virtual method that actually takes a PolyArchive at compile time.

Full Demo

See it Live On Coliru, printing:

Serialized: 22 serialization::archive 11 0 0 7 Morgana
Roundtripped: 22 serialization::archive 11 0 0 7 Morgana

Code:

#include <boost/serialization/serialization.hpp>
#include <boost/archive/polymorphic_oarchive.hpp>
#include <boost/archive/polymorphic_iarchive.hpp>
#include <boost/archive/polymorphic_text_oarchive.hpp>
#include <boost/archive/polymorphic_text_iarchive.hpp>
#include <boost/variant.hpp>
#include <sstream>

using PolyArchive = boost::variant<
            boost::archive::polymorphic_oarchive&,
            boost::archive::polymorphic_iarchive&
        >;

struct /*abstract*/ ISerializable {
    virtual void serialize(PolyArchive, unsigned) = 0;
};

struct MyClass : ISerializable {
    std::string data_member = "Morgana"; // something to serialize

    // the one method we need to implement
    virtual void serialize(PolyArchive ar, unsigned) override;
};

int main()
{
    std::stringstream ss;
    {
        // serialize:
        boost::archive::polymorphic_text_oarchive output(ss);

        MyClass object;
        output << object;
    }

    // Debug dump;
    std::cout << "Serialized: " << ss.str();

    {
        // read back:
        boost::archive::polymorphic_text_iarchive input(ss);

        MyClass cloned;
        input >> cloned;

        std::cout << "Roundtripped: ";
        boost::archive::polymorphic_text_oarchive pta(std::cout);
        pta << cloned;
    }
}

////////////////////////////////
// implementation:

namespace /*detail*/ {
    template <typename F> struct wrap_visitor : boost::static_visitor<> {

        wrap_visitor(F const& f) : f_(f) { }
        wrap_visitor(F&& f)      : f_(std::move(f)) { }

        template<typename... T> void operator()(T&&... t) const {
            f_(std::forward<T>(t)...);
        }

    private:
        F f_;
    };

    template <typename F> 
    wrap_visitor<F> make_visitor(F&& f) {
        return std::forward<F>(f);
    }
}

void MyClass::serialize(PolyArchive ar, unsigned) {

    boost::apply_visitor(make_visitor([=](auto& ar) { 
            ar & data_member;
        }), ar);
}