boost::serialization optional does not use boost::serialization::access

From: Janek Kozicki (janek.listy.mailowe_at_[hidden])
Date: 2019-07-21 20:24:53

Hi, this simple program will not compile. The only way to make it
work is to make a public default constructor. This defeats the purpose
of boost::serialization::access.

I tried to fix <boost/serialization/optional.hpp> but I couldn't get
it to correctly call the access::construct();

Please help.

// g++ -o serialize_optional /tmp/serialize_optional.cpp -lboost_serialization

#include <fstream>
#include <iostream>
#include <boost/serialization/nvp.hpp>
#include <boost/serialization/serialization.hpp>
#include <boost/archive/xml_iarchive.hpp>
#include <boost/archive/xml_oarchive.hpp>
#include <boost/serialization/access.hpp>
#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/serialization/optional.hpp>
#include <boost/optional.hpp>

class Foo {
        public:
                Foo(int k) : i(k) {}
                friend std::ostream& operator<<(std::ostream& os, Foo const& foo) {
                        return os << "Foo { " << foo.i << " }";
                }
        private:
                Foo() : i(-1) {};
                friend class boost::serialization::access;

                template<class Archive>
                void serialize(Archive & ar, const unsigned int){
                        #define N(a) BOOST_SERIALIZATION_NVP(a)
                        ar & N(i);
                        #undef N
                };
                int i;
};

int main() {
        using Data = boost::optional<Foo>;
        std::cout << "Boost version: " << BOOST_VERSION << "\n";
        {
                auto f = Data(133);
                std::ofstream os("test.xml");
                boost::archive::xml_oarchive oa(os);
                oa << boost::serialization::make_nvp("f", f);
        }
        {
                Data g;
                std::ifstream is("test.xml");
                boost::archive::xml_iarchive ia(is);
                ia >> boost::serialization::make_nvp("f", g);
                
                std::cout << g.get() << "\n";
        }
}

best regards
Janek Kozicki