Boost Serialization : Does the default constructor have to be public?

Just a tid bit of information that doesn't seem documented anywhere. Does anyone know? Because I would like to make it private, hoping that the cons开发者_运维知识库tructor would be called from boost::serialization::access which is declared as a friend.

Test example. Given that this works I'd assume it is a feature and would be upset if there was a future release that didn't allow the access granting mechanism to grant access to private default constructors.

#include <boost/serialization/access.hpp>
#include <boost/serialization/shared_ptr.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/archive/text_oarchive.hpp>
#include <sstream>
#include <iostream>

struct Colour {
    double colour[4];
    boost::shared_ptr<Colour> alt;
    static boost::shared_ptr<Colour> test() {
       return boost::shared_ptr<Colour>(new Colour);
    }
private:
    friend class boost::serialization::access;

    template<class Archive>
    void serialize(Archive & ar, const unsigned int /*file_version*/) {
        ar & colour;
    }

    Colour() {
        std::cout << "Getting called" << std::endl;
    }
};

int main() {
    boost::shared_ptr<Colour> c = Colour::test();
    c->alt = Colour::test();

    std::cout << "Created" << std::endl;

    std::stringstream str;
    boost::archive::text_oarchive oa(str);
    oa & c;

    std::cout << "Saved" << std::endl;

    c.reset();

    boost::archive::text_iarchive ia(str);
    ia & c;

    std::cout << "Restored" << std::endl;
}

(Interestingly it seems to default construct one and then copy construct another on my system).