Boost serialization with pointers and non-default

How would you serialize/deserialize this class using boost::serialization?

#include <vector>

struct Foo {
    struct Bar {
        std::vector<int> * data; // Must point to Foo::data

        Bar( std::vector<int> * d ) : data(d) { }
    };

    std::vector<int> data;
    std::vector<Bar> elements;

    Foo() {
        // do very time consuming calculation to populate "data" and "elements"
    }
};

The constructor in Foo must not be executed when the objected is loaded from the serialized data, but if the object is default constructed the constructor must be evaluated.

It is okay to add a default constructor to Bar, but after serialization the Foo::Bar::data must point to the Foo::data.

EDIT: Following is a non-working implementation of my attempt

This is my attempt based on the hints from @Matthieu. The problem is that when I deserialize Foo, I get no elements in Foo::data and Foo::elements.

struct Foo {
    struct Bar {
        std::vector<int> * data;

        Bar( ) : data( 0 ) { }
        Bar( std::vector<int> * d ) : data(d) { }

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

    std::vector<int> data;
    std::vector<Bar> elements;

    Foo() {
        std::cerr << "Running default constructor" << std::endl;
        data.push_back(1);
        data.push_back(2);
        data.push_back(3);
        data.push_back(4);
        data.push_back(5);
        elements.push_back( Bar( &data ) );
        elements.push_back( Bar( &data ) );
        elements.push_back( Bar( &data ) );
    }

    template<class Archive>
    Foo( Archive & ar ) {
        ar >> data; // is this corrent?
        ar >> elements;
    }

private:
    BOOST_SERIALIZATION_SPLIT_MEMBER();
    friend class boost::serialization::access;

    template<class Archive>
    void save(Archive & ar, const unsigned int version) const {
        const std::vector<int> * data_ptr = &data;

        // should data be seriliazed as pointer...
        // it is used as a pointer in Bar
        ar << data_ptr;
        ar << elements;
    }
};

int main(int argc, const char *argv[])
{
#if 0
    // serialize
    Foo foo;
    boost::archive::text_oarchive oar(std::cout);
    oar << foo;

#else
    // deserialize
    boost::archive::text_iarchive oar(std::cin);
    Foo foo(oar);

#endif
    std::cerr << foo.data.size() << std::endl;
    std::cerr << foo.elements.size() << std::endl;

    std::cerr << (&foo.data) << std::endl;
    for( const auto& a : foo.data )
        std::cerr << a << " ";
    std::cerr << std::endl;

    for( const auto& a : foo.elements)
        std::cerr << a.data << " ";
    std::cerr << std::endl;

    return 0;
}

There is a section in the documentation that describes how to (de)serialize classes with non-default constructors. See here.

Basically, you must implement two functions called save_construct_data and load_construct_data in the namespace boost::serialization to write out and read in the data used to construct instances of your class. You can then call a non-default constructor of Foo from the load_construct_data function with the parameters necessary to reconstruct a Foo object.

Here is a working example based on your updated code:

Note that I've used shared_ptr's to clarify that the data member serialized by Foo and Bar are referencing the same thing.

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

struct Foo {
    struct Bar {
        boost::shared_ptr< std::vector<int> > data; // Must point to Foo::data

        Bar( boost::shared_ptr< std::vector<int> > d ) : data(d) { }

        template<class Archive>
        void serialize(Archive & ar, const unsigned int version)
        {
          // ** note that this is empty **
        }
    };

    boost::shared_ptr< std::vector<int> > data;
    std::vector<Bar> elements;

    Foo() : data( new std::vector<int>() ) {
        std::cerr << "Running default constructor" << std::endl;
        data->push_back(1);
        data->push_back(2);
        data->push_back(3);
        data->push_back(4);
        data->push_back(5);
        elements.push_back( Bar( data ) );
        elements.push_back( Bar( data ) );
        elements.push_back( Bar( data ) );
    }

    template<class Archive>
    void serialize(Archive & ar, const unsigned int version)
    {
      // ** note that this is empty **
    }

    Foo(
      boost::shared_ptr< std::vector<int> > const & data_,
      std::vector<Bar> const & elements_ ) : data( data_ ), elements( elements_ )
    {
        std::cout << "cheap construction" << std::endl;
    }
};

namespace boost { namespace serialization {

template<class Archive>
inline void save_construct_data(
    Archive & ar, const Foo * foo, const unsigned int file_version
){
    ar << foo->data << foo->elements;
}

template<class Archive>
inline void load_construct_data(
    Archive & ar, Foo * foo, const unsigned int file_version
){
    boost::shared_ptr< std::vector<int> > data;
    std::vector<Foo::Bar> elements;

    ar >> data >> elements;

    ::new(foo)Foo(data, elements);
}

template<class Archive>
inline void save_construct_data(
    Archive & ar, const Foo::Bar * bar, const unsigned int file_version
){
    ar << bar->data;
}

template<class Archive>
inline void load_construct_data(
    Archive & ar, Foo::Bar * bar, const unsigned int file_version
){
    boost::shared_ptr< std::vector<int> > data;

    ar >> data;

    ::new(bar)Foo::Bar(data);
}

}}

int main()
{
  std::stringstream ss;

  {
    boost::scoped_ptr< Foo > foo( new Foo() );

    std::cout << "size before serialization is: " << foo->data->size() << std::endl;

    boost::archive::text_oarchive oa(ss);
    oa << foo;
  }

  {
    boost::scoped_ptr< Foo > foo;

    boost::archive::text_iarchive is(ss);
    is >> foo;

    std::cout << "size after deserialization is: " << foo->data->size() << std::endl;
  }

  return 0;
}