How about the following: template<class Archive> void save(Archive& ar, const unsigned int version) const { ar << _m; cout << "serialise A, _m = " << (unsigned long)_m << endl; } void save(Archive& ar, const unsigned int version) const { base * t; ar >> t; // deserialize to a temporary // modify _m according to t cout << "serialise A, _m = " << (unsigned long)_m << endl; delete t; } Robert Ramey Darryl Lawson wrote:

I realise that what I am doing is no doubt an unconventional usage of boost::serialization, but I wonder if any one has done a similar thing before.

I wish to serialize a derived class through a baseclass pointer, but on deserilize I do not want the object to be created (I do not want memory allocated) - as the object will have all ready been created.

An example, hopefully showing what I mean, is given at the end. The program will output something like,

construct A, _m = 155640808 serialise A, _m = 155640808 construct A, _m = 155649400 serialise A, _m = 155649808

But essentially I want the program to output something lke,

construct A, _m = 155640808 serialise A, _m = 155640808 construct A, _m = 155649400 serialise A, _m = 155649400

i.e. the second lot of construct A/serialise A will have the same address for _m.

The reason why I need this behaviour, is that only a sub-set of the derived class will be seriliazed, with the other parts of the class intialised differently depending on the session, and the initialisation of the class is always done _before_ deserialisation.

Is there anyway to get this behaviour from the boost::serialization library?

Many thanks, Darryl

Example -------

#include <fstream> #include <boost/archive/text_oarchive.hpp> #include <boost/archive/text_iarchive.hpp> #include <boost/serialization/export.hpp>

using namespace std;

struct base { virtual ~base() {} template<class Archive> void serialize(Archive& ar, const unsigned int version) {} };

struct derived : public base { derived() : base(), _value(0) {}

template<class Archive> void serialize(Archive& ar, const unsigned int version) { boost::serialization::void_cast_register<derived, base>(); ar & _value; }

int _value; };

BOOST_CLASS_EXPORT(derived);

struct A { A() { _m = new derived; cout << "construct A, _m = " << (unsigned long)_m << endl; }

template<class Archive> void serialize(Archive& ar, const unsigned int version) { // On deserialize this will create a new object, but I do not want it to, // rather I would like it to deserialize into the all ready created // object. ar & _m; cout << "serialise A, _m = " << (unsigned long)_m << endl; }

base* _m; };

void save(const A& a, const char* filename) { std::ofstream ofs(filename); boost::archive::text_oarchive oa(ofs); oa << a; }

void load(A& a, const char* filename) { std::ifstream ifs(filename); boost::archive::text_iarchive ia(ifs); ia >> a; }

int main(int argc, char* argv[]) { A a; save(a, "archive.txt"); A new_a; load(new_a, "archive.txt"); return 0; }