Re: [Boost-users] [serialization] Serialization through pointer to base class

From: Ruediger Berlich (ruediger.berlich_at_[hidden])
Date: 2007-04-04 11:44:43

Apologies - here is some info on the environment:

I'm running OpenSUSE 10.2 with g++ 4.1.2 and Boost 1.33.1 .

Thanks and Best Regards,
Ruediger

Ruediger Berlich wrote:

> Hi there,
> I am trying to serialize an object through a pointer to its base class.
> 
> My application is in the field of distributed processing. I have
> a "communicator" type of class, which receives pointers from other
> objects, serializes them, sends the generated string over a network for
> further, remote processing and then deals with the responses. It is
> supposed to know nothing about the objects it serializes, except that they
> contain a method "toString()", which is contained in the base class.
> 
> However, when I call that method, I only get a serialization of that base
> class, not of the derived class.
> 
> Here is some code to illustrate the problem. The output of the program is
> listed at the end of this posting.
> 
> How can I achieve serialization of the derived class (without explicit
> casting), if it is stored in a pointer to a base class ?
> 
> My assumption is that this has to do with the fact that the
> "serialization" method cannot be virtual, as it is a member function
> template.
> 
> Any help is appreciated.
> 
> Thanks and have a good day,
> Ruediger
> 
> /*************************************************************************/
> // headers and namespace statements removed ...
> 
> class base
>     :public vector<int>
> {
>     friend class boost::serialization::access;
>     
>     template<class Archive>
>     void serialize(Archive & ar, const unsigned int version){
>         using boost::serialization::make_nvp;
>         ar & make_nvp("vector",
>              boost::serialization::base_object<std::vector<int> >(*this));
>     }
> 
> public:
>     base(){ /* nothing */ }
>     virtual ~base() { /* nothing */ }
> 
>     virtual void appendValue(int val) = 0;
> 
>     string toString(void){
>         ostringstream ofs;
>         boost::archive::xml_oarchive oa(ofs);
>         // this should point to derived objects
>         oa << make_nvp("myp",*this);
>         return ofs.str();
>     }
> 
>     void printType(void){
>         cout << typeid(*this).name() << endl;
>     }
> };
> 
> BOOST_IS_ABSTRACT(base);
> 
> /****************************************************************************/
> 
> class derived
>     :public base
> {
>     friend class boost::serialization::access;
>     
>     template<class Archive>
>     void serialize(Archive & ar, const unsigned int version){
>         using boost::serialization::make_nvp;
>         ar & make_nvp("base",
>              boost::serialization::base_object<base>(*this));
>     }
> 
> public:
>     derived(){ /* nothing */ }
>     virtual ~derived() { /* nothing */ }
> 
>     void appendValue(int val){
>         this->push_back(val);
>     }
> };
> 
> 
> BOOST_CLASS_EXPORT(derived);
> 
> /****************************************************************************/
> 
> main()
> {    
>     base *myp = new derived();
>     myp->appendValue(1);
> 
>     // only serializes class "base"
>     cout << myp->toString();
> 
>     cout << "===========================" << endl;
> 
>     myp->printType();
>     // this in printType points to the correct type
>     cout << "===========================" << endl;
> 
>     // Explicit casting to the derived class yields the correct response
>     derived *mypp = (derived *)myp;
>     ostringstream ofs;
>     boost::archive::xml_oarchive oa(ofs);
>     oa << make_nvp("myp",*mypp);
>     cout << ofs.str() << endl;
> }
> 
> /****************************************************************************/
> 
> Here is the output of the program:
> 
> <?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
> <!DOCTYPE boost_serialization>
> <boost_serialization signature="serialization::archive" version="3">
> <myp class_id="0" tracking_level="0" version="0">
>         <vector>
>                 <count>1</count>
>                 <item>1</item>
>         </vector>
> </myp>
> ===========================
> 7derived
> ===========================
> <?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
> <!DOCTYPE boost_serialization>
> <boost_serialization signature="serialization::archive" version="3">
> <myp class_id="0" tracking_level="1" version="0" object_id="_0">
>         <base class_id="1" tracking_level="0" version="0">
>                 <vector>
>                         <count>1</count>
>                         <item>1</item>
>                 </vector>
>         </base>
> </myp>