Re: [Boost-users] [serialization] Serialization of derived classes

From: Robert Ramey (ramey_at_[hidden])
Date: 2006-10-23 12:20:06

For this two work, the base class MUST be polymorphic.
To fix this make one of your functions "virtual".  A simple
and common way to do this is to make the destructor
virtual.

Another good idea in my opinion is to make the
base class abstract by setting all functions "=0".
(you'll have to use BOOST_IS_ABSTRACT
as well).  This is not required for serializaton.
But I believe it helps clarify the design pattern
where the base class is an interface while the
derived classes are specific implementations.
In this case I find it helpful to make the constructors
(and destructor) "protected:") as well.

Robert Ramey

Christian Henning wrote:
> Hi there, I'm having problems when serializing derived classes from a
> non-polymorphic base class. I have read the section "Pointers to
> Objects of Derived Classes". But for some reasons I cannot make these
> tricks to work in my application. I have created a small sample
> application that shows my problem:
>
> class base
> {
> public:
>   base() : _type( 0 ) {}
>   base( unsigned int type) : _type( type ) {}
>
>   unsigned int type() { return _type; }
> private:
>
>   friend boost::serialization::access;
>
>   template< class ARCHIVE >
>   void serialize( ARCHIVE& ar, const unsigned int version )
>   {
>      ar & _type;
>   }
>
> private:
>
>   unsigned int _type;
> };
>
> class derived1 : public base
> {
> public:
>   derived1() : base( 1 ), _value( 2.1 ) {}
>
> private:
>
>   friend boost::serialization::access;
>
>   template< class ARCHIVE >
>   void serialize( ARCHIVE& ar, const unsigned int version )
>   {
>      ar & boost::serialization::base_object<base>( *this );
>      ar & _value;
>   }
>
> private:
>
>   double _value;
> };
>
> class derived2 : public base
> {
> public:
>   derived2() : base( 2 ), _value( 8 ) {}
>
> private:
>
>   friend boost::serialization::access;
>
>   template< class ARCHIVE >
>   void serialize( ARCHIVE& ar, const unsigned int version )
>   {
>      ar & boost::serialization::base_object<base>( *this );
>      ar & _value;
>   }
>
> private:
>
>   short _value;
> };
>
> BOOST_CLASS_EXPORT( derived1 );
> BOOST_CLASS_EXPORT( derived2 );
>
> int _tmain(int argc, _TCHAR* argv[])
> {
>   std::string data;
>
>   {
>      std::ostringstream archive_stream;
>      boost::archive::binary_oarchive archive( archive_stream );
>
>      derived2 d;
>      archive & d;
>
>      data = archive_stream.str();
>   }
>
>   {
>      // deserialize
>      std::string archive_data( &data[0], data.size() );
>      std::istringstream archive_stream( archive_data );
>      boost::archive::binary_iarchive archive( archive_stream );
>
>      // I don't know what derived class was serialized.
>      base* b;
>      archive & b;
>
>      if( b->type() == 1 )
>      {
>         derived1* d1 = static_cast<derived1*>( b );
>      }
>      else
>      {
>         derived2* d2 = static_cast<derived2*>( b );
>      }
>   }
>
> return 0;
> }
>
> Any help is more than welcome.
>
> Thanks ahead,
> Christian