Re: [Boost-users] [serialization] problem exporting derived classes

Subject: Re: [Boost-users] [serialization] problem exporting derived classes
From: Robert Ramey (ramey_at_[hidden])
Date: 2009-02-18 21:52:02

The problem is that you're saving the derived type and loading the base 
type.  If you want to load through a base class pointer, you have to save 
through a base class pointer.

change

  DerivedNode* n1 = new DerivedNode();
  DerivedNode* n2 = new DerivedNode();
  DerivedNode* n3 = new DerivedNode();
to  BaseNode* n1 = new DerivedNode();
  BaseNode* n2 = new DerivedNode();
  BaseNode* n3 = new DerivedNode();
Robert Ramey
Jens Weggemann wrote:
> Hello,
>
> I've been trying out boost::serialization and ran into a problem
> trying to serialize objects of derived classes.
> I set up this simple class hierarchy:
>
> class BaseNode {
>  std::vector<BaseNode*> children_;
>  ...
>  friend class boost::serialization::access;
>  template<class Archive>
>  void serialize(Archive &ar, unsigned version) {
>    ar & children_;
>  }
> };
> class DerivedNode : public BaseNode
> {
>  ...
>  friend class boost::serialization::access;
>  template<class Archive>
>  void serialize(Archive &ar, unsigned version) {
>    ar & boost::serialization::base_object<BaseNode>(*this);
>  }
> };
>
> All works fine as long as I register the types with the archives:
>
> arch.register_type(static_cast<BaseNode*>(NULL));
> arch.register_type(static_cast<DerivedNode*>(NULL));
>
> Now the documentation suggests exporting the classes as an alternative
> to that. But when I instead put
>
> BOOST_CLASS_EXPORT(BaseNode)
> BOOST_CLASS_EXPORT(DerivedNode)
>
> the polymorphism is not handled correctly and e.g. a tree of 3
> DerivedNodes is restored as a single BaseNode object.
> What am I missing here?
>
> The full code for this test is at
> http://noonti.de/tree_serialization.cpp.txt
>
>
> Thanks,
>
> Jens