[Boost-users] order of pointer serialisation

From: martin f krafft (madduck_at_[hidden])
Date: 2005-05-10 03:11:52

Consider a doubly-linked list. How do you serialise this?

The natural way would be to provide a serialize function such as:

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

This, however, causes "pointer conflict" exception to be thrown,
because the next pointers are serialised before the actual list node
is serialised.

The following is an abridged scneario illustrating the same dilemma:

  class Foo {
    friend class boost::serialization::access;
    template <typename AT> void serialize(AT& ar, unsigned int const)
    {}
  };
    
  int main(void)
  {
    Foo x;
    Foo *y = &x;

    boost::archive::text_oarchive oa(std::cout);
    oa & y; // serialise the pointer before the actual instance
    oa & x; // throws boost::archive::archive_exception(pointer_conflict)

    return 0;
  }

I could come up with many more examples in which it is now granted
that an instance has been serialised by the time that a pointer to
that very instance needs to be serialised. How does
boost::serialization expect this challenge to be handled?

Note that I understand very well what pointer_conflict is all about.
I do wonder whether this protection is always necessary. Worse yet:
there seems to be no way to disable it; catching the
pointer_conflict skips over the serialisation of the instance.

Is the only way to deal with this to ensure that instances are
always saved first?

-- 
martin;              (greetings from the heart of the sun.)
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