Re: [Boost-users] Re: Serialization of cross-pointing objects

11 May 2005

      Robert Ramey wrote:
...
This should work - How is your case different that test_cycle.cpp ?
I can't find test_cycle.cpp unfortunately, so I don't know. See my example below.
...
The pointer conflict means that something has been serializaed as first as a
pointer then as an object.  Normally this should be easily avoidable.  This
is flagged as an error because the de-serialization would be a problem.
First the deserialization of a pointer creates an object - next the object
itself is re-loaded - but now we have two objects when we started with one.
I see what you mean, but I fail how to see a convenient way to avoid it without 
changing all containers so that they have pointers instead of objects or 
something like that. Is perhaps BOOST_CLASS_TRACKING the key?

---

#include <vector>
#include <fstream>
#include 
#include 

class A {
   friend class boost::serialization::access;
public:
   A() : next(0) { };
   A* next;
private:
   template<class Archive>
   void serialize(Archive & ar, const unsigned int){
     ar & BOOST_SERIALIZATION_NVP(next);
   }
};

int
main()
{
   std::ofstream ofs("test_of_serialization.dat");
   boost::archive::xml_oarchive oa(ofs);

   std::vector<A> a1(4);
   a1[0].next = &a1[0];
   a1[1].next = &a1[1];
   a1[2].next = &a1[2];
   a1[3].next = &a1[3];

   oa << BOOST_SERIALIZATION_NVP(a1);

   // everything worked fine with a1...

   std::vector a2(4);
   a2[0] = new A;
   a2[1] = new A;
   a2[2] = new A;
   a2[3] = new A;
   a2[0]->next = a2[1];
   a2[1]->next = a2[2];
   a2[2]->next = a2[3];
   a2[3]->next = a2[0];

   oa << BOOST_SERIALIZATION_NVP(a2);

   // everything worked fine with a2...

   std::vector<A> a3(4);
   a3[0].next = &a3[1];
   a3[1].next = &a3[2];
   a3[2].next = &a3[3];
   a3[3].next = &a3[0];

   // but this throws "pointer conflict" exception
   oa << BOOST_SERIALIZATION_NVP(a3);
}

---

// Henrik Jacobsson
...
Robert Ramey
Henrik Jacobsson wrote:
...
Hi!
I want to serialize objects that link to each other through pointers,
like a linked list, but in this case a more complex graph with
several different classes and proxies (a graph which can contain
loops and and null-pointers).
But I get the "pointer conflict" exception since the serialization
discovers that an object have been serialized once before through its
pointer. The contents should be serialized/deserialized only once,
and then the pointers should just be kept consistent.
An alternative is of course to replace all the pointers with indeces
which are memory-independent. But that's a messy affair.
In the copy constructor I solved something similar through a
map which mapped from the old pointers to the newly
allocated ones, so that they could be properly replaced. Is something
like this hidden somewhere in the lib?
Any suggestions?
Best regards,
Henrik Jacobsson
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