Re: [Boost-users] Question about versioning of serialization

From: Robert Ramey (ramey_at_[hidden])
Date: 2007-10-24 16:59:18

look in 
basic_archive.cpp

Robert Ramey

  "Fred Lupien" <fred.lupien_at_[hidden]> wrote in message news:32e6d6bf0710241259r1eba133fofb924c86369498c4_at_mail.gmail.com...
  Yea I had thought about this but haven't tried it yet. I am working on this tonight. I have another question that could help me. Is their a documentation available on exactly what is serialized in the archive, the class format and related stuff seem to be serialized as a list of numbers (at least in text archive it appears so, forgive me if I am wrong). What is serialized exactly for each class (beside what the user specifies of course!)? 

  If found that on a forum but I am not sure how relevant it is (from basic_archive.cpp) :

  ////////////////////////////////////////////////////////////////////// 
  // 
  // class_information is stored as 
  // 
  // class_id* // -1 for a null pointer 
  // if a new class id 
  // [ 
  // exported key - class name* 
  // tracking level - always/never 
  // file version 
  // ] 
  // 
  // if tracking 
  // [ 
  // object_id 
  // ] 
  // 
  // [ // if a new object id 
  // data... 
  // ] 
  // 
  // * required only for pointers - optional for objects 

  Thank you!

  On 10/19/07, Robert Ramey <ramey_at_[hidden]> wrote:
    I've never thought about this, but my first attempt would be something like  the following:

    template<class Archive>
    void load(Archiv &ar , C &c, const unsigned int version){
        if(version < 4>)
            ar >> boost::serialization::base_object<A>(c);
        else
            ar >> boost::serialization::base_object<B>(c);
    }
      "Fred Lupien" <fred.lupien_at_[hidden]> wrote in message news:32e6d6bf0710190708x45191057ld2a23a92399f59e4_at_[hidden] ...
      Hello,

            we are currently implementing serialization versioning using boost and I am curious to know if their is a way to support a changing class hierarchy. Let's say C derives from A for a while and then a change requires C to derive from B instead, which derives from A. Is it possible to support such a change provided we know what default values are needed for the new class? I tried and even if don't serialize anything in class C I get stream errors because I believe boost saves some base info for each class. When it tries to reload those value from a old stream, obviously it desynchronizes it because the values are not in the stream. So is their a way around this? 

      Also is it possible to support the removal of a class. If, for exemple C derives from B which in turn derives from A and suddenly B is removed. Is their a way to deal with this without invalidating all previously saved data? 

      *** I doubt it.  If you want to load old archives, you have know how much to "skip" over.  The only thing that MIGHT be possible
      is something like:
      template<class Archive>
      void load(Archiv &ar , C &c, const unsigned int version){
          if(version < 4>){
              class a; //no longer used
              ar >> a;
              // throw away old data on leaving scope
          }
          else
              ar >> boost::serialization::base_object<B>(c);
      }

      Thank you for your help!

      -- 
      Frédérick Martel-Lupien
      Étudiant en Génie Informatique
      Université de Sherbrooke
      fred.lupien_at_[hidden] 

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  -- 
  Frédérick Martel-Lupien
  Étudiant en Génie Informatique
  Université de Sherbrooke
  fred.lupien_at_[hidden] 

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