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

From: Fred Lupien (fred.lupien_at_[hidden])
Date: 2007-10-24 15:59:06

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_mail.gmail.com...
> 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]