Re: [Boost-users] [Serialization] XML: Necessity of attribute class_id although class_name is given

Subject: Re: [Boost-users] [Serialization] XML: Necessity of attribute class_id although class_name is given
From: Robert Ramey (ramey_at_[hidden])
Date: 2014-07-28 15:28:15

Frank Stähr wrote
> Hello everybody,
> 
> I would like to know what the purpose of the xml attribute class_id is,

In the case of pointers - to know what kind of object to create

> how it works and in which cases it can be omitted.

archives contain only essential information.  Nothing can be omitted without
breaking the archive

> An example: “Base” is the (abstract) base class of _GMTF_ and _TTF_, and 
> _Hist_ also has got a similar base class.
> 
> If a pointer of Hist is a member of GMTF and you want to serialize a 
> pointer of Base, you get something like:
> <filter class_id="0" class_name="GMTF">
>    
> <m_bufHalfMaxSize>
> 4
> </m_bufHalfMaxSize>
>    
> <m_cutDetector class_id="1" class_name="Hist">
> </m_cutDetector>
> </filter>
> Here, the macros
> • BOOST_CLASS_EXPORT_GUID(...),
> • BOOST_CLASS_IMPLEMENTATION(..., 
> boost::serialization::object_serializable) and
> • BOOST_CLASS_TRACKING(..., boost::serialization::track_never)
> are used.
> 
> The problem now is: Although the exact classes are given by 
> human-readable class names (and used!), you cannot omit or change the 
> class_id. If you edit the XML file by hand, it can become invalid (-> 
> exception on reading). It seems that the class ids are just ascending 
> non-negative numbers.
> 
> What are the rules for choosing the class ids? Why are they necessary? 
> Is there a trick to remove this attribute or at least to ignore it?
> 
> 
> Thanks,
> Frank
> 
> 
> -- 
> Frank Stähr
> Technische Universität Berlin
> Communication Systems Group
> Sekr. EN1 Einsteinufer 17
> 10587 Berlin, Germany
> _______________________________________________
> Boost-users mailing list

> Boost-users_at_.boost

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