Re: [Boost-users] XML Serialization Traits Question

From: William Bardon (wbardon_at_[hidden])
Date: 2006-07-28 10:57:17

Robert,

        I forced a crude solution by defining a custom XML output
archive and defining My_Archive::save_override(const nvp<T>&, int) as
below. This would double the class information in some objects, so I
also set the BOOST_CLASS_IMPLEMENTATION to object_serializable for my
serialized classes.  It is crude, but seems to give me the results I
need.

class My_Archive : public
boost::archive::xml_oarchive_impl<OArchive_Versioned>
{
        ...

        template<class T>
        void save_override(
                    #ifndef BOOST_NO_FUNCTION_TEMPLATE_ORDERING
                    const
                    #endif
                    ::boost::serialization::nvp<T> & t, int)
        {
            save_start(t.name());
            if (boost::serialization::implementation_level<T>::value >=
boost::serialization::object_serializable)
            {
                write_attribute("version",
boost::serialization::version<T>::value, "=\"");
            }
            boost::archive::save(* this->This(), t.const_value()); 
            save_end(t.name());
        }

        // redeclared fall thru versions of all specialized versions of
save_override omitted for space
};

        By examining the library's source code, I have determined that
the tracking of class traits in serialization occurs in
basic_oarchive_impl::save_object, defined in basic_oarchive.cpp.  It
uses the class cobject_type to track classes it has seen.  Because
basic_archive_impl and cobject_type are completely defined within
basic_archive.cpp, the ability to customize XML attributes is limited.
Perhaps future versions of Serialization could look into making the XML
optionally generate an XML schema for interoperation with other
XML-based applications.

Thanks,
Bill

-----Original Message-----
From: boost-users-bounces_at_[hidden]
[mailto:boost-users-bounces_at_[hidden]] On Behalf Of Robert Ramey
Sent: Wednesday, July 26, 2006 5:57 PM
To: boost-users_at_[hidden]
Subject: Re: [Boost-users] XML Serialization Traits Question

I would have to go back and study the code again to give a good answer
and I'm not up to this right now.   But I can offer a suggestion in the 
short term.

I use VC 7.1 debugger.

Set a trap at save_override(const version ...

When the program traps in the debugger.  examine the stack trace to see
the chain of overrides and specializations whch got you there.

Robert Ramey

William Bardon wrote:
> Robert,
>
> Thanks for the example; it helped clear up some uncertainty about the 
> template arguments in the derived class.
>
> Regarding save_override, I see where specialized versions for the 
> different trait types are defined in basic_xml_oarchive.ipp.  I also 
> see that save_override is called from the operator<< redefinition in 
> interface_oarchive.hpp.  What I'm missing is the link between 
> serializing a user-defined class object and the traits of that object.
>
> template<class Archive>
> BOOST_ARCHIVE_OR_WARCHIVE_DECL(void)
> basic_xml_oarchive<Archive>::save_override(const version_type & t,
> int) {
>    int i = t.t; // extra .t is for borland
>    write_attribute(VERSION(), i);
> }
>
> This function already displays a version attribute in the XML.  How do

> I make sure this is called for every element?  I'm sorry, but I can't 
> figure out how to modify chain of control to force output of class 
> traits.
>
> Starting in oserializer.hpp, boost::archive::save<Archive,T> calls 
> static save_non_pointer_type<Archive, T>::invoke calls serialize_adl 
> calls boost::serialization::serialize<Archive, T> calls 
> access::serialize calls myClass::serialize<Archive> (Archive& ar, 
> unsigned int version)
>
> There is also a function oserializer<Archive,T>::save_object_data that

> calls serialize_adl, but I can't find anything that calls it.
>
> The struct save_non_pointer_type<Archive, T> in oserializer.hpp may be

> the key, but I'm not sure how it works.
>
> In oserializer.hpp, struct
> save_non_pointer_type<Archive,T>::save_standard::invoke calls 
> Archive::save_object, but I do not see an implementation of the 
> function anywhere:
>
> $ find /usr/include/boost -type f -exec grep -H save_object {} \;| 
> grep -v save_object_ | more
> /usr/include/boost/archive/polymorphic_oarchive.hpp:    virtual void
> save_object(
> /usr/include/boost/archive/detail/oserializer.hpp:
> ar.save_object(& t, oserializer<Archive, T>::instantiate());
> /usr/include/boost/archive/detail/basic_oarchive.hpp:    void
> save_object(
> /usr/include/boost/archive/detail/polymorphic_oarchive_impl.hpp:
> virtual void save_object(
> /usr/include/boost/archive/detail/polymorphic_oarchive_impl.hpp:
> ArchiveImplementation::save_object(x, bos);
>
> I'd appreciate any suggestions.  Am I even looking in the right place?
>
> Thanks in advance,
> Bill
>
> -----Original Message-----
> From: boost-users-bounces_at_[hidden]
> [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Robert Ramey
> Sent: Wednesday, July 26, 2006 12:14 PM
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] XML Serialization Traits Question
>
>
> "William Bardon" <wbardon_at_[hidden]> wrote in message 
> news:C73E689C9FCF884B9A550ECFADA0721C087D3E_at_exchange.cab-cds.org...
> I have read both the Boost Serialization online documentation and the 
> archives of the Boost-Users mailing list looking for guidance, but I 
> haven't found anything relevant.  I see how to use 
> BOOST_CLASS_TRACKING and BOOST_CLASS_IMPLEMENTATION to remove version 
> tracking attributes from the XML tags.
>
> Usage of these macros does a lot more than remove attributes from the 
> tags.
>
> I want to go in the opposite direction.  I notice that the class_id 
> and version attributes only appear the first time an object of a 
> particular class is serialized.  I want to force the version attribute

> to appear in every object's attribute list.
>
> Do I need to derive a custom archive to do this?
>
> yes
>
> I have looked at the source code for serialization, but I have trouble

> plowing through all of the macros, template expansions, and overloaded

> functions to locate the specific part that controls whether class 
> traits are stored as attributes in a particular XML tag.
>
> I sympathize - its hard to follow.  And I wrote it.  Here are some
> hints:
>
> a) look at demo_fast_archive.cpp to see a model of how one can derive 
> from an existing archive.
> b) You'll want to derive from xml_oarchive_impl<...> to create 
> my_xml_oarchive
> c) add a "save_override" function to my_xml_oarchive which replaces 
> the "save_override" function in
>    basic_xml_oarchive.ipp.  Implement this using functions defined 
> basic_xml_oarchive.ipp
>
> Good luck -
>
> Robert Ramey
>
>
> Thanks in advance,
> Bill
>
>
>
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