Hi Heather,
By specifying the option "-" in the pragma, you simply tell rootcint
to not generate the code for Streamer because you want to plug in
your own version.
The recipee I gave to you will be able to read your old files
written before
any schema evolution was introduced in Root I/O and will be able to read
all current and future versions of your class without changing Streamer.
What we write in the Users Guide about case C is correct. This chapter
describes the migration procedure to Root version 3. This assumes
that you are not changing your class at the same time. The new system
is able to read the old files (with the restriction of WriteArray).
So, please try my recipee. It should work.
Rene Brun
On Tue, 6 Aug 2002, Heather Kelly wrote:
> Hi Rene,
>
> Sorry..I still have a question. I have read the User's Guide, and I
> thought I would fall under Case C:
>
> Case C: You use the automatic streamer in the dictionary file and you already
> use the option "+" in the LinkDef file. If the old automatic Streamer does
> not
> contain any statement using the function WriteArray, you have nothing to do,
> except running rootcint again to regenerate the new form of the Streamer
> function, otherwise proceed like for case B.
>
> I already use the automatic streamer in the dictionary file, the "+" option
> in the LinkDef file, and there is no use of WriteArray in the Streamer.
> All of this was true with the initial implementation of the classes - so I
> thought I could use automatic schema evolution from the start and not have
> any Streamer routines in my implementation files.
>
> >From your description - I'm in Case B, like the TAxis class..and it sounds
> to me as though I would have to keep updating the Streamer each time I
> update the class - so that all previous versions can be handled...and use
> the "-" option in the LinkDef file.
>
> How would I ever graduate to using the "+" option if I cannot do that right
> now?
>
> Thanks,
> Heather
>
> At 11:28 PM 8/5/02 +0200, Rene Brun wrote:
> >Hi Heather,
> >
> >We explain this process in the Users Guide.
> >You can see an example in TAxis::Streamer.
> >
> >You must have somewhere the old McPositionHit::Streamer.
> >If you lost it, you can regenerate it by running rootcint
> >on your old class and specifying
> >#pragma link C++ class McPositionHit;
> >
> >Now to generate the new Streamer able to read both the old
> >class version (before Schema evolution was introduced)
> >and have the automatic schema evolution for your new versions,
> >run rootcint with
> >#pragma link C++ class McPositionHit-;
> >and implement the following Streamer function:
> >
> >void McPositionHit::Streamer(TBuffer &R__b)
> >{
> > // Stream an object of class McPositionHit.
> >
> > if (R__b.IsReading()) {
> > UInt_t R__s, R__c;
> > Version_t R__v = R__b.ReadVersion(&R__s, &R__c);
> > //we use the automatic algorithm for 9eg) class version > 2
> > if (R__v > 2) {
> > TMcPositionHit::Class()->ReadBuffer(R__b, this, R__v, R__s,
> >R__c);
> > return;
> > }
> > //====process old versions before automatic schema evolution
> > //put here the old read part of the old Streamer
> > //====end of old versions
> >
> > } else {
> > McPositionHit::Class()->WriteBuffer(R__b,this);
> > }
> >}
> >
> >
> >Rene Brun
> >
> >On Mon, 5 Aug 2002, Heather Kelly wrote:
> >
> >> Hi,
> >>
> >> I am using ROOT 3.02.07 on Windows 2000.
> >>
> >> I have a set of ROOT classes, where one class has been updated - where one
> >> data member was removed and another one added - and the version
> incremented.
> >> My LinkDef file looks like:
> >> >#ifdef __CINT__
> >> >
> >> >#pragma link off all globals;
> >> >#pragma link off all classes;
> >> >#pragma link off all functions;
> >> >
> >> >#pragma link C++ class VolumeIdentifier+;
> >> >#pragma link C++ class McParticle+;
> >> >#pragma link C++ class McPositionHit+;
> >> >#pragma link C++ class McIntegratingHit+;
> >> >#pragma link C++ class McEvent+;
> >> >
> >> >#endif
> >> So I am using the "new" Streamer system, and used it all along - starting
> >> with the original version of these classes.
> >>
> >> I have an older ROOT file, written with the first version of the classes,
> >> and I want to be able to read that file using the new version of the
> shared
> >> library - which contains the updated version of one class. However,
> when I
> >> do so, using TTree::GetEvent, I receive the following error and warning
> >> messages:
> >> Error in <TBuffer::CheckByteCount>: object of class McPositionHit read too
> >> few bytes
> >> Warning in <TBuffer::CheckByteCount>: McPositionHit::Streamer() not in
> sync
> >> with data on file, fix Streamer()
> >>
> >> I was hoping that upon reading, when it was found that the file contained
> >> version 1 of McPositionHit, and the library contained version 3, that the
> >> description of the class in the input file would be used to retrieve the
> >> data. Am I doing something wrong? Or am I just confused? Must I
> >> regenerate the original class library using MakeProject on the old file in
> >> order to read this file without problems?
> >>
> >> Thanks,
> >> Heather
> >>
> >
> >
>
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