Re: version numbers for classes in ClassDef

Rene Brun (Rene.Brun@cern.ch)
Thu, 24 Sep 1998 18:05:15 +0200

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Chris Jillings wrote:
>
> Hello,
> We may need to use the version number in ClassDef. We have have a data
> structure that contains classes defined with ClassDef(ClassName,1). I want
> to expand the data in the class but still be able to read the data already
> written.
> Is there a doc which explain how to do this? The InputOutput.html
> indicates it is possible but doesn't explain how to make both versions
> accessible to the linker.
>

Hi Chris,
Sorry for this late reply. I am pretty busy these days at FNAL.
Yes, you should use the version_id in the ClassDef macro if you
make changes in your class (adding/deleting members).
You must also modify the Streamer function for your class
by testing on the version_id when you read the object.
See an example in the Root class TAxis at URL:
http://root.cern.ch/root/html/src/TAxis.cxx.html

Rene Brun

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