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This is pulled from an e-mail on the gnustep-dev mailing list:

The way to do versioning of objects encoded using General/NSCoder is to set the current version of the class in the initialize method. Then when the class is encoded it will be written out with that version number. When you decode the class, you can check for old version numbers.

For example:

static const int currentVersion = xxx; // Current version

Typically, this sort of thing should be done to handle changes in archiving formats between and within releases of the libraries.

When we make a release of the base library, we update the system version … and this makes it possible for us to write much more efficient code: [Author’s note: I don’t understand the code below…Anyone care to fill in details or explanation?]

Yeah – I don’t get it either. The “systemVersion” message returns the version of the system used to make the archive. By system I mean Operating System, like General/NextStep, General/OpenStep, Rhapsody, General/MacOSX, etc. Seems to me that information is only relevant to test for the existence of the data required by “versionForClassName:”. And since that’s always available for non-keyed archives on Mac OS X, why bother?

Of course, they could be using their own General/NSCoder subclasses and can redefine systemVersion to mean whatever they want. In which case we won’t understand your example until they include the General/NSCoder subclasses. I think that might be the case from some of the other context in the message

Finally, it’s worth mentioning that the built-in versioning scheme (“versionForClassName:” etc.) only seems to work with non-keyed archives. When using keyed archives you’ll need to supply your own versioning mechanism.

Oh – also, General/PierreYves writes:

I agree, and have cleaned up the original post so it reads correctly.

Also, the original code doesn’t account for superclasses. As in any “init” routine, you must call “super” to set the value of “self”. Again, I have adjusted the original post so it reads correctly.

– General/MikeTrent