Object databases routinely go away and routinely come back.

Ten years ago I worked with a database called Versant OODBMS (from Actian). I was a junior sysadmin so i was essentially administering it at a very surface level but skimming the documentation (and trying some of the samples) it was very cool that you could pick essentially any random class, implement an interface (and hence a few method) and that was it, you had a database-serializable object.

The main issue was really scaling out (as in, multiple machines) but otherwise was a really great database.

Yes, POET for C++, for example.

http://www.kevra.org/TheBestOfNext/ThirdPartyProducts/ThirdP...