For example, if I change the signature in a function in either the header or the cpp, I'd like it to automatically change in the other one. If I add a new function in either, it should appear in both. If I delete a function, it could probably comment out the other one. Manually having to duplicate one's changes seems silly.
Some people have mentioned http://www.lazycplusplus.com/ in response to a similar question, but it seems that that's a command line tool which would require saving and then running this external tool on a particular file. That's still more manual steps than I would have thought were necessary; I'd like this to apply changes as I type.