What am I doing wrong?
1. I write some code in Qt Creator, it works.
2. I save all, copy the folder with all the files in it to another location for safekeeping.
3. Then I write more code. This additional code doesn't work and I've gone too far to undo it.
4. So I close Qt, delete that folder, move the earlier one back to where it was originally, and load the older version.
But Qt still gives me the same errors that the newer code gave me, and won't run. It seems to remember this newer (bad) code even though it no longer appears anywhere in Qt - and the problem persists over computer reboot. I feel I'm missing something very obvious, but Mr Google can't help at all.
Build->Rebuild all. Your IDE is probably caching some of the previous results for performance reasons. I had the problem once using Qt Creator, and it solved it for me.