At some point recently (not sure but since March 2013?) my Eclipse CDT stopped auto completion. I figured out that at Editor->Content Assist->Advanced checking Parsing-based Proposals lets auto completion work on my machine.
Ubuntu 12.10, Eclipse 4.2.2, CDT 8.1.2
This snapshot shows Task-Focused, which I don’t see on my another Eclipse instance. In that case I just checked Parsing-based Proposals and completion still works.