One of the main reasons why autocomplete features are generally lacking in enhanced text editors (like Emacs or Kate) is that it requires some knowledge of the build configuration (source files included in the build, include paths, environment variables and such things). So, the bare minimum that you are going to need is an integration of a build-system into the text editor, and once you have that, it is pretty much an IDE.
The other reason why auto-completion is lacking in light-weight code editors is that a good and intelligent code-completion system is NOT a light-weight piece of software.
I don't know of anything that is like that, half-way between an enhanced text editor and a full-blown IDE.
Personally, I use KDevelop (for Linux, you didn't specify the OS). The code-completion engine it uses is really good. In my experience, I find that it outperforms even IntelliSense (the Visual Studio code-completion engine). KDevelop uses Kate to manage the text-editing part of the IDE (code-highlighting, auto-formatting, folding, and all that stuff), but it supplements it with code-completion and great support for external build-systems. I also write heavily templated code which usually is too much to handle for the code-completion systems of CodeBlocks, Eclipse, and Visual Studio, but, amazingly, the code-completion system of KDevelop is mostly able to keep up and resolve to good suggestions most of the time. KDevelop is a full-blown IDE (albeit a very light-weight one), and I don't know if there are any ways to get an even lighter version of it.
Kate does advertise code-completion features but I doubt it is the same strength as KDevelop, if it is, then it would be worth a try. Also, given the plugin-based system that the KDevelop team is using (with KDevPlatform) I would imagine that it would be quite possible to create a kind of KDevelop-Lite edition fairly easily, one that would just include Kate and the KDevelop code-completion system.