Fill a Feature Request.
A code completion Ctags based wouldn't be difficult to implement.
About Ctags:
http://ctags.sourceforge.net/ * Supports 33 programming languages
* It is capable of generating tags for all types of C/C++ language tags
* Compiles on UNIX, MSDOS, Windows 95/98/NT, OS/2, QNX, Amiga, QDOS, VMS, Macintosh, and Cray.
* Included in major Linux distributions
* Tag index files are supported by most major editors, including: vi, Emacs, JED, jEdit, UltraEdit
* Fast: Even for an unsorted 24MB tag file, tag searches take about one second on a 650 MHz Athlon Linux machine.