While vim is a true Swiss-knife I still prefer external tools for some jobs. This approach is some times much more intuitive and easy to remember than using the built-in equivalent.
In the case of indenting, I filter the whole file buffer through astyle. The astyle parameters are much easier to grasp in a couple of minutes, especially if you are not a vim guru. Also astyle provides much more flexibility in fine-tuning the output.
First install astyle:# apt-get install astyle
Then inside vim:
:%!astyle (simple case – astyle default mode is C/C++)
or
:%!astyle --mode=c --style=ansi -s2 (ansi C++ style, use two spaces per indent level)
or
:1,40!astyle --mode=c --style=ansi (ansi C++ style, filter only lines 1-40)