We use Understand for C++ (link is to the index of all "Understand for..." family members) when reviewing and designing formal unit tests for our clients' code. It's extremely useful for manual static analysis: understanding structure and inter-relatedness, so to speak.

However, to understand dynamic behavior you should look at various tracing options, even the lowly printf(), or try stepping through in a debugger. The larger, more complex and the more object-oriented the code, the more important understanding the dynamics are.

Anyway, Understand for C++ is much more interactive than any of the free comment extraction or cross reference tools and the database has a Perl API, though we've not had a chance to use it. It's worth the price if your doing this as part of your job.