I'm still gathering interest, comments, questions, suggestions and reviews for a library I'm developing.
It basically allows you dynamic type composition and mutation in C++. It's called Boost.Mixin (it's for boost, but not part of boost yet) and it's vaguely similar to Ruby's mixins.
Code is here: https://github.com/iboB/boost.mixin
Docs are here: http://ibob.github.io/boost.mixin/
There has been a forum topic on the subject before here but it failed to generate much interest. Hopefully I'll get more this time