How difficult is it to wrap a large C++ library with R?

From what I read from R website, it is easy to wrap individual C/C+ 
+ functions but not C++ classes.

There was a couple of posts about this recently:

https://stat.ethz.ch/pipermail/r-devel/2005-April/subject.html

Look for the posts concerning C++, overloading methods and objects in  
R. If I remember correctly "Ali" had some prototype automatic wrapper  
for large classes (he wasn't happy about the performance, but that's  
another story ;) - see his 'speeding up the library' posts).

Cheers,
Simon