One can hide internal representations just fine in C using pointers to incomplete struct types with the definition of the struct and the implementation of the functions operating on it in a separate file. I like this much better than what C++ does, because only the interface and not also implementation details end up in the header. This keeps things nicely separated and built times very short.

I see no real benefit from -xc++ and I would also miss some stuff such as variably modified types, designated initializes, etc.