Marc, Just uploaded v0.6 which supports building of two separate class hierarchies -- separately for interfaces and for implementations as described in GoF Pattern Bible for the Bridge pattern. The interface hierarchy is built as struct Base : public pimpl<Base>::pointer_semantics {...}; struct Derived : public Base {...} etc. The implementation hierarchy is still hidden and is built as template<> struct pimpl<Base>::implementation {...}; template<> struct pimpl<Derived>::implementation : public pimpl<Base>::implementation {...}; etc. Marc, let me know if that looks right for your purpose as I myself have no use for deep inheritance trees to test them out. Best, Vladimir.
Marc,
I have re-read the Bridge pattern in GoF Pattern Bible. Their example is just what you describe in your example. I agree that having/supporting two separate hierarchies is an elegant solution to a few problems. Let me think how pimpl<> might support that.
Best, Vladimir.