Why use a boost::shared_ptr?
shared_ptr is an extremely versatile and poweful gadget. One (among others) thing it handles well is incomplete classes for implementation hiding. Maybe there are other smart pointers doing that sort of thing cheaper. I am not sure.
You want your implementation details to be shared across all 'Test's? What you need is more like a deep-copying smart pointer.
It certainly depends on application. Like if I have a GUI widget, then I do not want to deep-copy it. If I have static read-only configuration data, I do not want deep-copying those either. Deep-copying vs. shallow-copying is a separate issue that is likely to be addressed via an additional policy template parameter. I did not want to blow up my example with all the bells and whistles for smaller code foot-print and so that the main idea stands out clearer. Best, Vladimir.