Is there a way to copy from shared_ptr<Widget> to shared_ptr<const Widget>? I think not, which would make one of the entries in that table completely impossible to use.
Why not? In fact shared_ptr even has an aliasing constructor that allows you to refer an pointer whose lifetime is tied to an unrelated shared_ptr.
Of course the only sane use case is that that unrelated shared_ptr will definitely outlive that other pointer. But you get the idea, shared_ptr is incredibly flexible.
That's for the reverse (unsafe) conversion. A shared ptr can always be implicitly converted to a shared ptr to const. In fact a shared_ptr<T> can always be implicitly converted to a shared_ptr<T2> as long as T* is convertible to T2*. So for example derived to base and anything to void are allowed.
They will share the same reference count. In fact you can make two shared_ptrs to arbitrary types share the same reference count (although in this case there is no implicit conversion and you need to use the explicit aliasing constructor); it is useful for interior pointers for example.
