What is boost's shared_ptr(shared_ptr const & r, T * p) used for?

Question

boost::shared_ptr has an unusual constructor

template shared_ptr(shared_ptr const & r, T * p);

and I a

Answer 1

For "shared_ptr b(a, dynamic_cast(a.get()));"

I think it is not the recommended way using smart pointer.

The recommended way of doing this type conversion should be:

shared_ptr b(a);

Since in Boost document it is mentioned that:

shared_ptr can be implicitly converted to shared_ptr whenever T* can be implicitly converted to U*. In particular, shared_ptr is implicitly convertible to shared_ptr const, to shared_ptr where U is an accessible base of T, and to shared_ptr.

In addition to that, we also have dynamic_pointer_cast which could directly do conversion on Smart Pointer object and both of these two methods would be much safer than the manually casting raw pointer way.