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

You can also use this to keep dynamic casted pointers, i.e.:

class A {};
class B: public A {};

shared_ptr a(new B);
shared_ptr b(a, dynamic_cast(a.get()));