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

It is useful when you want to share a class member and an instance of the class is already a shared_ptr, like the following:

struct A
{
  int *B; // managed inside A
};

shared_ptr   a( new A );
shared_ptr b( a, a->B );

they share the use count and stuff. It is optimization for memory usage.