shared_ptr and weak_ptr differences

Question

I am reading Scott Meyers \"Effective C++\" book. It was mentioned that there are tr1::shared_ptr and tr1::weak_ptr act like built-in pointers, but

Answer 1

For future readers.
Just want to point out that explanation given by Atom is excellent, here is working code

#include  // and others
using namespace std;

    class B; // forward declaration 
    // for clarity, add explicit destructor to see that they are not called
    class A { public: shared_ptr b; ~A() {cout << "~A()" << endl; } };  
    class B { public: shared_ptr a; ~B() {cout << "~B()" << endl; } };     
    shared_ptr x(new A);  //x->b share_ptr is default initialized
    x->b = make_shared(); // you can't do "= new B" on shared_ptr                      
    x->b->a = x;
    cout << x.use_count() << endl;