Is it safe to use such smart pointer casting?

Question

Is it safe to use such smart pointer casting?

APtr a(new A());
BPtr & b = (Bptr&)a; // this is it

there,

class A
{
public:
   virtual ~A(){}
   virtual void methodA() = 0;
}
typedef std::tr1::shared_ptr<A> APtr;

class B : public A
{
public:
   virtual ~B(){}
   virtual void methodB() = 0;
}
typedef std::tr1::shared_ptr<B> BPtr;

/////////////////////////////////////////////////////////////////////////////////

BPtr & b = a; //this way doesn't work

Answer 1

To downcast a smart pointer, you should use the xxxx_pointer_cast functions, e.g. a static cast

BPtr b = std::tr1::static_pointer_cast<B>(a);

or dynamic cast

BPtr b = std::tr1::dynamic_pointer_cast<B>(a);