[boost] Re: shifted_ptr review

From: cppljevans_at_[hidden]
Date: 2003-10-10 18:02:05

On 10/01/2003 02:49 PM, Gregory Colvin wrote:
> It's an important feature of shared_ptr that it doesn't care how the
> pointee gets allocated.  So one could use the shifted_ptr allocation
> strategy with shared_ptr, and get the advantages of co-allocating the
> pointee and the count.  But what you couldn't get is the smaller
> size of shifted_ptr, because there is no way to tell shared_ptr how
> to navigate from the pointer to the associated count.
Which raises a con not mentioned in the "Rationale" in
   shifted_ptr/libs/smart_ptr/doc/index.html
in
   http://groups.yahoo.com/group/boost/files/shifted_ptr.zip
This con is that shifted_address<I,J> will work correctly only
for the case where I != J.  This is because when I == J,
the class is not polymorphic and then:
  template<typename U> shifted_address<I,I>::get(U*a_p)
returns:
   (a_p - offset(sizeof(shifted_header) + shifted_header::s_padding));
which is incorrect when a_p does not point to the start of the
memory block, which is the case in the following code where pb is
created:

   struct A{int a;};
   struct B{int b;};
   struct C: public A, public B{virtual ~C(){}};
   shifted_ptr<C> pc(new (so) C);
   shifted_ptr<B> pb(pc);

another con is that when the instance is polymorphic, dynamic_cast
is called to get the header, which is a virtual function call.
This call is made each time the reference count is decremented
or incremented for a polymorphic instance, AFAICT.