Re: [boost] [shifted_ptr] __normal_iterator ..?

From: Larry Evans (cppljevans_at_[hidden])
Date: 2008-06-20 13:36:15

On 06/20/08 03:36, Phil Bouchard wrote:
> Hi,
> 
> Upon trying to create the new shifted_allocator class that is instead to be 
> passed as a template parameter on STL containers, I got stuck in a circle or 
> errors.  I came to realize there was not way to replace internal pointers 
> used by containers with smart pointers... which is very unfortunate.  There 
> are different solution I could take but hopefully I won't need them.
> 
> By trying compiling shifted_ptr_test2.cpp in the Sandbox we'll get the 
> following errors.  I believe shifted_allocator (in shifted_allocator.hpp) 
> should be represented the way it is but I may likely got something wrong 
> because I haven't followed the verdict on this in previous posts:
> 
> ...stl_vector.h: In member function `__gnu_cxx::__normal_iterator<typename 
> _Alloc::const_pointer, std::vector<_Tp, _Alloc> > std::vector<_Tp, 
> _Alloc>::begin() const [with _Tp = boost::detail::sh::shifted_ptr<vector>, 
> _Alloc = boost::shifted_allocator<boost::detail::sh::shifted_ptr<vector> 
>  >]':
> ...stl_vector.h:221:   instantiated from `std::vector<_Tp, 

Hi Phil,

Even if you could find some way for your allocator to work for vector, I
don't think it would work for other stl containers.  These other
containers, e.g. std::list<_Tp>, use Allocator<_Tp>::pointer to point to
other elements in the container.  AFAICT, what's needed for garbage
collections is Allocator<_Tp>::root_pointer.  This would be
a *smart* pointer to the head of the list in the case of std::list<_Tp>
and all other pointer would just be _Tp*.  In the case of 
std::vector<_Tp>, I guess the start of the vector would be a *smart* 
pointer to _Tp[].  The iterators for a GC'ed vector would have to
retain a copy of the root_pointer, I guess, in order to keep the
whole vector alive if even 1 iterator were still around.

Anyway, I encountered a similar problem with the policy_ptr
library I worked on years ago.  The code used to be in
the sandbox; hoever, I can't find it now.  However, FWIW, there's
this post:

http://article.gmane.org/gmane.comp.lib.boost.devel/155407/match=policy_ptr

-regards,
Larry