Re: [Boost-users] boost::weak_ptr and boost::intrusive_ptr

From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2006-12-01 15:35:03

Nat Goodspeed wrote:
> [Nat] Not to be argumentative, but...
>
> It seems possible to implement weak_ptr notification by building a list
> of weak_ptr instances referencing a given object. If we don't want that
> list to consume additional heap memory, the list could itself be
> intrusive in the weak_ptr objects. If we want it to be efficient, we
> build a doubly-linked list. (This may call for a policy-based
> implementation so the consumer can decide which overhead is least
> noxious.)
So basically, within the referenced object, you have a pointer to the
first weak pointer, which holds a pointer to the next weak pointer, and
so on, linked in both directions?

Interesting idea. I can't find any problems with it.

template<class Pointee, class Access = weak_member_access<Pointee,
&Pointee::weak_head> >
class intrusive_weak_ptr
{
  intrusive_weak_ptr *prev, *next;
  Pointee *ptr;

public:
  typedef intrusive_ptr<Pointee, ...> strong_ptr;

  intrusive_weak_ptr(const strong_ptr &o)
    : prev(0), ptr(o.get())
  {
    intrusive_weak_ptr * head = Access::get_head(*o);
    head->prev = this;
    next = head;
    Access::set_head(*o, this);
  }

  strong_ptr lock()
  {
    if(!ptr) {
      throw lock_error();
    }
    return strong_ptr(ptr);
  }

  void signal_destruction()
  {
    ptr = 0;
    if(next) {
      next->signal_destruction();
    }
    next = 0;
    prev = 0;
  }
};

With the destructor of intrusive_ptr starting the signal_destruction
chain, of course.
Some TMP would be required to make intrusive_ptr only require weak_ptr
if it's actually used.
intrusive_list could be used to make the weak_ptr list more robust.

Sebastian