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

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2006-12-01 14:22:34

Just a small question:

Isn't it so, that linked lists (double or single), can be spreaded in the memory, so that cache
misses and page faults are subject to happen? I think deque is a better choice (which can be
better optimized in terms of locality)...

With Kind Regards,

Ovanes Markarian

On Fri, December 1, 2006 20:13, Nat Goodspeed wrote:
>> -----Original Message-----
>> From: boost-users-bounces_at_[hidden] [mailto:boost-users-
>> bounces_at_[hidden]] On Behalf Of Sebastian Redl
>> Sent: Friday, December 01, 2006 1:23 PM
>> To: boost-users_at_[hidden]
>> Subject: Re: [Boost-users] boost::weak_ptr and boost::intrusive_ptr
>>
>> The use case here is this:
>>
>> NodePtr base = make_tree();
>> NodePtr child = base.get("path/to/child");
>> base.reset();
>> Now, unless the parent knows that it owns the child and notifies it,
> the
>> parent pointer of child is dangling. A weak pointer can detect this
>> situation.
>
> [Nat] Okay.
>
>> The reason I see for there being no weak_intrusive_ptr is that it is
>> quite simply impossible to implement while staying intrusive. Let's
>> remember what a weak pointer does: after the death of the object it
>> points to, it knows that it is now invalid. There being no
> notification
>> of pointers when other pointers get destructed, there's only one way
> to
>> do this: to keep the refcount for the object alive, so that the weak
>> pointer can check it.
>> But if the refcount is part of the object, as is the case for
>> intrusive_ptr, it's impossible to keep the refcount alive while
>> destroying the object.
>>
>> In other words, the appeal of intrusive_ptr is that it doesn't require
>> any resources outside the object it points to. However, this
> restriction
>> makes it impossible to implement an efficient weak pointer.
>
> [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.)
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