Re: [Boost-users] seeking comments on 'weak_key_map' experiments

Subject: Re: [Boost-users] seeking comments on 'weak_key_map' experiments
From: Nicolas Lelong (rotoglup_at_[hidden])
Date: 2008-12-12 10:11:20

> Is there some way that this can interface with boost::shared_ptr/weak_ptr
better?
> weak_key_map<int, int> m;
> boost::shared_ptr<int> k(new int(1));
> m.insert(std::make_pair(k,5));
>
> assert(*m.find(k) == 5);
>
> k.reset();
>
> assert(m.empty());

I'd also like that, but in this approach, I can't figure how to hook into
the key shared_ptr deleter to have access to a notification of the smart_ptr
deletion. I think we can currently get the current deleter of a shared_ptr,
but we can't change it to combine it with another deleter.

I rather suspect that the data structure would need to be implemented from
> scratch.  One of the nastier problems is how to deal with the case of:
> iterator invalidation.  The safest solution is to make the presence of an
> iterator to a particular element guarantee that the key continues to exist.

That's a good point ! To make a parallel with Python, the iteration seems to
be an issue :

quote from Python doc
*Note: Caution: Because a WeakValueDictionary is built on top of a Python
dictionary, it must not change size when iterating over it. This can be
difficult to ensure for a WeakValueDictionary because actions performed by
the program during iteration may cause items in the dictionary to vanish "by
magic" (as a side effect of garbage collection).
*

Reimplementing the whole map structure brings the task to another level of
complexity...