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

Subject: Re: [Boost-users] seeking comments on 'weak_key_map' experiments
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-12-11 13:24:16

AMDG

Nicolas Lelong wrote:
> I've been doing too much Python recently - thanks to boost.python - 
> and my return in the c++ world in quite painful. To help me manage 
> some of my objects lifetime dependencies, I wanted an access to a 
> system similar to the Python WeakKeyDictionary class :
>
>     Mapping class that references keys weakly. Entries in the
>     dictionary will be discarded when there is no longer a strong
>     reference to the key. This can be used to associate additional
>     data with an object owned by other parts of an application without
>     adding attributes to those objects.
>
>
> I drafted a small 'weak_key_map' class based on boost weak_ptr, 
> shared_ptr deleters, and a base class enabling classes to be used as 
> keys in the weak_key_map.
>
> A small use case example is shown after, and source of weak_key_map & 
> co are attached.
>
> I was wandering if someone had experience to share about a similar 
> system ?
>
> For a first draft, it already serves me well - but, I'm not very happy 
> with to "pollute" keys with the base class 'enable_master_ptr'...
>
> Any idea to improve all this ?

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 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.

struct node {
    boost::weak_ptr<K> key;
    V value;
    node* parent;
    node* left;
    node* right;
};

struct iterator {
    boost::shared_ptr<K> key; // force the key to stay alive
    node* pos;
};

In Christ,
Steven Watanabe