Re: [boost] weak_ptr triple states

From: Joe Gottman (jgottman_at_[hidden])
Date: 2005-07-14 18:20:34

"Jaap Suter" <boost_at_[hidden]> wrote in message 
news:003501c58833$f508a020$4119059a_at_unknown...
> Hi,
>
> a weak_ptr can be said to have three states.
>
> 1. Never been assigned to (default constructed)
> 2. Assigned to, lock will return a shared_ptr to a valid object.
> 3. Assigned to, but the object pointed to is dead and lock will return 
> null.
>
> As far as I know it is currently impossible to distinguish between case 1
> and 3. Am I overlooking something? If not, can somebody explain the reason
> why it is not possible to distinguish the two cases? Admittedly, I cannot
> come up with any elegant design in which such a distinction is useful, so
> the question is merely theoretical.
>

   Actually, it is possible to distinguish case 1 from case 3:

   weak_ptr<Foo> wp = get_weak_ptr();
   if (wp.expired()) {
        weak_ptr emptyPtr; // Default constructed weak_ptr
        if (!(emptyPtr < wp) && !(wp < emptyPtr)) {
            cout << "Case 1" << endl
        } else {
            cout << "Case 3" << endl;
        }
    } else { // Not expired
        cout << "Case 2" << endl;
    }
}

It is vital that a weak_ptr that is put into a set and later expires does 
not change its ordering relative to the other elements of the set.

Joe Gottman