Re: [boost] weak_ptr triple states

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-07-14 20:28:20

Joe Gottman wrote:

>   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)) {

Right... you can thank Joe for that, with the latest changes to TR1 (*) 
operator< can tell you whether a weak_ptr is empty. The original version did 
not guarantee that all empty weak pointers compared equivalent. :-)

So it's possible.

template<class T> bool is_empty( weak_ptr<T> const & wp )
{
    weak_ptr<T> empty;
    return !(empty < wp) && !(wp < empty);
}

Whether it should be used is another story. :-)

(*) See issue 2.4 in

http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1837.pdf