Re: [boost] weak_ptr, maybe empty -- doc bug?

From: Andy Thomas-Cramer (Andy_Thomas-Cramer_at_[hidden])
Date: 2005-08-04 10:53:17

Is this a documentation bug?

  template<class Y> explicit shared_ptr(weak_ptr<Y> const & r);
  Effects: If r is empty, constructs an empty shared_ptr ...

It seems to be throwing an exception, rather than returning an empty
shared_ptr.

> -----Original Message-----
> From: boost-bounces_at_[hidden]
[mailto:boost-bounces_at_[hidden]]
> On Behalf Of Joe Gottman
> Sent: Wednesday, August 03, 2005 7:16 PM
> To: boost_at_[hidden]
> Subject: Re: [boost] weak_ptr, maybe empty
> 
> 
> "Andy Thomas-Cramer" <Andy_Thomas-Cramer_at_[hidden]> wrote in message
> news:2F6133743473D04B8685415F8243F4765347D5_at_madison-
> msg1.global.avidww.com...
> >
> > I have a weak_ptr that might be empty. If not empty, I'd like to
make a
> > smart pointer from it.
> >
> > But the code below throws in the shared_ptr constructor, and I don't
see
> > how to test whether a weak_ptr is empty.
> >
> >    boost::weak_ptr<int> wp;
> >    boost::shared_ptr<int> sp(wp);
> >    if ( sp.get() ) {
> >        ...
> >    }
> >
> > Suggestions?
> >
> 
> It depends what you mean by empty.  A weak_ptr can either
>     a) be associated with a valid shared_ptr
>     b) be associated with a shared_ptr that has since expired
> or c) be default initialized and never associated with a shared_ptr
> 
> If you don't need to differentiate between case b) and c), the most
> elegant
> way is
> if (shared_ptr<int> sp = wp.lock()) {
>     //do work with valid shared_ptr
> }
> 
> If you need to distinguish between b) and c) you can continue with
>  } else {
>     weak_ptr empty;
>     if (!(wp < empty) && !(empty < wp)) {
>         //case c, default-constructed weak_ptr
>     } else {
>         //case b, expired weak_ptr
>     }
> }
> 
> In practice, most people don't need to distinguish between case b) and
c)
> 
> Joe Gottman
> 
> 
> 
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