Re: [boost] [interest] rich-typed smart pointers

Subject: Re: [boost] [interest] rich-typed smart pointers
From: Julian Gonggrijp (j.gonggrijp_at_[hidden])
Date: 2013-10-07 09:34:48

Jonathan Wakely wrote:

> On 7 October 2013 11:01, Julian Gonggrijp wrote:
>> 
>> The std::weak_ptr detects the dangling pointer and changes it into a
>> null pointer. This makes sense, because null pointers are easier to
>> detect. However, as the surrounding code probably relies on a live
>> pointer (because dangling pointers are never planned) the program is
>> still going to fail. This is what I meant by "disaster".
> 
> Why is it going to fail?  Expired pointers (not dangling ones) most
> certainly are planned, and weak_ptr is designed to support (and
> detect) that case.  Users of std::weak_ptr know that it needs to be
> checked, and the explicit conversion that is needed makes it hard to
> forget to do that. Either you say:
> 
>    std::shared_ptr<X> sp(wp);
> 
> which will throw if weak_ptr.expired() is true, or you use the
> non-throwing form in a conditional:
> 
>    if (auto sp = wp.lock())
>      /* ... */ ;
>    else
>      /* deal with it */ ;

Of course a programmer can avoid the problem by doing the right thing.
That's not what my paragraph above was about. As I stated in the
follow-up, the same reasoning applies to the rtp pointers.

Your own quick example clearly suggests how the programmer may do the
wrong thing:

auto sp = wp.lock();
// use without checking for null

Please note that I'm not disputing that reference counting has a clear
advantage to single ownership in cases like these. I'm just saying
that dangling or expired pointers can still be a pitfall despite that
advantage.

-Julian