Re: [boost] [Smart Ptr][Bind] Implementaion of Weak Callback Function

Subject: Re: [boost] [Smart Ptr][Bind] Implementaion of Weak Callback Function
From: Christopher Currie (christopher_at_[hidden])
Date: 2009-03-21 19:21:03

On Sat, Mar 21, 2009 at 2:39 PM, bnv <bnv_at_[hidden]> wrote:
> Artyom <artyomtnk <at> yahoo.com> writes:
>
>> Hello,
>>
>> Today boost::bind allows to use shared_ptr to ensure that
>> when the function is called the object exists:
>>
>> shared_ptr<bar> bar_ptr;
>> function<void()> foo;
>>
>> foo=bind(&bar::slot,bar_ptr)
>>
>> Now even if we call bar_ptr.reset() the object is still owned by foo.
>
> However, I seem to recall something like this coming up before and the rationale
> for not supporting it is was that the behavior to follow when the weak pointer
> is expired is subjective. Â That is, in some cases you may want an exception, and
> in other you may want nothing to happen.

I tried doing what my intuition told me would be the moral equivalent,
which would be to use composition to give bind a weak_ptr instead of a
smart one:

  foo = bind( &bar::slot, bind( &weak_ptr<bar>::lock, weak_ptr<bar>(
bar_ptr ) ) );

However, one of those binds must be persisting the smart pointer
somewhere, because calling foo always has an effect for me, where I
would expect it to segfault after the shared_ptr is reset, since
lock() would return a null shared_ptr. You might be able to see the
problem though, and get where you want to be.

HTH,
Christopher