Re: [Boost-users] Casting a shared_ptr as a prototyped (void *)?

From: Leon Mlakar (leon.mlakar_at_[hidden])
Date: 2008-01-16 04:13:17

Wouldn't that open a potential gap for original shared_ptr to destroy
whatever is pointed to by raw pointer before thread manages to bind it back
to the shared count?
.
 
Cheers,
 
Leon

  _____  

From: boost-users-bounces_at_[hidden]
[mailto:boost-users-bounces_at_[hidden]] On Behalf Of Ovanes Markarian
Sent: Wednesday, January 16, 2008 8:31 AM
To: boost-users_at_[hidden]
Subject: Re: [Boost-users] Casting a shared_ptr as a prototyped (void *)?

Try using the enable_shared_from_this idiom:
http://www.boost.org/libs/smart_ptr/sp_techniques.html#from_this

This will allow you to pass a raw pointer to your thread and later on bind
the raw pointer to the correct shared counter. 

Good Luck,
Ovanes

On Jan 16, 2008 6:45 AM, Elli Barasch <comptonsw_at_[hidden]> wrote:

Richard Dingwall wrote: 

On Jan 16, 2008 5:22 PM, Elli Barasch  <mailto:comptonsw_at_[hidden]>
<comptonsw_at_[hidden]> wrote:

  

 Richard Dingwall wrote:

 On Jan 16, 2008 1:11 PM, Elli Barasch  <mailto:comptonsw_at_[hidden]>
<comptonsw_at_[hidden]> wrote:

 I'd like to pass a shared_ptr as the entry argument to a thread function

via pthread_create. However,

the entry argument is prototyped as void *. I can't simply cast the

shared pointer as a (void *). How do I go about this?

 Try passing:

static_cast<void *>(your_ptr.get())

Richard

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 A follow up question.  Does get() increase the reference count, or does it

simply return the underlying pointer without doing so?  If so, I'd be

worried that a race exists such that the object could be destroyed before

the thread gets to run.

    

get() does not incremement the reference count, as it returns a raw

pointer, and there is no way of knowing how a raw pointer is being

consumed or stored.

If you the function you are passing it to retains a reference to the

pointer anywhere, there is no way this can be tracked by the

shared_ptr. Furthermore, if the shared_ptr(s) go out of scope the raw

pointer references will be left dangling.

HTH,

Richard

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Is there a way I can "new" a copy of the shared_ptr onto the stack?
Something like:

pthread_create(&t,&attr,&func,(void *) new shared_ptr<T>(shp));    //mangled
syntax, I know

Something like this would guarantee the object stays in scope?

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