Re: [Boost-users] How can I have an object-specific and thread-specific pointer?

Subject: Re: [Boost-users] How can I have an object-specific and thread-specific pointer?
From: Phillip Hellewell (sshock_at_[hidden])
Date: 2011-11-16 23:00:32

Anyone?  Should I try boost-devel?

On Sat, Nov 12, 2011 at 4:00 PM, Phillip Hellewell <sshock_at_[hidden]> wrote:

> Hi,
>
> My class contains a member var that is a little buffer used as a cache.
> Instances of this class are used concurrently (referenced through a
> shared_ptr).
>
> I would like to switch my buffer to a thread_specific_ptr, but there are
> two big problems with using thread_specific_ptr as a member var:
> 1. There is a memory leak when my object goes away, because by design only
> one of the pointers gets cleaned up (the one for the thread that happens to
> delete the object).
> 2. There is undefined behavior when a new object gets created that happens
> to be allocated at the same address as a previously freed one.  get( )
> could return a pointer to a buffer that goes with an old (now freed) object.
>
> I try not to question the wisdom of the thread_specific_ptr authors, but
> given these two issues, especially #2, I have to conclude that
> thread_specific_ptr is only suitable for statics / globals.  It is wholly
> unfit to be used as a member variable (except in a singleton class I guess).
>
> The next best thing I can think of for my class is to just have a
> std::map<thread_id, void*>, with synchronized access of course, and I clean
> up all the pointers in the map in the DTOR.  That would work just great,
> except I really do need the pointers to also get cleaned up (and removed
> from my map) when the corresponding threads go away.  Some of my objects
> can live a very long time, whilst threads get created and destroyed off and
> on, so without this functionality the map could grow indefinitely.
>
> So I'm stuck.  I can't use thread_specific_ptr because it won't clean up
> everything when my object goes away, and I can't use a map because it won't
> clean up anything when a thread goes away.  Is there an easy solution to
> this?
>
> If there were some way for me to hook up a callback directly to the thread
> cleanup function, I could add a callback to remove it from my map when the
> thread goes away.  But I'd also need the ability to remove that callback in
> my DTOR.  Does boost provide any way to do this?
>
> I can't be the first person to want a thread-specific, object-specific
> pointer, can I?
>
> Phillip
>