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

Subject: [Boost-users] How can I have an object-specific and thread-specific pointer?
From: Phillip Hellewell (sshock_at_[hidden])
Date: 2011-11-12 18:00:19

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