ref count, thread id's

here we go again i''m still "working" on shared data. each data object has a ref counter and a status bitset. what i wanna do is add support for MT. nothing complicated, just when object is being shared it is shifted into "shared" state (so it uses internal locking). now, to find out if "the user", who got (aquired) a data object reference causes the shift from not_shared to shared state, we need some kind of a thread id. would work something like this: - give first reference to a user, query the user for thread id, remember the id - give second/other reference, again query the user for thread id, if id differs from that one we remembered, switch the state however, this creates some problems. we need to maintain a database of ref2thread_ids. so, i''m looking for a way on how to store these "items" an example: say we have one shared data object X, five threads (a,b,c,d,e), each thread has 10 "users". now it would be a waste of resources, to keep a thread ID for every reference, since some references come from the same thread. what kind of a container do u recommend?