In my Cocoa Mac app I have an instance A which contains an unordered set of instances B which in turn has an ordered set of instances C. An instance of C can only be in one instance B and B only in one A.  

I would like to have an unordered set of all instances C available on instance A. I could enumerate over all instances B each time, but that seems expensive for something I need to do often. However, I am a bit worried that keeping track of instances C in A could become cumbersome and be the cause of  inconsistencies, for example if an instance C gets removed from B but not from A. 

Solution 1 Use a NSMutableSet in A and add or remove C instances whenever I do the same operation in B. 

Solution 2 Use a weak referenced NSHashTable in A. When deleting a C from B, it should disappear for A as well. 

Solution 3 Use key value observing in A to keep track of changes in B, and update a NSMutableSet in A accordingly. 

Solution 4 Simply iterate over all instances B to create the set whenever I need it.  

Which way is best? Are there any other approaches that I missed? 

NB I don't and won't use CoreData for this app.