The PersistentHeapCollectionBase<T>-derived collection types provide a separate "family" of collection types from the heap collection ones (HeapVector<T> etc), which again provide a separate family from non-heap collection ones (Vector<T> etc.) An abundance of riches :)

There are a moderate amount of PersistentHeapCollectionBase-derived types in the codebase, but just as often the codebase ends up using Persistent<Heap*<T>> instead.

While its implementation trick saves a pointer word, which given the objects involved doesn't seem too important, it is worth considering switching to Persisent<Heap*<T>> instead. A smaller Oilpan programming model/universe without it.