Method num_nonempty

libCacheSim/dataStructure/sparsepp/spp.h:1287–1287

We also may want to know how many *used* buckets there are

    // We also may want to know how many *used* buckets there are
    size_type num_nonempty() const   { return (size_type)_num_items(); }

    // TODO(csilvers): make protected + friend
    // This is used by sparse_hashtable to get an element from the table