Class RWHashTable is the easiest Smalltalk-like collection class to understand. It uses a simple hashed lookup to find the bucket where a particular object occurs, then does a linear search of the bucket to find the object. A key concept is that multiple objects that test isEqual to each other can be inserted into a hash table.

Class RWBag is similar to RWHashTable, except that it counts occurrences of multiple objects with the same value; that is, it retains only the first occurrence and merely increments an occurrence count for subsequent ones. RWBag is implemented as a dictionary, where the key is the inserted object and the value is the occurrence count. This is the same way the Smalltalk Bag object is implemented. Note that this implementation differs significantly from many other C++ Bag classes which are closer to the RWHashTable class and not true Bags.

Class RWSet is similar to its base class RWHashTable, except that it doesn't allow duplicates. If you try to insert an object that isEqual to an object already in RWSet, the object will be rejected.

Class RWIdentitySet, which inherits from RWSet, retrieves objects on the basis of identity instead of value. Because RWIdentitySet is a Set, it can take only one instance of a given object.

Note that the ordering of objects in any of these classes based on hash tables is not meaningful. If ordering is important, you should choose a sequenceable class.