Sometimes referred to as maps, dictionaries use an external key to find a value. The key and value may be of different types, and in fact usually are. You can think of dictionaries as associating a given key with a given value. For example, if you were building a symbol table in a compiler, you might use the symbol name as the key, and its relocation address as the value. This approach would contrast with your approach for a Set, where the name and address would have to be encapsulated into one object.

The Essential Tools Module provides two dictionary classes: RWHashDictionary, implemented as a hash table, and RWBTreeDictionary, implemented as a B-tree. For these classes, both keys and values must inherit from the abstract base class RWCollectable.