The USet (Unordered Set) Interface.

1. A class supports the USet interface if it provides these functions.
   • int size()
   • bool add(T x)
   • T remove(T x)
   • T find(x)
   See Chapter 1.2.3 for the requirements on these functions.
2. Suppose template<T> class USetImplementation is a class implementing USet. Then the following could be done.

   // establish empty USet H and designate -1 as "null".
   USetImplementation<int> H(-1); 
   H.size(); // returns 0.
   
   H.add(2); // returns true.
   H.add(5); // returns true.
   H.size(); // returns 2.
   
   H.add(2); // returns false. 2 is already in the USet.
   H.size(); // returns 2.
   
   H.remove(3); // returns null
   H.size(); // returns 2.
   
   H.find(2); // returns 2
   H.find(3); // returns null
   H.size(); // returns 2.
   
   H.remove(2); // returns 2
   H.size(); // returns 1.
   

3. But what good is this, really? Does find really return to the caller what the caller already has? ...well, there are two uses of a USet: (1) as an oracle to answer whether a known item has the property the set represents, and (2) as a map (an association). In the map case we use the definition of == on type T to capture a useful notion of ``equivalent".

   class record 
   { public: 
       string name; 
       int phone;
       record(string n, int p): name(n), phone(p) {}
   };
   
   ///// support for USet implementations on T = record /////
   
   
   // Tell USet the meaning of 'equal' for records.
   bool operator==(record a, record b) { return a.name == b.name; }
   //////////////////////////////////////////////////////////
   
   // usage examples //
   
   record dummy("null", 0);
   // establish USet H, designate dummy as "null".
   USetImplementation<record> H(dummy); 
   
   H.size(); // returns 0.
   record me("saunders", 8316238);
   record pres("obama", 1234567);
   record vp("biden", 7654321);
   
   H.add(me); // returns true.
   H.add(pres); // returns true.
   H.size(); // returns 2.
   
   record me2("saunders", 0);
   H.add(me2); // returns false. 
               // record with name "saunders" is already in.
   H.size(); // returns 2.
   
   H.remove(vp); // returns dummy record
   H.size(); // returns 2.
   
   record rec;
   rec = H.find(me2); // returns me.  
   				   // IMPORTANT: phone # is now available in rec.
   H.find(vp); // returns dummy
   H.size(); // returns 2.
   
   H.remove(me2); // returns me.  
                  // Note that caller did not know me fully.
   H.size(); // returns 1.
   

4. Bottom line, regardless of record type: if operator== is defined in terms of part 1 of the records, then find and remove are returning information about part 2 of the records to the caller. Also USet is insuring there is only one record stored per possible part 1 data.