From: kaba@nowhere.com   
      
   14.7.2012 3:23, Kaba wrote:   
   > ### Option 3: Transferred ownership   
   >   
   > In this option A is moved into B. This guarantees that B can not be   
   > modified or destructed, as long as A exists and refers to B.  The A   
   > can release the ownership of B to outside, given that it also clears   
   > its state. The problem with this approach is that there can be only   
   > one data structure B which can refer to A.   
      
   Clearly there is an error here, since I had the roles of A and B   
   reversed. It should have been:   
      
   ### Option 3: Transferred ownership   
      
   In this option A is moved into B. This guarantees that A can not be   
   modified or destructed, as long as B exists and refers to A.  The B   
   can release the ownership of A to outside, given that it also clears   
   its state. The problem with this approach is that there can be only   
   one data structure B which can refer to A.   
      
   I will also add:   
      
   ### Option 5: Locking   
      
   In this option B notifies A that it is being referred to. In each   
   mutating operation of A, or when destructing A, A checks whether it is   
   being referred to. If this is the case, an error is generated at   
   run-time.   
      
   --   
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