From: marhardwick@googlemail.com   
      
   I have a need for a class to be used as a type of ID, say an ID for   
   apples, I also need another class for IDs for oranges, another for   
   pears and so on. I also need it to be the case that you cannot assign   
   an apple ID to a pear ID. For example:   
      
   AppleID a(7); // ID for an apple.   
   AppleID b;   
      
   b = a;        // Allowed.   
      
   PearID p(5);  // ID for a pear obviously.   
      
   b = p;        // Oops, not allowed. Would like to be a compile error.   
      
   I recall reading about a templated class that gave this behaviour,   
   where some sort of signature differentiated between what effectively   
   were the same class. Unfortunately my memory has failed me and my   
   googling skills are inadequate. Does anyone have suggestion or can   
   point me in the right direction?   
      
   Thank you.   
      
      
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