callback.h

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// callback.h 
//      Data structure to allow an object to register a "callback".
//      On an asynchronous operation, the call to start the operation
//      returns immediately.  When the operation completes, the called 
//      object must somehow notify the caller of the completion.
//      In the general case, the called object doesn't know the type
//      of the caller.
//
//      We implement this using virtual functions in C++.  An object
//      that needs to register a callback is set up as a derived class of
//      the abstract base class "CallbackObj".  When we pass a
//      pointer to the object to a lower level module, that module
//      calls back via "obj->CallBack()", without knowing the
//      type of the object being called back.
//
//      Note that this isn't a general-purpose mechanism, 
//      because a class can only register a single callback.
//
//  DO NOT CHANGE -- part of the machine emulation
//
// Copyright (c) 1992-1996 The Regents of the University of California.
// All rights reserved.  See copyright.h for copyright notice and limitation 
// of liability and disclaimer of warranty provisions.
//

#ifndef CALLBACK_H
#define CALLBACK_H 

#include "copyright.h"

// Abstract base class for objects that register callbacks

class CallBackObj {
   public:
     virtual void CallBack() = 0;
   protected:
     CallBackObj() {};  // to prevent anyone from creating
                                // an instance of this class.  Only
                                // allow creation of instances of
                                // classes derived from this class.
     virtual ~CallBackObj() {};
};

#endif

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