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From: patty@chmsr (Patty Jones)
Newsgroups: comp.lang.c++
Subject: contravariance confusion
Message-ID: <5063@hydra.gatech.EDU>
Date: 19 Jan 90 14:04:00 GMT
Sender: news@prism.gatech.EDU
Reply-To: patty@chmsr.gatech.edu (Patty Jones)
Organization: Center for Human-Machine Systems Research - Ga Tech
Lines: 93


What does the following 1.36.1 g++ compiler warning mean??

warning: contravariance violation for method types ignored

and could this become a serious problem later on as our program grows?

Also, I'm not sure I understand what contravariance means -- is that when
you pass a pointer to member of class "base" to a function that expects
a pointer to a member of class "derived"?  But in our situation we have
defined a function (SchedEvent; see below) to expect pointers to class "base" 
and we actually use the SchedEvent function with pointers to class "derived".  

A bit of background:

We are working on an interactive real-time simulation in C++ (in the
domain of satellite ground control) using the g++ 1.36.1
compiler on Suns.

We have a function that creates event objects which are maintained
on an event list; the function prototype is as follows:

  SchedEvent(int, ActiveSimulationObject *, void_fp, char *)

  where
  1) the first argument is an integer which represents the time 
	  the event is scheduled to occur,
  2) a pointer to an ActiveSimulationObject object (ActiveSimulationObject is
     our base class); this points to the object to whom something is going to 
	 occur,
  3) void_fp is a pointer to a member function in class ActiveSimulationObject 
       (i.e., typedef void (ActiveSimulationObject :: *void_fp) (...);  )
  4) the fourth argument is a character string that represents the 
	  arguments to the function pointed to by void_fp.

Our program works but we are getting the "contravariance" compiler warning 
on lines where we call SchedEvent.  We are calling SchedEvent with
pointers to derived classes and their members.   For example,

part of the class hierarchy we have right now is as follows:

		ActiveSimulationObject
                |
                |
          SpacecraftComponent
                |
                |
            TapeRecorder

and part of the class definition for TapeRecorder is as follows:

class TapeRecorder : public SpacecraftComponent {
	char *mode;  // can be "record", "standby", "playback", "fast forward"
	.
	.

 public:
	 void setMode(char *m) { mode = m; }
	.
	.
};

And finally, we declare:

TapeRecorder TR1;

To schedule TR1 to go into "record" mode at time 5, we do the following:

   SchedEvent(5, &TR1, &TR1.setMode, "record"); 

and as I said, we get the "contravariance" warning on lines such as this
where we call SchedEvent.

thanks for your help!

patty

Patty Jones                                        
Center for Human-Machine Systems Research
School of Industrial and Systems Engineering
Georgia Institute of Technology,  Atlanta, GA 30332

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Patty Jones                                        
Center for Human-Machine Systems Research
School of Industrial and Systems Engineering
Georgia Institute of Technology,  Atlanta, GA 30332

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