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From: avoncampe@ant.enet.dec.com (Alfred von Campe)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Problem with Instance Variables in THINK C 4.0
Message-ID: <430@ryn.esg.dec.com>
Date: 29 Aug 89 13:33:09 GMT
Sender: guest@ryn.esg.dec.com
Organization: Digital Equipment Corporation
Lines: 129

Digital has been having some problems with its gateway (DECWRL) recently.
I'm reposting this since I don't know if it made it out the first time.

    From: avoncampe@ant.enet.dec.com (Alfred von Campe)
    Subject: Problem with Instance Variables in THINK C 4.0
    Date: 17 Aug 89 01:21:21 GMT

Posted for a friend who currently doesn't have access to the net:
--------------------------------------------------------------------------------

    OK, I've run into a problem, no doubt easily solved, but it has
    eluded me for three days now.

    1) I've declared a subclass of CScooterSamplePane (which is a subclass
    of CPanorama - See the Think C Class Library):

    struct CScooterSampleDial : CScooterSamplePane{	/* Class Declaration */
     				
    	/* Instance Variables */
    			      
    Point	theStart,
    		theEnd;
    			
        /*      Methods      */

    int		Init(				/* initialize the dial */
    	Point		center,
    	Point		end,
    	int		length,
    	int		zeroOffset,
    	double	scale
    	);
    	                                                    
    int		Create(				/* draw the pointer*/ 
    	int	value
    	);    
    	                                                           
    int		Update(				/* update  position*/
    	int	delta
    	);                                             

    }


    2) I define the  Methods:
    int	CScooterSampleDial::Init(  /* initialize pointer position values */
    		Point	center,
    		Point	end,
    		int		length,
    		int		zeroOffset,
    		double	scale)
    {
    	...	
    	theStart.h = 50; theStart.v = 50;
    	theEnd.h = 150; theEnd.v = 150;
    	...
    }

    int	CScooterSampleDial::Create(  /* create/display pointer at 1st position*/
    	int		value)
    {           
    	...                                       
    	MoveTo(theStart.h,theStart.v);
    	LineTo(theEnd.h, theEnd.v);
    	...
    }

    int	CScooterSampleDial::Update(	/* update the pointer */
    	int		value)
    {
    	...
    }


    3) finally I Declare an instance and send some messages to the ...Dial
    object:


    ...
    CScooterSampleDial *theDial;		/* declare an instance */
    ...                                                          
    theDial = new(CScooterSampleDial);          /* create an instance*/
    theDial->Init(center, end, 15, 1, 1.0);     /* send the configuration*/
    theDial->Create(35);                        /* make the pointer*/


    /* of course large chunks of irrelevant code have been omitted */


    The Problem:

    When the Init object is finished, theStart and theEnd points are
    properly assigned as described in the Init method above.

    When I send to the Create method, theStart and theEnd are set to
    0 for each member of those two Point structs.

    In the Think C Manual on Page 188 (Ch. 15 - Using Objects in Think
    C) it states that there is an implicit declaration

    register CLASS *this;

    and within a method definition one can omit the 'this->' from an
    instance variable. However, whether or not I use 

    this->theStart.h    or   theStart.h

    in the Create method, I do not get the values established in the
    Init method.

    I've been through Chapters 14,15,&16 of the manual and have tried
    many different variations, to no effect as of yet.

    Any ideas ??

    Scooter
--------------------------------------------------------------------------------

Alfred

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