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From: mwizard@eecs.cs.pdx.edu (Craig Nelson)
Newsgroups: comp.lang.pascal
Subject: Re: Overwrite TApplication.Init?
Message-ID: <2337@pdxgate.UUCP>
Date: 12 Apr 91 06:06:28 GMT
References: <11841@j.cc.purdue.edu> <1991Apr12.012228.3581@maytag.waterloo.edu>
Sender: news@pdxgate.UUCP
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dmurdoch@watstat.waterloo.edu (Duncan Murdoch) writes:

>In article <11841@j.cc.purdue.edu> zhou@brazil.psych.purdue.edu (Albert Zhou) writes:
>>It seems that overwriting TApplication.Init is very hazardous.
>> I have tried to overwrite it in order to get a different background. 
>>Here is the code:
>>
>>procedure mine.init; virtual;
>>begin
>>	Tapplication.init;
>>	write a different background;
>>end;
>>
>>Odd enough, this modification caused the system to be rebooted.

>I don't think the problem is with TApplication, it's with your override.  
>You should declare mine.init to be a constructor, not a virtual method.  
>Virtual methods can't be called until after the object is constructed.

	I KNOW the problem is with the fact it's a method and not a constructor.
I ran into the same problem when I was learning how to use TStreams because
I cas declaring the MyObject.Load(var S:TStream) to be a method and not a 
constructor.  Don't know exactly how it is done, and Sydney, feel free to
but in and tell me I'm wrong,  but all objects have a link to their virtual
methods through their contructors.  Normally if you do not call a constructor
with an object before you use a virtual method a runtime error will occur. 
This does not seem to happen if you override the constructor of the derive'
class with a virtual method instead.  It's like it used to be a constructor,
so maybe it still is.  Anyway, I stay away from that problem by using the
extended sytnax of the New() function.  If you pass New(PMyObject,Init) and
Init is not a constructor, you will not get a working program, but a COMPILER
error instead of a runtime error.  I know it uses the heap alot more, but
welcome to OOP.

Cheers!

 []====================================================================[]
 || Craig R. Nelson                | CCSofD Software Inc.              ||
 || Programmer                     | Beaverton, OR, 97005              ||
 || mwizard@eecs.ee.pdx.edu        | (unlisted on the net)             ||