Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!wuarchive!zaphod.mps.ohio-state.edu!think.com!hsdndev!husc6!endor!siegel
From: siegel@endor.uucp (Rich Siegel)
Newsgroups: comp.sys.mac.programmer
Subject: Re: C/C++ difference?
Message-ID: <5720@husc6.harvard.edu>
Date: 13 Feb 91 16:22:19 GMT
References: <cwmaX1w163w@shark.cs.fau.edu> <49114@apple.Apple.COM>
Sender: news@husc6.harvard.edu
Reply-To: siegel@endor.UUCP (Rich Siegel)
Organization: Symantec Language Products Group
Lines: 62

In article <49114@apple.Apple.COM> marc@Apple.COM (Mark Dawson) writes:

>>1)  my think C manual says it is upwards compatable with c++, meaning
>>  think C is lacking something in c++.  what exactly is it lacking?
>C++ supports 
>  o data abstraction  the data needed by that code is local to that
>                      code; if you want access to that data, you have to ask 
>                      the code ("object") to get it for you)
>  o inheritance  you can derive a new user-defined type from an old one
>                 and make changes only where you need them--in theory this
>                 makes for easier code reuse)
>  o polymorphism  the actual creating of a new user-defined type that can
>                  inherit (or not) properties of the old type, plus add
>                  something.  For example, you could have a type "oval"
>                  that draws an oval.  Using polymorphism, you could derive
>                  a sub-type called "circle" that uses (inherits) all of
>                  "oval"'s drawing routines, but defines that the x & y
>                  axises (sp?) are the same.

	Actually, THINK C does support all three of these things; you can
write your own methods to abstract the instance variables of an object.
What THINK C does not provide is compiler-enforced abstraction through the
use of keywords such as "private", "protected", and "friend".

	THINK C supports the Object Pascal model of inheritance, that is,
a class can inherit its characteristics from only one superclass. C++ supports
multiple inheritance, in which a class can derive its characteristics from
more than one superclass.

	In your description, you've confused inheritance and polymorphism;
in fact, both of the things you describe (inheriting characteristics from
an ancester class and overriding the inherited behavior) are features of
the inheritance model. Polymorphism means that you can have to disjoint types
of object which both have a message of the same name, but the name means
different things for each class. Consider:

TYPE Shape = object
	
	procedure Draw;

end;

	Square = object (Shape)
		sideLength : Integer;
		procedure Draw;
		Override;
	end;

	Oval = object(Shape)
		majorAxis : Integer;
		minorAxis : Integer;
		procedure Draw;
		Override;
	end;

The "Draw" message has different meaning to an Oval object than it has to
the "Square" object.

R.
 Rich Siegel	Symantec Languages Group  Internet: siegel@endor.harvard.edu

"I was just trying to be subtle. That's my job, isn't it?"