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From: gordon@meaddata.com (Gordon Edwards)
Newsgroups: comp.object
Subject: Re: ??Bidirectional Inheritence??
Message-ID: <1737@meaddata.meaddata.com>
Date: 23 Oct 90 12:54:27 GMT
References: <6957@uwm.edu> <304@mtiame.oz>
Sender: usenet@meaddata.com
Reply-To: gordon@meaddata.com (Gordon Edwards)
Distribution: comp
Organization: Mead Data Central, Dayton OH
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In article <304@mtiame.oz>, jep@mtiame.oz (Jesper Peterson) writes:
|> In article <6957@uwm.edu> ctumey@csd4.csd.uwm.edu (Carol T Tumey) writes:
|> >[....]
|> >In the small amount of reading I have done on OOP/OOD I have
|> >not seen anything on the subject of parents inheriting
|> >values from their children nor can I see the benfits of
|> >such hierarchical incest :-)
|> 
|> Do virtual functions in C++ qualify? (I'm asking)
|> An example given in Bjarne Stroustrup's book is: [page 37]
|> 	A "Shape" class which does not have the attributes required to
|> 		sensibly respond ro a "draw" message. It does, however,
|> 		declare the "draw" function as "virtual".
|> 	Derive some classes from Shape, such as Circle, Square etc. Each
|> 		of these properly define the "draw" function.

The word virtual tells the compiler that the function draw() should be
bound at run-time.  The default (no virtual keyword) binds the draw()
function at compile time.

|> 	You can now store a mixture of Circles and Squares in an array
|> 		of Shapes.

What you are describing here is a polymorphic array.  The array of Shape&
can store Shapes and Circles and Squares (which are also Shapes).

|> 	You can loop through this array and sensibly send "draw"
|> 		messages to instances of Shape rather than the specialised
|> 		classes.

If I have an instance X of Circle, then I also have an instance of Shape.
When X calls draw(), the class definition of Circle is checked and if draw()
exists X is bound to it.  If not, the class definition of Shape is checked 
and if draw() exists, X is bound to it.  This continues up the class 
hierarchy.

|> 
|> As I understand it, Shape effectively "inherits" "draw" from it's children.
|> 

Not really, Shape's draw is never examined.  This is hard to explain without
being able to talk face-to-face and draw pictures. Try to remember that the 
order of evaluation starts at the sub-class (or leaf) and works up through the 
class hierarchy.


-- Gordon     (gordon@meaddata.com)