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From: cjmchale@cs.tcd.ie (Ciaran McHale)
Newsgroups: comp.object,comp.lang.eiffel
Subject: Re: Inheritance and Information Hiding
Message-ID: <1991Feb8.171341.19413@cs.tcd.ie>
Date: 8 Feb 91 17:13:41 GMT
Organization: DSG, Dept. of Comp. Sci., Trinity College, Dublin.
Lines: 58

Here's my $0.02 worth on the topic concerning rectangles and
polygons.

Some people have pointed out that since rectangle doesn't
support all the operations which might be performed on a
polygon, rectangle should *not* be a subtype of polygon.
(Other approaches were also suggested.) Instead, the graphics
hierarchy should look like:

		graphic (base class)
		  /\
		 /  \
		/    \
	  rectangle  polygon

A problem with this approach is that rectangle is certainly
similar to polygon in many respects and so we would like to
have it derived from polygon so that we can reuse code.

Bascially we're torn between two seemingly conflicting issues:

	1. We can't have rectangle as a subtype of polygon
	   since add_vertex doesn't make sense when applied
	   to a polygon.
	2. We do not want to duplicate code which is common
	   between rectangle and polygon.

One way to solve this dilemma is for the language to have two
hierarchies rather than just one. There would be a TYPE
hierarchy and an IMPLEMENTATION (CODE) hierarchy. Using this
scheme, rectangle and polygon can be siblings in the TYPE
hierarchy, but rectangle could (if the programmer wanted to)
inherit code from polygon in the IMPLEMENTATION heierarchy.


    TYPE HIERARCHY			IMPLEMENTATION HIERARCHY
    --------------			------------------------
	graphic					 graphic
	  /\					    |
	 /  \					    |
	/    \					 polygon
  rectangle  polygon				    |
						    |
						rectangle

Note that the IMPLEMENTATION hierarchy does not have to
mirror the TYPE hierarchy. The IMPLEMENTATION of rectangle
will include the add_vertex operation (inherited from the
IMPLEMENTATION of polygon) but this will be "private" since
it is not specified in rectangle's TYPE.


Ciaran.
-- 
Ciaran McHale		"Verbosity says it all"			      ____
Department of Computer Science, Trinity College, Dublin 2, Ireland.   \  /
Telephone: +353-1-772941 ext 1538	FAX: +353-1-772204	       \/
Telex: 93782 TCD EI			email: cjmchale@cs.tcd.ie