Re: Anybody in US using ADA ? One silly idea..

[Mark Biggar <mark.a.biggar@attbi.com>]

Hyman Rosen wrote:
> Dmitry A. Kazakov wrote:
> 
>> That's the whole point. We want to inherit from Ellipse (non-abstract)
>> some of its methods, but we do not want to inherit the representation
>> (two doubles).
> 
> 
> I don't understand what you are trying to accomplish.
> If you want to inherit method implementation, how can
> this not be tied to object representation?
> 

What is going on here is the fundamential paradox of OO systems.
It often happens that the direction of substutibility inheritence and
implementation inheritence are opposite to each other.  For example,
you would normally like class Integer to be a subclass of class Float
because you want to be able to use an Integer anywhere you can use
a Float.  But, given methods in class Float like Get_Exponent you
want to implement class Float using Class Integer.  Oops, you can't
do both.

In general there are four ways two classes can be related to each
other in an OO system.

I.  class A isa class B
	you can use a A anywhere you can use an B

In Ada terms this is "type A is new B ..."

II. class A is implemented as class B
	each instance of class A is implemeted under the covers as
	an instance of class B, but you have provided a completely
	different interface.  E.G., class Stack is implemented using
	class Array, but you access it via push and pop instead of
	indexing.

	type A is private;
     private
	type A is new B ...;

III. class A contains class B.  Class A has a visible member
	of type class B plus some other stuff.

	type A is record B_mem: B; ... end record;

1V.  class A is not related ot class B at all.  included of
	completeness.

Unfortunately I and II can be in conflict like Integer and Float above
and you end up wiht an unresolvable circular dependency and so
must give up one or the other.

-- 
Mark Biggar
mark.a.biggar@attbi.com