Re: Ada95 OOP Questions

[jsa@alexandria (Jon S Anthony)]

In article <3219EFDE.7161@cmass.com> Farshad Nayeri <farshad@cmass.com> writes:

> Eric Newton said:
[...]
> > In C++/Modula-3 I must list the operations on
> > the private state of Matrix within the Matrix class. 
> > I must list the operations on the private state of 
> > Vector within the Vector class.
> > There is visible association of dispatch methods and type.
[...]
> Modula-3 has separate mechanisms for encapsulation (modules & 
> interfaces) vs. inheritance and dispatching (object types). 
> An implementor can define an interface which reveals partial 
> information about one or more object types; clients can import
> only the necessary features by importing the interface that
> reveals the right kind of information about (one or more) types.
> Anyone with the right level of access to a group of classes, 
> that is, anyone who imports the right "private" interface 
> to a module, can access the private state of those classes
> as prescribed by the implementor of the interface.

This is done in Ada95 by use of child packages and/or use of
constrained generics.  In fact, it is "cake" and one of the primary
uses of these constructs.


> Steve Freeman has written a nice article for Dr. Dobb's Journal,
> comparing the encapsulation and grouping facilities of Ada95, 
> C++, Eiffel, and Modula-3. He addresses precisely the kinds of issues that
> Eric mentioned and goes through code examples to illustrate his
> point. A copy of the article is available via the Modula-3 home page.

Well, I read the article and I note that since Freeman does not
mention how this sort of stuff is quite easily done in Ada, perhaps he
does not understand the Ada model.  He does a very good job with C++.

In fact, at the first M3 (Association) example, he only mentions the
problems that C++ or Eiffel would have in so "subtyping".  Neither
problem is applicable to Ada.

/Jon
-- 
Jon Anthony
Organon Motives, Inc.
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