Re: Ada95 OOP Questions

[Farshad Nayeri <farshad@cmass.com>]

Eric Newton said:

> The point of the (deleted) example was to show that 
> a package is just a namespace, and that multiple classes 
> can live in that namespace, and that the features of 
> the class do not need to be visibly associated
> with the dispatch object.  Matrixes encapsulates 
> Matrix *and* Vector types. I can list the Matrix and 
> Vector operations willy-nilly within this namespace.  

> 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.

I believe this is an inaccurate representation of the way Modula-3 
organizes namespaces and visibility. Much care went into designing 
namespaces and avoiding problems in the C++ "private" state model. 

Modula-3 has separate mechanisms for encapsulation (modules & 
interfaces) and inheritance and dispatching (object types). 
Using partial revelation--where one interface can reveal partial 
information about various types; others can import only the necessary 
features by importing that interface--Eric can do the kinds of 
groupings he was talking about.

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 web:

http://www.research.digital.com/SRC/modula-3/html/partial-rev/index.html

Regards, -- Farshad

--
Farshad Nayeri
Critical Mass, Inc.
http://www.cmass.com/

Disclaimer: I am biased!