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From: mbk@jt3ws1.etd.ornl.gov (Kennel)
Newsgroups: comp.lang.ada
Subject: Re: Type extension questions
Date: 27 Mar 1995 20:37:52 GMT
Organization: Oak Ridge National Lab, Oak Ridge, TN
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Date: 1995-03-27T20:37:52+00:00
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Harry Koehnemann (koehnema@enuxsa.eas.asu.edu) wrote:
> Well, yes and no.  What I really want to do is restrict the interface
> to a type.  It might help to see an example.  Let's pretend we have 2
> abstract data types, where one is an extension of the other:

>     class List {
> 	public:  Set (...) { ...}
> 	private: ...;
>     };

>     class Stack : private List {
> 	public:  Push() { ...}
> 	private: ...;
>     };

> It is not appropriate for a Stack to invoke Set.  One could argue that
> Stack has-a List instead of Stack is-a list (I think Tucker Taft made
> this argument earlier).  Bottom line - I guess there is no way to
> perform private inheritance, as shown in the above example, in Ada95,
> which was my question.  Thanks for the input.

I wouldn't call this 'has-a' *or* 'is-a'.

I would call it 'is-implemented-as-a'.

For example, we may want many features from List to be *public*, and
yet still *not* allow a Stack to be substituted at run-time for a List.
{i.e. divorce subtyping from inheritance, a convenient idea i've found}

How ought one do this in Ada?

cheers
matt