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From: danmcleran@hotmail.com
Newsgroups: comp.lang.ada
Subject: Re: Private area and child packages
Date: 31 Dec 2004 09:55:09 -0800
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Date: 2004-12-31T09:55:09-08:00
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Jeffrey Carter wrote:
> danmcleran@hotmail.com wrote:
>
> > Is there any way to hide implementation detail from child packages?
An
> > example, say I have a parent package like this:
> >
> > package Some_Package is
> > type Secret_Type is private;
> > private
> > type Secret_Type is record
> > Secret_Value : Integer := 0;
> > end record;
> > end Some_Package;
> >
> > I don't want any other component to be able to manipulate the
> > Secret_Value record component, not even a child package of
> > Some_Package.
>
> If you don't need to make the type visible at all, you can simply
> declare it in the package body. Items in package bodies are hidden
from
> all other scopes.
>
> If you want the type to be visible, but its implementation to be
hidden,
> you can use a pointer:
>
> package P is
>     type Something is [limited] private;
>
>     -- operations on Something
> private -- P
>     type Implementation;
>     type Something is access [all] Implementation;
> end P;
>
> The full view of Implementation must then be given in the body of P.
If
> you do this, you're responsible for the memory management of
Something.
> This is usually easier if you make Something limited, since the
client
> cannot then make copies of the access values, eliminating
opportunities
> for dangling references.
>
> --
> Jeff Carter
> "Beyond 100,000 lines of code you
> should probably be coding in Ada."
> P. J. Plauger
> 26


But then how do I know when the access to the Implementation object
goes out of scope? It seems to me that I need to either derive from
Ada.Finalization.Limited_Controlled or define Something as a smart
pointer to Implementation.