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From: "Frank J. Lhota" <NOSPAM.Lhota.adarose@verizon.net>
Newsgroups: comp.lang.ada
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Subject: Re: visibility of private incomplete types
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Date: Fri, 29 Oct 2004 15:33:04 GMT
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Date: 2004-10-29T15:33:04+00:00
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"Nick Roberts" <nick.roberts@acm.org> wrote in message 
news:2uf1loF27r1iqU1@uni-berlin.de...
> Randy Brukardt wrote:
> Also, would I be right in saying that, in Ada 95, it is not permitted
> for a type declared (by an incomplete type declaration) in the private
> part of a package to be completed in the package's body?

No, this was permitted as early as Ada 83, and I used this technique many 
times. For example, assume that we are writing a package that implements 
widgets. Widgets will be implemented as pointers to a record, but of course 
we will hide this implementation detail using a private type. We can also 
hide the structure of this record type as follows:

    package Widgets is

        type Widget_Type is limited private;

        procedure Open( Widget : in out Widget_Type; Name : in String );
        procedure Close( Widget : in out Widget_Type );
        ...

    private

        type Widget_Data;
        type Widget_Type is access Widget_Data;

    end Widget;

This type of package specification in legal in both Ada 83 and Ada 95 (and 
presumably in Ada 200x), for Widget_Data is never used in a way that 
requires a full declaration. Of course, Widget_Data must be fully declared 
in the package body.