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From: Ryan Tarpine <rtarpine@hotmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Hiding a type
Date: Sat, 06 Jul 2002 18:16:20 +0000
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Date: 2002-07-06T18:16:20+00:00
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Pat Rogers wrote:

> Do check out a textbook from the library -- it is worth the time -- or
> take one of the on-line tutorials.  See www.adapower.com.

I hope to get a textbook soon.  I'll have to buy one, as my local 
library is quite small.  I've read most of Ada Distilled and use it as a 
quick reference, and I don't find the reference manual too difficult to 
look through because I've used much more cumbersome languages before 
**ahem**c++**cough**.  The Lovelace tutorial is also wonderful.

>>I want to write functions that will return something of type Hide in a
>>way so clients can't do anything except pass it to other functions.  I
>>also don't want them to know if I change how I implement it.  (In my
>>real program, I'm trying to hide that Foo returns an access type.)
>>Please tell how I should actually do this
> 
> 
> package Opaque is
>     type Hidden is limited private;
>     -- declare the operations here that you want to export to clients
> private
>     type Hidden is access all Integer;
> end Opaque;

I'm sorry but I didn't make it clear enough.  It's more like this:

package Test is
     type Public_Name is limited private;
     function Initialize( File_Name : String ) return Public_Name;
     procedure Process( Input : Public_Name );
private
     type Private_Type is record ... end record;
     type Private_Type_Ptr is access all Private_Type;
     subtype Public_Name is Private_Type_Ptr; -- or what?
end Test;

I don't want to expose the name Private_Type_Ptr because I know that it 
might change in the future.  I know it sounds silly so it's probably 
just poor planning on my part.

To give more specific details as to what I'm doing, I'm trying to return 
a pointer to a node in a tree structure.  There are several types of 
nodes and I'm concerned that in the future I will want to return a 
different type of node.  I want to hide this from the client.

Thank you again,
Ryan