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From: "David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>
Subject: Opaque Types (was Elimination of "use" clauses)
Date: 1999/07/20
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Date: 1999-07-20T00:00:00+00:00
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Richard D Riehle <laoXhai@ix.netcom.com> wrote in message
news:7n0hr7$180@dfw-ixnews21.ix.netcom.com...
> In article <FF2ECA.76@stuyts.nl>,
> jerry@jvdsys.stuyts.nl wrote:
>
> >I am unfamiliar with the term 'opaque type', perhaps because I never
looked
> >at Modula-3. Can you explain what it means ?
>
<large snip>
> The second Ada example is designed as an
> "opaque type".
>
> generic
>   type StackElementType is private;
>   Max  : Positive := 100;
> package Stack is
>   type Stack_Type is limited private;
>   -- declarations for push, pop, etc.
> private
>   type Stack_Data;
>   type Stack is access all Stack_Data;
> end Stack;
>
> where there is no information about the structure of the stack,
> even in the private part.  The details of the stack are deferred
> to the package body.  For example,
>
> package body Stack is
>   type Stack_Array is array(1..Max) of StackElementType;
>   subtype Stack_Index is range 0..Max;
>   type Stack_Item is record
>     Top : Stack_Index := 0;
>     Data : Stack_Array;
>   end record;
>   -- implement push, pop, etc.
> end Stack;
>

Didn't you mean something more like the following?
(The type declared in the body is not a completion of the
incomplete type Stack declared in the private part).

generic
  type StackElementType is private;
  Max  : Positive := 100;
package Stack is
  type Stack_Type is limited private;
  -- declarations for push, pop, etc.
private
  type Stack_Item;
  type Stack_Type is access all Stack_Item;
end Stack;

package body Stack is
  type Stack_Array is array(1..Max) of StackElementType;
  subtype Stack_Index is Integer range 0 .. Max;
  type Stack_Item is record
    Top : Stack_Index := 0;
    Data : Stack_Array;
  end record;
  -- implement push, pop, etc.
end Stack;

That asked, I now have another question, or two, viz.:

  1. Is the value of an opaque type worth giving up the
     flexibility of a discriminated type wherein the
     stack size does not need to be a formal parameter
     of the generic package, or have I missed a way to
     have a discriminated opaque type?

  2. In a similar vein, it seems that opaque controlled
     types are not possible.  Is this true.

Perhaps a more general question is -- is there really
any value in making a limited private type opaque?  If so,
what is that advantage?

David C. Hoos, Sr.