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From: jsa@alexandria.organon.com (Jon S Anthony)
Subject: Re: Which compiler is correct?
Date: 1997/06/20
Message-ID: <JSA.97Jun20185414@alexandria.organon.com>#1/1
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Distribution: world
References: <33A77C54.5484@bix.com> <EBz083.9EB@world.std.com>
 <ud205z1ifw.fsf@tanana.llnl.gov>
Organization: PSI Public Usenet Link
Newsgroups: comp.lang.ada
Date: 1997-06-20T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <ud205z1ifw.fsf@tanana.llnl.gov> woodruff@tanana.llnl.gov (John Woodruff) writes:

> Here is another inquiry of the type "which compiler is correct." It is
> so similar to the issue Bob answered for Tom that I retain the thread
> (and the answer Bob gave is so clear that I imagine that this one will
> be equally easy to understand once I know the answer.)
> 
> Let me define a package with a tagged private type:
> package Parent is
>    type Obj is tagged private ;
> private
>    type Obj is tagged record
>       I : Integer ;
>    end record ;
> end Parent ;
> 
> The package has just one child that extends the type.  Depending on the
> specifics of how I derive the new type, I get correct code or code that
> evokes an error message that I don't understand:
> 
> package Parent.Child is
>    type Good_Obj is new Parent.Obj with private ;
>    type Bad_Obj  is new Parent.Obj with null record ;

NOTE: at this point

a) Parent's i component is _not_ visible and so not available to this
view of the children.

b) Bad_Obj is _fully_ declared at this point and so will never get the
implicit declaration of Parent's i component.


>    procedure Do_Nothing ;  -- just to require body
> private
>    type Good_Obj is new Parent.Obj with null record ;

At this point, Parent.I is visible and the full declaration of
Good_Obj implicitly declares this component.


> end ;
> 
> One of the compilers (compiler "r") is able to compile the body without
> error; the other compiler reports an error in the body when I try to
> reference the component of the parent type.
> 
> package body Parent.Child is
>    good : Good_Obj ;
>    Bad  : Bad_Obj ;
>    procedure Do_Nothing is
>    begin
>       null;
>    end ;
> begin
>    Good.I := 2 ;
>    Bad.I := 2 ;  -- compiler "g" reports error
> end ;

So, Bad.I is not directly available and compiler "g", I believe, is
correct.

> selector".  I am unable to think of a reason why the body of the child
> package can reference the parent's private declaration of Obj for one,
> but not for the other derived types.

I think this all falls out of the rules in 7.3.1, and in particular
pay close attention to what is said in (3).

/Jon

-- 
Jon Anthony
OMI, Belmont, MA 02178
617.484.3383
"Nightmares - Ha!  The way my life's been going lately,
 Who'd notice?"  -- Londo Mollari