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From: Robert A Duff <bobduff@world.std.com>
Subject: Re: meaning of "current instance"
Date: 1999/11/11
Message-ID: <wcc7ljo3ld8.fsf@world.std.com>#1/1
X-Deja-AN: 547429611
Sender: bobduff@world.std.com (Robert A Duff)
References: <382b0da1_2@news1.prserv.net>
Organization: The World Public Access UNIX, Brookline, MA
Newsgroups: comp.lang.ada
Date: 1999-11-11T00:00:00+00:00
List-Id: <comp.lang.ada>

"Matthew Heaney" <matthew_heaney@acm.org> writes:

> I have some code below that I thought had a bug (um, I mean "error"), 

;-)

> but now I'm not so sure.
> 
> Suppose I have this:
> 
>   type T is tagged private;
> ...
>   function Init (O : access T'Class)
> 
>   type T is
>     tagged record
>       I : Integer := Init (T'Access);
>     end record;

I think you want T to be limited, or else use 'Unchecked_Access.

> If I have an object of type T:
> 
> declare
>   O1 : T;
> begin
> 
> then it's clear that the "current instance" to which T'Access refers has
> type T.
> 
> But now suppose I derive from T:
> 
>   type NT is new T with null record;
> 
> and declare an instance of type NT:
> 
> declare
>   O2 : NT;
> begin
> 
> Object O2 (like O1) has a component I, which is initialized with the
> "current instance."  But what is the type of the "current instance"
> here, passed to Init?

Um, I guess it's T, but what difference does it make?  It's *tag* is
NT'Tag.  Inside Init, we have a value of an anonymous
access-to-class-wide.  So if Init dispatches, it will dispatch to
an NT operation.

> --STX
> with Ada.Text_IO;  use Ada.Text_IO;
> 
> package body P.C is
> 
>   function Do_Init (O : access NT) return Integer is
>   begin
>     Put_Line ("P.C.Do_Init");
>     return 1;
>   end;
> 
> end P.C;
> package P.C is
> 
>   type NT is new T with null record;
> 
> private
> 
>   function Do_Init (O : access NT) return Integer;
> 
> end P.C;
> with Ada.Text_IO;  use Ada.Text_IO;
> 
> package body P is
> 
>   procedure Put (O : in T'Class) is
>   begin
>     Put_Line ("I is" & Integer'Image (O.I));
>   end;
> 
>   function Init (O : access T'Class) return Integer is
>   begin
>     Put_Line ("P.Init");
>     return Do_Init (O);
>   end;
> 
> 
>   function Do_Init (O : access T) return Integer is
>   begin
>     Put_Line ("P.Do_Init");
>     return 0;
>   end;
> 
> 
> end P;
> package P is
> 
>   type T is tagged limited private;
> 
>   procedure Put (O : in T'Class);
> 
> private
> 
>   function Init (O : access T'Class) return Integer;
> 
>   function Do_Init (O : access T) return Integer;
> 
>   type T is
>     tagged limited record
>       I : Integer := Init (T'Access);
>     end record;
> 
> end P;
> with P;   use P;
> with P.C; use P.C;
> 
> procedure Test_P is
>   OT : T;
>   ON : NT;
> begin
>   Put (OT);
>   Put (ON);
> end;

Seems to me it should print:

    I is 0
    I is 1

But I don't see what the type of the expression T'Access has to do with
it, so perhaps I'm misunderstanding your question.

- Bob