From: Robert A Duff <bobduff@world.std.com>
Subject: Re: meaning of "current instance"
Date: 1999/11/11
Date: 1999-11-11T00:00:00+00:00 [thread overview]
Message-ID: <wcc7ljo3ld8.fsf@world.std.com> (raw)
In-Reply-To: 382b0da1_2@news1.prserv.net
"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
next prev parent reply other threads:[~1999-11-11 0:00 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
1999-11-11 0:00 meaning of "current instance" Matthew Heaney
1999-11-11 0:00 ` Robert A Duff [this message]
1999-11-11 0:00 ` Matthew Heaney
1999-11-11 0:00 ` Tucker Taft
1999-11-15 0:00 ` Tucker Taft
1999-11-15 0:00 ` tmoran
1999-11-15 0:00 ` Tucker Taft
1999-11-11 0:00 ` tmoran
1999-11-11 0:00 ` Matthew Heaney
1999-11-12 0:00 ` tmoran
1999-11-11 0:00 ` Matthew Heaney
1999-11-12 0:00 ` tmoran
1999-11-12 0:00 ` Matthew Heaney
1999-11-12 0:00 ` tmoran
1999-11-12 0:00 ` Matthew Heaney
1999-11-13 0:00 ` Robert A Duff
1999-11-13 0:00 ` Nick Roberts
1999-11-13 0:00 ` Robert A Duff
1999-11-14 0:00 ` tmoran
1999-11-13 0:00 ` Matthew Heaney
1999-11-15 0:00 ` John English
1999-11-15 0:00 ` Matthew Heaney
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