From: Richard Riehle <richard@adaworks.com>
Subject: Re: Problems with tagged records and inheritance
Date: Mon, 14 Jul 2003 18:34:06 -0700
Date: 2003-07-15T01:30:40+00:00 [thread overview]
Message-ID: <3F135A0E.A46C33D1@adaworks.com> (raw)
In-Reply-To: bekkpr$12eg$1@ulysses.noc.ntua.gr
I am including your posting at the end of this reply.
First, you have a visibility problem because type Base
is private and its contents are not visible. Also, you have
a design problem. Consider,
package The_Base is
type Base is tagged private;
procedure Put(B : Base);
private
type Base is tagged record
AAA : Integer := 42;
end record;
end Base;
Now you have a design with a primitive operation for Base. You
would need more of these, of course.
package Derived_From_Base is
type Derived is new The_Base.Base with private;
procedure Put(D : Derived);
private
type Derived is new The_Base.Base with record
BBB : Integer := 451;
end record;
end Derived_From_Base;
Now you have a derived type along with a primitive Put
method for it. You implement the Put in terms of the existing
Put in The_Base.
with Ada.Integer_Text_IO;
package body Derived_From_Base is
procedure Put(D : Derived) is
begin
The_Base.Put(The_Base.Base(D));
Ada.Integer_Text_IO.Put(BBB);
end Put;
end Derived_From_Base;
One of the key ideas behind object-oriented programming
is that of reuse. In the Put method for Derived you are
first reusing the implementation of the Put for its parent and
then coding the part for the immediately visible type.
Hope this helps. I did not compile this so there might be some typos,
but the fundamental idea is there.
Richard Riehle
Papastefanos Serafeim wrote:
> I have using a base type like this:
> ...
> type Base is tagged private;
> ...
> type Base is tagged record
> AAA: Integer:=1;
> end record;
>
> and a child type like this
>
> type Child is new Base with private;
> ....
> type Child is new Base with record
> BBB: Integer:=5;
> end record;
>
> The problem is that the following is not working:
>
> procedure Test(Ch: in Child) is
> begin
> Put(Ch.AAA); --<- This line has an error, it says no selector AAA for
> type Child
> Put(Ch.BBB);
> end Test;
> The procedure Test is declared in the same package
> as the type Child and defined at the package's Body.
>
> The error is becouse AAA is not part of Child.
> Why is that ? I thought that Child would contain
> AAA and BBB, and not only BBB...
>
> --
> Papastefanos Serafeim
> serafeim@otenet.gr
prev parent reply other threads:[~2003-07-15 1:34 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-07-10 20:56 Problems with tagged records and inheritance Papastefanos Serafeim
2003-07-10 22:15 ` Re; " tmoran
2003-07-10 22:24 ` Robert I. Eachus
2003-07-10 22:29 ` Ludovic Brenta
2003-07-11 11:56 ` Marin David Condic
2003-07-11 11:49 ` Papastefanos Serafeim
2003-07-15 1:34 ` Richard Riehle [this message]
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