Re: Problem with dynamic dispatching (Polymorphism)

[Bo Sanden <bsanden@acm.org>]

Mattias wrote:

> Hi!
>
> I�m developing a simple linked list to hold different data types..
> I have to one superclass with two derived classes..
>
> I have declared a 'Class type which I use to access all of my structures,
>
> I have no trouble adding data to the list. The error occurs when I want to
> extract the data and Print it to the screen..
>
> look at the following code example:
>
> program.ads:
> type Tecken is tagged null record;
> type Siffra is new Tecken with
>   Record
>       Element : Integer;
>   end record;
> procedure Print(S : Siffra);
>
> type Bokstav is new Tecken with
>   record
>          Element : Character;
>    end record;
>   procedure Print(B : Bokstav);
>
> type pTecken is access all Tecken'Class;
>
> program.adb
> -- I have to simple procedures which Print the values of Class bokstav,
> '' notice .. pekare is of type pTecken ..
>
> procedure Print(B : Bokstav) is
>    begin
>       Put(B.Element);
>    end Print;
>
>    procedure Print(S : Siffra)  is
>    package Iio is new Integer_Io(Integer);
>       use Iio;
>    begin
>       put(S.Element);
>    end Print;
>
> -- Then I have this procedure to extract the elements from the list. Pekare
> is
> Procedure Display is
>    begin
>       while Lista /= null loop
>
>          Print(Lista.Pekare);
>          Lista := Lista.Next;
>       end loop;
>    end Display;
>
> Really need help on this one
>
> thx
> Mattias

You need a procedure Print (T : Tecken). It would make sense to make it
and the
type Tecken abstract.

Bo Sanden
Colorado Tech
Colorado Springs