Tagged types (beginner's question) !

Tagged types (beginner's question) !

[Vincent DIEMUNSCH]

Hello,

I have a simple question concerning tagged types. Suppose that we have :

    type Rationnel is abstract new Ada.Finalization.Controlled with null
record;

    type Fraction is new Rationnel with record
          Num�rateur    : integer;
          D�nominateur : positive;
       end record;

    type Entier is new Fraction with null record;

First Question :
    Can I override the primitive operation "Adjust" of the controlled
type with the following :
        procedure Adjust (F : in out Fraction'Class);
    instead of :
        procedure Adjust( F : in out Fraction);

Second question :
    How can Adjust (F : in out Fraction'Class) return an object of type
Entier instead of Fraction ?
    I tried something like
        F := E; --(with E : Entier)
    and many other more complicated things but I always have the same
error message from GNAT :
        "Dynamiclly tagged expression required".

Looking in the ARM and the user guide from GNAT, I could not get what
this message meant !
Any help would be greatly appreciated. Thanks in advance.

Vincent DIEMUNSCH

Re: Tagged types (beginner's question) !

[Ted Dennison]

Vincent DIEMUNSCH <"vincent.diemunsch"@edf.fr@NO.SPAM.PLEASE> wrote in message news:<akkq54$kcq$1@clnews.edf.fr>...

>     and many other more complicated things but I always have the same
> error message from GNAT :
>         "Dynamiclly tagged expression required".

This message generally means that the expression in question needs to
have to have a 'Class in it somewhere.

Re: Tagged types (beginner's question) !

[Pat Rogers]

> First Question :
>     Can I override the primitive operation "Adjust" of the
controlled
> type with the following :
>         procedure Adjust (F : in out Fraction'Class);
>     instead of :
>         procedure Adjust( F : in out Fraction);

No.  That would be a different procedure and not an overriding.

> Second question :
>     How can Adjust (F : in out Fraction'Class) return an object of
type
> Entier instead of Fraction ?
>     I tried something like
>         F := E; --(with E : Entier)
>     and many other more complicated things but I always have the
same
> error message from GNAT :
>         "Dynamiclly tagged expression required".

Better deal with the problem above first.

Re: Tagged types (beginner's question) !

[Stephen Leake]

Vincent DIEMUNSCH <"vincent.diemunsch"@edf.fr@NO.SPAM.PLEASE> writes:

> Hello,
> 
> I have a simple question concerning tagged types. Suppose that we have :
> 
>     type Rationnel is abstract new Ada.Finalization.Controlled with null
> record;
> 
>     type Fraction is new Rationnel with record
>           Num�rateur    : integer;
>           D�nominateur : positive;
>        end record;
> 
>     type Entier is new Fraction with null record;
> 
> First Question :
>     Can I override the primitive operation "Adjust" of the controlled
> type with the following :
>         procedure Adjust (F : in out Fraction'Class);
>     instead of :
>         procedure Adjust( F : in out Fraction);

No. This would be "overloading", not "overriding". Calls to Adjust on
an object of type Rationnel or Fraction made by the run-time system
will not dispatch to this procedure.

Adjust has a very specific meaning for Controlled types. Why do you
want to do this instead?

> Second question : How can Adjust (F : in out Fraction'Class) return
> an object of type Entier instead of Fraction ? 

Note that Adjust is a procedure, so it does not "return" anything. Not
clear what you really mean here.

Again, why do you want to do this? 

If you are trying to emulate C++ constructors (just a wild guess), try
defining:

function New_Entier (...) return Entier;

-- 
-- Stephe