Re: Extending a type and Finalization

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

Pascal Malaise wrote on comp.lang.ada:
> I declare P.T as "new Ada.Finalization.Controlled with record ..." and
> I don't define Finalize for it.
> Then in Pp I declare Tt as "new P.T with record ..." and I declare an
> overriding procedure Finalize (V : in out Tt) and its body in Pp body.
> It does not compile, with error: 'subprogram "Finalize" is not
> overriding'
>
> Same if I define in P a Finalize for T.
>
> Any idea?
>
> ------------------------------
> with Ada.Finalization;
> package P is
>   type T is tagged private;
> private
>   type T is new Ada.Finalization.Controlled with record
>     I : Integer;
>   end record;
> end P;
>
> with P;
> package Pp is
>   type Tt is tagged private;
> private
>   type Tt is new P.T with record
>     Ii : Integer;
>   end record;
>   overriding procedure Finalize (V : in out Tt);
> end Pp;
>
> package body Pp is
>   overriding procedure Finalize (V : in out Tt) is
>   begin
>     null;
>   end Finalize;
> end Pp;

That's because P.T is a *private* record extension of
Ada.Finalization.Controlled. Package PP cannot see that P.T derives
from it.

Solution 1: rename PP to P.P, a child package of P so it can see the
private part of P.

Solution 2: make P.T a public record extension:

type T is new Ada.Finalization.Controlled with private;

But I recommend that you *not* derive P.T from any other type; use a
mixin instead. Making a type controlled is not a good enough reason
for making it tagged.

--
Ludovic Brenta.