Re: virtual destructors

[mheaney@on2.com (Matthew Heaney)]

"kat-Zygfryd" <6667@wp.pl> wrote in message news:<b83l69$7bk$1@news.onet.pl>...
>
> I want to have destructors working on access' to class wide types,
> so that when passed a variable, an actual type destructor was
> called, not a one overloaded for the specific type.
> 
> example:
> 
> type Base is tagged record ... end record;
> type PBase is access Base;
> type CBase is access Base'Class;
> 
> type Derived is new Base with record ... end record;
> type PDerived is access Derived;
> type CDerived is access Derived'Class;
> 
> procedure Free is new Ada.Unchecked_Deallocation(Base,PBase);
> procedure Free is new Ada.Unchecked_Deallocation(Derived,PDerived);
> 
> procedure Dispose(self: in out CBase) is
> begin
>    --?
>    self := null;
> end Dispose;
> 
> procedure Dispose(self: in out CDerived) is
> begin
>     -- ?
>    self := null;
> end Dispose;
> 
> declare
>     B: CBase := new Base;
>     D: CBase := new Derived;
> begin
>     Dispose(B);
>     Dispose(D);
> end;
> 
> I want both Disposes to work as they should, what should I fill them with?

Give the class a primitive operation that accepts an access parameter:

procedure Do_Dispose (B : access Base) is ...;
procedure Do_Dispose (D : access Derived) is ...;

procedure Dispose (B : in out CBase) is
begin
   if B /= null then
      Do_Dispose (B);  --dispatches
      B := null;
   end if;
end Dispose;