Re: Extending a type and Finalization

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Thu, 4 Jun 2009 14:18:41 -0700 (PDT), Adam Beneschan wrote:

> On Jun 4, 1:06�pm, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>> On Thu, 4 Jun 2009 12:33:04 -0700 (PDT), Adam Beneschan wrote:
>>> In addition to making an individual component a Limited_Controlled
>>> type, you can use trickery using access discriminants to allow the
>>> Initialize and Finalize routines to access the entire record.
>>> Assuming P.T is limited, you can define something like:
>>
>>> � �type Control_Type (Ref : access P.T'Class) is new
>>> � � � �Ada.Finalization.Limited_Controlled with null record;
>>
>>> � �overriding procedure Initialize (Obj : in out Control_Type);
>>> � �overriding procedure Finalize � (Obj : in out Control_Type);
>>
>>> And then when defining TT:
>>
>>> � �type TT is new P.T with record
>>> � � � Control : Control_Type (TT'Access);
>>> � � � ...
>>> � �end record;
>>
>>> Then when you declare an object of type TT, Initialize will be called
>>> on the Control component, and the access discriminant Ref will give
>>> you access to the entire record inside the Initialize and Finalize
>>> routines.
>>
>>> (This idea isn't mine. �It might be Matthew Heaney's, but I'm not
>>> sure. �The Control_Type declaration was adapted from a "shapes"
>>> demonstration program, but I'm not sure where it came from.)
>>
>> I am not sure if this pattern is safe. What gives a guaranty that other
>> components of TT are initialized before the Initialize gets called on
>> Control and finalized after the Finalize gets called?
> 
> RM 7.6(12)

Then it should rather be:

 � �type TT is new P.T with record
 � � � ...
 � � � Control : Control_Type (TT'Access);  -- The last component
 � �end record;

BTW, it seems that this pattern has a more consistent behavior upon
extension than Ada.Finalization itself:

   type TTT;
 � type TTT_Hook (Ref : not null access TTT'Class) is new
 � � �Ada.Finalization.Limited_Controlled with null record;
 � overriding procedure Initialize (Obj : in out TTT_Hook);
 � overriding procedure Finalize � (Obj : in out TTT_Hook);

 � type TTT is new TT with record
 � � �...
 � � �Hook : TTT_Hook (TTT'Access);
 � end record;

Unlikely to Ada.Finalize from the Initialize of TTT_Hook we do not call to
the Initialize of Control_Type, because it has been called.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de