Re: Self pointer in limited record

[Maciej Sobczak <see.my.homepage@gmail.com>]

On 5 Lip, 10:22, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:

> > What is the use for Self?
>
> The pattern (also called Rosen trick) is used:

That's cheating - I was told there are no tricks in Ada! ;-)

> 1. to having mutable arguments of functions:
>
>    function Search (X : T; K : Key) return Value is
>       Object : T renames X.Self.all;
>    begin
>       ... -- Modify the search cache associated with X via
>           -- mutable Object view

OK, makes sense. I have seen it used with rendezvous and I guess the
motivation was similar.

> 2. to re-dispatch from primitive operations:
>
>    type T_Access is access all T'Class;  -- Note 'Class
>    type T is  new Ada.Finalization.Limited_Controlled with record
>       Self : T_Access := T'Unchecked_Access;
>
>    procedure Bar (X : T); -- Primitive
>    procedure Foo (X : T); -- Primitive
>
>    procedure Foo (X : T) is
>       Object : T'Class renames X.Self.all;
>    begin
>       Bar (X); -- This does not dispatch!
>       Bar (Object.Self.all); -- This dispatches

I guess Bar (Object) should be enough?

Yes, it makes sense as well.

> Both defeat the type system in some sense and potentially indicate a
> design problem.

I have seen it applied *regularly* in some Ada project, so it looked
like "default" idiom for OO programming.

> > And why is it Unchecked?
>
> Because of accessibility rules.

You mean - the access type is defined in the same scope as the record
and there is a potential danger that the access component in the
record can point to some other record that may disappear leading to
dangling pointer?

Thank you for explanation.

--
Maciej Sobczak
http://www.msobczak.com/