Re: Self-referential types

[Robert A Duff <bobduff@world.std.com>]

Ted Dennison <dennison@telepath.com> writes:

> I'm trying to create my first self referential type. But there's
> obviously something about this technique I'm missing, because all
> attempts produce compiler errors in multiple compilers.
> 
> Below is a code sample that is taken directly from Cohen's AAASL,
> section 11.8.3:
> 
> procedure Test is
> 
>    type Cell_Type;
> 
>    type List_Type is access all Cell_Type;
> 
>    type Cell_Type is
>      limited record
>         Contents_Part      : String (1..20);
>         Next_Cell_Part     : List_Type := Cell_Type'Access;
>         Previous_Cell_Part : List_Type := Cell_Type'Access;
>      end record;
> begin
>    null;
> end Test;
> 
> Compiling this with Gnat gives me:
> Test.adb:10:43: non-local pointer cannot point to local object
> Test.adb:11:43: non-local pointer cannot point to local object
> 
> I don't see an accesability level problem here, ...

    X: List_Type;

    procedure Nested is
        Local: Cell_Type;
    begin
        X := Local.Next_Cell_Part;
    end Nested;

    Nested;
    
    ... X.all ... -- Here, X is a dangling pointer.

That's the accessibility level problem.  You need to use
'Unchecked_Access, and you need to be careful not to do the above.
The language design tries to prevent dangling pointers
unless you do something "unchecked".

>...and it looks like it
> thinks Cell_Type is an object, not a type.

It *is* an object -- the "current instance" of the type (sort of like
"self" or "this" in some other languages).

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.