Re: unexpected behaviour of finalize

[Adam Beneschan <adam@irvine.com>]

On Apr 4, 2:33 am, stefan-lu...@see.the.signature wrote:
> Hi,
>
> I have written a little program to demonstrate finalisation. The idea is
> to locally declare a singelton object of a class derived from
> Ada.Finalization.Limited_Controlled, and to override the Finalize method.
> When the sigelton object leaves scope, Finalize than cleans up whatever
> needs to be cleaned up. At a first look this appeared to work nicely. But
> when done in a recursive subprogram, Finalize rather unexpectedly seems to
> always read the local parameters of the innermost scope.
>
> Here is a test case:
>
> ---begin fin_test_case.adb
>
> with Ada.Text_IO, Ada.Finalization;
>
> procedure Fin_Test_Case is
>    type Fin is new Ada.Finalization.Limited_Controlled with null record;
>
>    procedure Do_Something (I: Natural) is
>
>       type My_Fin is new Fin with null record;
>
>       overriding procedure Finalize(Self: in out My_Fin) is
>         S: String := Natural'Image(I);
>       begin
>          Ada.Text_IO.Put_Line("  Do_Something Finalization " & S);
>       end Finalize;
>
>       Singleton: My_Fin;
>
>    begin -- Do_Something
>       Ada.Text_IO.Put_Line("  I = " & Natural'Image(I));
>       if I>0 then
>          Do_Something(I-1);
>          Do_Something(I-1);
>       end if;
>    end Do_Something;
>
> begin
>    Ada.Text_IO.Put_Line("Start Do_Something (2)");
>    Do_Something(2);
>    Ada.Text_IO.Put_Line("Stop Do_Something (2)");
> end Fin_Test_Case;
>
> ---end fin_test_case.adb
>
> This is what I got:
>
> ---begin output
>
> Start Do_Something (2)
>   I =  2
>   I =  1
>   I =  0
>   Do_Something Finalization  0
>   I =  0
>   Do_Something Finalization  0
>   Do_Something Finalization  0
>   I =  1
>   I =  0
>   Do_Something Finalization  0
>   I =  0
>   Do_Something Finalization  0
>   Do_Something Finalization  0
>   Do_Something Finalization  0
> Stop Do_Something (2)
>
> ---end output
>
> Somehow, the local parameter I is always 0 when finalize is called. What I
> expected was the following:
>
> ---begin expected output
>
> Start Do_Something (2)
>   I =  2
>   I =  1
>   I =  0
>   Do_Something Finalization  0
>   I =  0
>   Do_Something Finalization  0
>   Do_Something Finalization  1
>   I =  1
>   I =  0
>   Do_Something Finalization  0
>   I =  0
>   Do_Something Finalization  0
>   Do_Something Finalization  1
>   Do_Something Finalization  2
> Stop Do_Something (2)
>
> ---end expected output
>
> Now, is my expectation wrong? Or is this a compiler bug? I am using
> GNAT GPL 2007 (20070405-41).

This looks like a pretty clear compiler bug, especially since by the
time Finalize is called by the outermost Do_Something, it's using, for
the value of I, the parameter from a Do_Something call that is no
longer active.  It shouldn't be hard to modify this slightly, using
(say) a string or record as a parameter to Do_Something, to create a
test where utter garbage is displayed; then it would be clearer that
this is a compiler bug.

                                   -- Adam