Re: Seemingly wierd conversion for in out parameter

[aklee@spam.this.interport.net (Albert K. Lee)]

which version of GNAT were you using?


On Fri, 20 Feb 1998 06:41:02 GMT, Corey Minyard <minyard@acm.org> wrote:
>
>I've been playing with some techniques with derived types and I ran
>across a rather unusual things.  The following program:
>
>with Text_IO; use Text_IO;
>with Ada.Tags; use Ada.Tags;
>
>------------------------------------------------------------------------
>procedure Tester is
>
>   type Base is abstract tagged null record;
>   type Base_Class is access all Base'Class;
>
>   type Der1 is new Base with null record;
>   type Der1_Class is access all Der1'Class;
>
>   type Der2 is new Base with null record;
>   type Der2_Class is access all Der2'Class;
>
>   procedure Process (Var : in out Base_Class) is
>   begin
>      Var := new Der2;
>   end Process;
>
>   V1 : Der1_Class := new Der1;
>-- This will be a compile error.
>--   U1 : Der1_Class := new Der2;
>
>begin
>
>   Put_Line("Base tag = " & Expanded_Name(Base'Tag));
>   Put_Line("Der1 tag = " & Expanded_Name(Der1'Tag));
>   Put_Line("Der2 tag = " & Expanded_Name(Der2'Tag));
>
>   Put_Line("V1 tag = " &  Expanded_Name(V1'Tag));
>
>   Process(Base_Class(V1));
>
>   Put_Line("V1 tag = " &  Expanded_Name(V1'Tag));
>
>end Tester;
>------------------------------------------------------------------------
>
>will compile and produce the following output:
>
>------------------------------------------------------------------------
>Base tag = TESTER.BASE
>Der1 tag = TESTER.DER1
>Der2 tag = TESTER.DER2
>V1 tag = TESTER.DER1
>V1 tag = TESTER.DER2
>------------------------------------------------------------------------
>
>As you can see, the variable V1 should not be able to reference a
>tagged type of Der2, but this program does that without any errors
>under GNAT (the only compiler I have right now).  I would think that
>the compiler wouldn't allow a class conversion passed to an in out
>parameter, but it seems to.
>
>I'm certainly no language lawyer, but I would image that doing a cast
>would make something not a "variable" any more (RM 6.4.1(5)), thus
>this would not be legal.  Is GNAT wrong, or is this legal, or is this
>a grey area?
>
>Thanks,
>
>-- 
>Corey Minyard               Internet:  minyard@acm.org
>  Work: minyard@nortel.ca       UUCP:  minyard@wf-rch.cirr.com