Re: Seemingly wierd conversion for in out parameter

[Stephen Leake <Stephen.Leake@gsfc.nasa.gov>]

Bernd Holzmueller wrote:
> If no Ada rule is violated in the program presented by Corey Minyard,
> they should perhaps be reconsidered as the following, slightly extended
> program shows:
> 
> 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 record X: Integer; end record;
>    type Der1_Class is access all Der1'Class;
> 
>    type Der2 is new Base with record Y: Float; end record;
>    type Der2_Class is access all Der2'Class;
> 
>    procedure Process (Var : in out Base_Class) is
>    begin
>       Var := new Der2'(Y => 3.45);
>    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));
> 
>    V1.X := 6;
> 
>    Process(Base_Class(V1));
> 
>    Put_Line("V1 tag = " &  Expanded_Name(V1'Tag));
> 
>    Text_Io.Put_Line("V1.X = " & Integer'IMAGE(V1.X));
> 
> end Tester;
> 
> This program outputs
> 
> Base tag = TESTER.BASE
> Der1 tag = TESTER.DER1
> Der2 tag = TESTER.DER2
> V1 tag = TESTER.DER1
> V1 tag = TESTER.DER2
> V1.X =  1079823565
> 
> and the last line of the output is a typical output of an uninitialized
> variable
> which is rather strange for a program that has all objects properly
> initialized!

Running this with ObjectAda 7.1 on Windows 95, we get Constraint_Error
at the call " Process(Base_Class(V1)); ", which is expected since we are
attempting to store a pointer to Der2 in a variable of type access Der1.
GNAT 3.10p on Windows 95 does not raise Constraint_Error here, so I
suspect it has a bug. What compiler are you using? please submit a bug
report!
 
-- 
- Stephe