Re: Upcasting

[Lutz Donnerhacke <lutz@iks-jena.de>]

* Matthew Baulch wrote:
> My question is: How would you do this 'upcasting' if varying
> types existed? Obviously the compiler could tell where each
> value must go based on their type if only one of their type
> was needed in the transition. How does this type of thing work?
> (or does the A95 compiler not allow it)

procedure t is
   type Base is tagged null record;
   type Derivated is new Base with null record;

   d  : Derivated;
   dc : Base'Class := d;
   b  : Base       := Base(d);
   bc : Base'Class := b;
begin
-- d := Base (b);       -- will not compile
   d := Derivated (dc);
   d := Derivated (bc); -- will raised CONSTRAINT_ERROR on run time
end t;