Re: Discriminants of tagged types

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

Maciej Sobczak wrote on comp.lang.ada:
> I'm not sure if I have already complatined about it, but as I have hit
> that problem again, here it goes:
>
> GNAT says that discriminants of tagged types cannot have default
> values.
> So this is OK:
>
>    type T (A : Integer := 0) is null record;
>
> but this is an error:
>
>    type T (A : Integer := 0) is tagged null record;
>
> I find it disturbing and artificial, but perhaps I don't see the big
> picture. What is the rationale for this limitation?

If one discriminant has a default value, then all discriminants must
have default values (ARM 3.7(9.1/2)).  The reason is explained in the
Annotated Ada Reference Manual[1].

http://www.adaic.com/standards/05aarm/html/AA-3-7.html

Tagged types have, by definition, a hidden discriminant which is the
tag.  The tag has no default value and cannot have one, for that would
allow changing the type of an object at run time (through assignment
of an aggregate).  Therefore, *additional* discriminants cannot have
default values either.

--
Ludovic Brenta.