Re: Strange error message

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Charly <carl.weierstrass@googlemail.com> writes:

> Hi,
>
> when I defined a class hierarchy I encountered a problem, that I could reduce 
> to the following few lines:
>
> This version compiles without problems:
> -----------------
> package Test is
>
>    type Base is tagged private;
>    function Create return Base;
>
>    type High is new Base with private;
>
> private
>
>    type Base is tagged null record;
>
>    type High is new Base with null record;
>
> end Test;
> -----------------
>
> but this one
>
> -----------------
> package Test is
>
>    type Base is tagged private;
>    function Create return Base;
>
>    type High is new Base with private;
>
> private
>
>    type Base is tagged null record;
>
>    type High is new Base with record     --
>       H_Val : Natural := 0;              -- these lines are changed
>    end record;                           --
>
> end Test;
> -----------------
>
> does not compile an I get an error:
>
>    type must be declared abstract or "Create" overridden
>
> Why do I have to overide the function Create for type High.
> I don't see the necessity.

Because whatever Create does, it doesn't set any value for H_Val,
because H_Val doesn't exist in Base.  So it makes no sense to inherit
it as is.  [*] But see below.

In your first version, High adds no new components.  Ada 95 used to
require overriding in that case, too, but the overriding is always:

    function Create return High is
    begin
        return (Base with null record);
    end Create;

What else would it be?  So the language was changed to provide that
overriding by default in the "null record" case.

I don't know why Dmitry and Randy want us to write that out explicitly;
it seems like an obviously useful and safe default.

[*] Your H_Val has a default of 0, so it might make sense to define
the language to provide an automatic default overriding:

    function Create return High is
    begin
        return (Base with H_Val => 0);
    end Create;

That's not how the language is now, and I suspect Dmitry and Randy
would be appalled at the idea.  I don't really understand their
point of view -- I mean, I love consistency and all, but "A FOOLISH
consistency is the hobgloblin... etc".

By the way, Tucker wanted the current rule for Ada 95.  I argued him
out of it.  That is, in 1993 or so, I agreed with Dmitry's and Randy's
current position.  I've since changed my mind.

- Bob