Re: statically compatible access discriminants in derivation

["Nick Roberts" <Nick.Roberts@dial.pipex.com>]

I think if you look closely at the wording of 4.9.1(4), you will see that
in the derivation definition

   type object( board : access device_instance.object'class ) is new
unit_if.object( board ) with null record;

the subtype of the board in the parent type (unit_if.object) is 'statically
compatible' with the subtype of the new discriminant, because (a) it is an
access type, and (b) the parent type's discriminant's subtype is
unconstrained.  All class-wide types are unconstrained.

So I believe you can do what you put in the example.

I cannot claim to be authoritative here: others may know better than me
(please, don't laugh, I have my pride you know :-).

-- 

Nick Roberts
Croydon, UK

Proprietor, ThoughtWing Software; Independent Software Development
Consultant
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Pat Rogers <progers@acm.org> wrote in article
<69gucb$l9k$1@uuneo.neosoft.com>...
> Given a tagged type with an access discriminant designating a
> classwide type, and a child type derived from this parent type, is it
> possible to declare an access discriminant in the child type that is
> statically compatible with the parent's access discriminant, while having
> the child's access discriminant designate a classwide type further down
in
> the parent's designated derivation class? 
> [etc...]