Re: Declaring constants for a abstract tagged type and concrete sub classes

[Simon Wright <simon@pushface.org>]

Serge Robyns <serge.robyns@gmail.com> writes:

> On Tuesday, 7 July 2015 19:21:46 UTC+2, G.B.  wrote:
>> On 07.07.15 18:27, Serge Robyns wrote:
>> 
>> > Now I'm facing my other issue, how to "merge" the root and child
>> > part in a new No_Element ...  This is about valid Ada syntax.
>> 
>> > No_Element : constant T_Concrete := ( T_Root.No_Element, 789);
>> 
>> If the derived type stays within the package hierarchy,
>> i.e. in a child package (that's Abstract_Root.No_Element
>> in the extension aggregate):
>> 
>> ...
>> private
>>    No_Element : constant Non_Abstract :=
>>       Non_Abstract'(No_Element with
>>                     Abc => -42);
>> 
>
> This one did the trick!  I'm marveled by the syntax and the meaning of
> the standalone tick (').

In this case you really don't need the Non_Abstract'; as evidenced by
my post in this thread yesterday, which I wouldn't have posted had it
not compiled.

   with Abstract_Root;
   package Concrete is
      type T_Concrete is new Abstract_Root.T_Root with private;
      No_Element : constant T_Concrete;
   private
      type T_Concrete is new Abstract_Root.T_Root with record
         ABC : Integer;
      end record;
      No_Element : constant T_Concrete :=
        (Abstract_Root.T_Root with ABC => -42);
   end Concrete;