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From: Mart van de Wege <mvdwege@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Can't access record attribute in derived type
Date: Wed, 08 Mar 2017 22:16:45 +0100
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Date: 2017-03-08T22:16:45+01:00
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Shark8 <onewingedshark@gmail.com> writes:

> On Wednesday, March 8, 2017 at 7:15:22 AM UTC-7, Mart van de Wege wrote:
>> Hi,
>> 
>> I have the following definitions
>> 
>> private
>> type Creature is new Base_Creature with record
>>       Attributes	 : Attribute_Array;
>>       Gender             : Possible_Gender := Unknown;
>>       Current_Hit_Points : Integer;
>>       Parents            : Group_Of_Creatures.List;
>>       --  An instance of Ada.Containers.Indefinite_Doubly_Linked_Lists
>> end record;
>> 
>> In child package #1 (Persons):
>> 
>> private
>>    type Person is new Creature with record
>>    [...]
>> and in child package #2 (Knights):
>>    type Knight is new Person with record
>>    [...]
>> 
>> I try to read the first element of the Parents attribute in knights.adb
>> like this:
>> 
>>    function Father (K : in Knight) return Knight is
>>    begin
>>       return K.Parents.First_Element;
>>    end Father;
>
> One Problem is that K.Parents.First_Element *isn't* of type Knight --
> It's of type Creature -- that is surely one problem.
>
Yeah, that is solved by some explicit conversions. I am not happy with
that, but this is still the first phase of my attempt at implementing
families, and getting my head around the intricacies of OO in Ada.

> Secondly, the construct 
>> type Creature is new Base_Creature with record
>>       Attributes	 : Attribute_Array;
>>       Gender             : Possible_Gender := Unknown;
>>       Current_Hit_Points : Integer;
>>       Parents            : Group_Of_Creatures.List;
>>       --  An instance of Ada.Containers.Indefinite_Doubly_Linked_Lists
>> end record;
> cannot exist, as you cannot instantiate the Container without completing the type which can't be complete without the instantiation of the Container (because of the Parents element).
>
> I would recommend something like this:
>
>     Type Base_Creature is abstract tagged record
>        Attributes         : Attribute_Array;
>        Gender             : Possible_Gender := Unknown;
>        Current_Hit_Points : Integer;
>     end record;
>
> --...
>     Package Creature_Grouping is new Ada.Containers.Indefinite_Doubly_Linked_Lists( Base_Creature'Class );
> --...
That's more or less what I did. Creature is derived from abstract
Base_Creature, and Group_Of_Creatures is defined as new
Ada.Containers.Indefinite_Doubly_Linked_list (Element_Type =>
Base_Creature'Class).

Mart

-- 
"We will need a longer wall when the revolution comes."
    --- AJS, quoting an uncertain source.