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Newsgroups: comp.lang.ada
Date: Wed, 8 Mar 2017 13:03:08 -0800 (PST)
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Subject: Re: Can't access record attribute in derived type
From: Shark8 <onewingedshark@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:33497
Date: 2017-03-08T13:03:08-08:00
List-Id: <comp.lang.ada>

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.

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 );
--...

    Type Creature is new Base_Creature with record
       Parents : Creature_Grouping.List;
    end record;

--...
    Type Person is new Creature with --[...]
    Function Father( Input : Person ) return Creature'Class is
      ( Input.Parents.First_Element );
--...
    Type Knight is new Person with --[...]