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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Can't access record attribute in derived type
Date: Wed, 8 Mar 2017 14:08:05 -0600
Organization: JSA Research & Innovation
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References: <86mvcv4zyu.fsf@gaheris.avalon.lan>
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Date: 2017-03-08T14:08:05-06:00
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"Mart van de Wege" <mvdwege@gmail.com> wrote in message 
news:86mvcv4zyu.fsf@gaheris.avalon.lan...
> 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;
>
> And the compiler complains:
>
> "no selector "Parents" for type "Knight" defined"
>
> What am I missing here? The declarations are private, but shouldn't a
> tagged type inherit the entire record of its parent type? That means
> that it should have a Parents selector, shouldn't it?

I suspect we'll need to see a more complete example to definitively say what 
the problem is.

But if I had to guess, I'd suggest that you've run afoul of the "sibling 
inheritance problem". The rule for visibility for child components is that 
*all* of components of the *all* of the ancestors have to be visible at the 
point of declaration of the child type. If even one of the ancestors is not 
visible (as happens when deriving from a type defined in a sibling child 
package), then the components aren't visible, either.

Note that in such a case, if you have visiblity on some but not all of the 
ancestors, you can type convert the object to make them visible. In your 
example above:

     return Creature(K).Parents.First_Element;

will work if you have some intermediate ancestor without visible components. 
So, if the above works, you have a sibling inheritance problem, and your 
choices are either to use conversions like the above, or move the types so 
that all of the ancestors are visible.

                                     Randy.