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Newsgroups: comp.lang.ada
Date: Wed, 15 Jan 2020 15:19:55 -0800 (PST)
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Subject: Re: How to stop inheritance in a good way?
From: Jere <jhb.chat@gmail.com>
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Xref: reader01.eternal-september.org comp.lang.ada:57849
Date: 2020-01-15T15:19:55-08:00
List-Id: <comp.lang.ada>

On Wednesday, January 15, 2020 at 3:32:14 AM UTC-5, reinert wrote:
> Assume I define a tagged record (or interface) + derived types:
> 
>  type int1_t is interface;
>  function A(x : int1_t) return Integer is abstract;
> 
>  type object1_t is abstract new int1_t with private;
>  function A(x : object1_t) return Integer;
>  function B(x : object1_t) return Integer;
> 
>  type object2_1_t is new object1_t with private;
>  function C(x : object2_1_t) return Integer;
> 
>  type object2_2_t is new object1_t with private;
>  function D(x : object2_2_t) return Integer;
> 
> and I want object2_2_t *not* to inherit the function B (from type object1_t).
> I can make a dummy function B for object2_2_t (to override), but is it a more elegant/proper way? Or I here somehow break the concept of inheritance and enter a "dead end" ?
> 
> reinert

It really depends on what you want object2_2_t to "be".  If it truly has a
public "is a" relationship to object1_t, then you have to accept B since
that is part of the "contract" (bad wording I know) of object1_t.  However
if you are ok with object2_2_t being a descendant of int1_t (the origin of
function A), then you can do something like the following (I added
completions to your function so I could get it all to compile):

    package Test is
        type int1_t is interface;
        function A(x : int1_t) return Integer is abstract;
        
        type object1_t is abstract new int1_t with private;
        function A(x : object1_t) return Integer is (0);
        function B(x : object1_t) return Integer is (0);
        
        type object2_1_t is new object1_t with private;
        function C(x : object2_1_t) return Integer is (0);
        
        -- Note the change here
        type object2_2_t is new int1_t with private;
        function D(x : object2_2_t) return Integer is (0); 
    private
    
        type object1_t is abstract new int1_t with null record;
    
        type object2_1_t is new object1_t with null record;

        -- See how the inheritance is listed differently
        type object2_2_t is new object1_t with null record;
    
    end Test;