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From: "G.B." <bauhaus@futureapps.invalid>
Newsgroups: comp.lang.ada
Subject: Re: A bad counterintuitive behaviour of Ada about OO
Date: Mon, 11 Aug 2014 13:35:22 +0200
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Xref: number.nntp.dca.giganews.com comp.lang.ada:188362
Date: 2014-08-11T13:35:22+02:00
List-Id: <comp.lang.ada>

On 11.08.14 09:58, Maciej Sobczak wrote:
> procedure Use_Specific_Factory (F : in Specific_Factory) is
>
>     SP : Specific_Product := F.Make_Product;
>
> begin
>     -- ...
> end;
>
> And there is no way to make*both*  work in Ada. You can get one or the other but not both.


Is this a correct paraphrase?

"To make this practically work in Ada, one has to write

procedure Use_Specific_Factory (F : in Specific_Factory) is

     SP : Specific_Product := Specific_Product (F.Make_Product);

begin
     -- ...
end;

"and watch for run-time dispatching."

Where

   function Make_Product (Shop : access Factory)
     return Product'Class is abstract;

can be overridden as a prim op of Factory.
The variance becomes a property of its invocation context.