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From: Philippe Tarroux <philippe.tarroux@limsi.fr>
Newsgroups: comp.lang.ada
Subject: Re: tagged type as generic parameter
Date: Fri, 04 Jan 2008 14:08:52 +0100
Organization: University Paris-Sud, France.
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Date: 2008-01-04T14:08:52+01:00
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Dmitry A. Kazakov a �crit :
>
> The function A_Data returns Data. It is a primitive operation of Data and
> thus covariant in the result. So when you derive anything from Data, you
> have to override it in order to provide a correct implementation that would
> return the derived type rather than the base.
>   
Ok. But does it mean that only the constructor is a primitive operation 
of the data type, not the print procedure? In this case why Print 
doesn't need to be redefined?
> There are many ways to resolve the issue, the choice depends on other
> factors:
>
> 1. Class-wide A_Data:
>
>    function A_Data (V : Integer) return Data'Class;
>
> This will be same (contravariant) for all descendants of A_Data and thus
> need not to overridden.
>
> 2. Data-specific List package:
>
> generic
>    type Data is abstract new Data_Handler.Data with private;
> package Data_List is
>    ...
>    type Item is new Data with record
>       Next : Item_Ptr;
>    end record;
>    function A_Data (V : Integer) return Item;
>
> We know here that Data is a descendant of Data_Handler.Data, so we can
> provide an override for A_Data.
>
> 3. Aggregation instead of inheritance:
>
>    type Item is record
>       Next  : Item_Ptr;
>       Value : Data;
>    end record;
>
> No inheritance, no problem.
>
> 4. Non-generic implementation of lists. You can define a list interface and
> derive Data_Handler.Data from a list item. (In Ada 2005 it is easier to do
> than it was in Ada 95) That would reverse the inheritance order and thus
> solve the problem with A_Data.
>
> 5. Storage-pool based implementation of lists. You can design a storage
> pool that would maintain lists of allocated there items. In this case,
> unlikely to all other variants, the list item is not a fully separate type
> but just a pool-specific access to Data type. Because there again is no
> inheritance involved, the problem is solved.
>   
Thanks for all these suggestions that bring to me interesting insights. 
Especially i know that #4 solves the problem (it is the alternative 
proposed in Barnes for the construction of list items). I won't be aware 
of #5. I will try it.

Regards

Philippe Tarroux