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From: Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>
Newsgroups: comp.lang.ada
Subject: Re: Smarter Generics
Date: Sun, 15 Aug 2004 01:14:03 +0000 (UTC)
Organization: GMUGHDU
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Date: 2004-08-15T01:14:03+00:00
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Frank J. Lhota <NOSPAM.lhota.adarose@verizon.net> wrote:
: What would be great is if there were some way to write Limited_Stacks so
: that the implementation strategy could be chosen based on the generic
: parameters, so that we could do something like this:

The following can be compiled, don't know if it is sufficiently
general, though.


generic
   type Stack_Element is private;
   Max_Length : in Positive;
package Dyn_Stacks is

   type Stack_Type is limited private;
   Stack_Overflow, Stack_Underflow : exception;

   procedure Push ( Item : in Stack_Element; Onto : in out Stack_Type );
   procedure Pop ( Item : out Stack_Element; Onto : in out Stack_Type );

private

   Some_Threshold: constant := 1_000;

   subtype Stack_Array_Index is Natural range 0 .. Some_Threshold;
   type Stack_Array is array(Stack_Array_Index) of Stack_Element;

   type Implementation is tagged null record;
   --   not yet decided what strategy to use, maybe abstract?

   type Array_Implementation is new Implementation with record
      items: Stack_Array;
   end record;

   type List_Implementation is new Implementation with record
      null;  -- replace with a list type
   end record;

   type Strategy(which: Boolean) is limited record
      case which is
         when False =>
            a_items: Array_Implementation;
         when true =>
            l_item: List_Implementation;
      end case;
   end record;

   type Stack_Type is new Strategy(Max_Length > Some_Threshold);
   --   there we are ;-)

end Dyn_Stacks;


-- Georg