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From: "Matthew Heaney" <mheaney@on2.com>
Newsgroups: comp.lang.ada
Subject: Re: List container strawman
Date: Fri, 2 Nov 2001 19:24:01 -0500
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Date: 2001-11-02T19:24:01-05:00
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"Jeffrey Carter" <jeffrey.carter@boeing.com> wrote in message
news:3BE3235D.E292B890@boeing.com...
> Another place where some compiler writers use tricks is the interaction
> between a random number Generator value and the Random function. The
> generator holds the state data for the algorithm, which are supposed to
> be updated when a number is generated. You can implement Generator as a
> controlled type with a pointer to the state data, but again other tricks
> are possible from within the compiler.

Why would you need to implement type Generator as a Controlled type?
Generator state is allocated on the stack, so Finalization is not required.
Allocating state on the heap and using Controlled Finalization would be a
horribly inefficient way to implement that type.

No heap allocation or compiler magic is needed to implement Generator; just
use the Rosen Trick:

package P is
   type Generator is limited private;
   function Random (G : Generator) return Integer;
private
   type Handle_Type (G : access Generator) is null record;
   type Generator is limited record
      Handle : Handle_Type (G'Access);
      State : State_Type;  --whatever
   end record;
end P;

package body P is
   function Random (G : Generator) return Integer is
      State : State_Type renames G.Handle.G.all;
   begin
      -- modify State as necessary
      return X;
    end;
end P;

This is all perfectly legal.