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From: malaise@magic.fr
Newsgroups: comp.lang.ada
Subject: Legal usage of downward closures
Date: Thu, 16 Aug 2007 09:11:18 -0700
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Date: 2007-08-16T09:11:18-07:00
List-Id: <comp.lang.ada>

Hi,

I have problems understanding the RAT05 section on downward closures.

I thought that anonymous access to a sub-program, as parameter of a
sub-program, would prevent local copies and further usage out of
scope.
Surprisingly, the following example compiles an runs (on GNAT GPL
2007).

package Pack is
  procedure Store (Arg : access procedure);
  procedure Call;
end Pack;

package body Pack is
  Acc1 : access procedure;
  type Acc_Type is access procedure;
  Acc2 : Acc_Type;

  procedure Store (Arg : access procedure) is
  begin
    Acc1 := Arg;
    Acc2 := Arg;
  end Store;
  procedure Call is
  begin
    Acc1.all;
    Acc2.all;
  end Call;

end Pack;
--------------------------------------
with Ada.Text_Io;
with Pack;
procedure Tes is
begin
  declare
    I : Integer := 21;
    procedure Proc is
    begin
      Ada.Text_Io.Put_Line ("In Proc" & I'Img);
    end Proc;
  begin
    Pack.Store (Proc'Access);
  end;

  declare
    I : Integer := 22;
  begin
    Pack.Call;
  end;
end Tes;

This produces the output:
In Proc 21
In Proc 21
despite my attempt to "overwrite" 21 by 22 :-)


So it seems that the feature is rather to guarantee persistence of the
referenced sub-program and its scope as long as a reference to it is
stored somewhere.
Is it so much "magic"?

How long are Proc and I=21 kept in memory for further calls to "Call".
Are they cleared if I call Pack.Store with an access to another
procedure?

Thank's