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From: johnherro@aol.com (John Herro)
Subject: Re: access variable Q:
Date: 1996/06/19
Message-ID: <4q8bnm$3eh@newsbf02.news.aol.com>#1/1
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sender: root@newsbf02.news.aol.com
references: <31C43F54.74E62073@jinx.sckans.edu>
organization: America Online, Inc. (1-800-827-6364)
newsgroups: comp.lang.ada
Date: 1996-06-19T00:00:00+00:00
List-Id: <comp.lang.ada>


David Morton <dmorton@jinx.sckans.edu> writes:
> type int_ptr is access integer;
> type foo is record
>    a, b, c : integer;
> end record;
> bar : foo;
> a_ptr := ??????  -- how do I make this point at bar.a  ????
     In Ada 83, you can't point at an object that has a name, but only at
(nameless) objects created with "new."  But in Ada 95 we can do what you
want.  When declaring the object, give the type as "ALIASED Integer"
instead of just Integer.  When declaring the access type, use "access ALL"
instead of just access.  Then set your pointer to <object name>'ACCESS. 
For example, here's a complete program that displays 316 by making A_Ptr
point at Bar.A:
with Ada.Text_IO; use Ada.Text_IO;
procedure Test is
   type Int_Ptr is access all Integer;
   type Foo is record
      A, B, C : aliased Integer;
   end record;
   Bar   : Foo;
   A_Ptr : Int_Ptr := Bar.A'Access;
begin
   Bar.A := 316;
   Put_Line(Integer'Image(A_Ptr.all));
end Test;
     There's much more to this topic.  For example, because Ada is
designed to catch as many errors as possible at compile time, Ada will
make sure that the lifetime of the object pointed at is at least as long
as that of the access type.  This is called an "accessibility check," and
it avoids the danger of dangling references which plague many C/C++
programs.  If you like to live dangerously, you can deliberately override
the accessibility check.  Also, it's possible to request read-only access
to the object pointed at, and to point at *subprograms*.  Thus, dynamic
dispatching and callback are both possible.  There's more information in
my Ada Tutor program, available for download at the Web and FTP sites
below my signature.
     I hope this helps.
- John Herro
Software Innovations Technology
http://members.aol.com/AdaTutor
ftp://members.aol.com/AdaTutor