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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Comparing Access Types
Date: Thu, 9 Nov 2017 09:33:49 +0100
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Xref: feeder.eternal-september.org comp.lang.ada:48779
Date: 2017-11-09T09:33:49+01:00
List-Id: <comp.lang.ada>

On 09/11/2017 06:37, Jere wrote:
> I have a package which uses a type to wrap an access type.  I thought it
> might be nice to provide the basic comparison functions, "=", "<", ">", etc.
> but access types can only be compared using "=" and "/=".  I figured that
> way, my types could be used in some sort of sorted container, like a map.
> 
> I want to keep the incomplete type for other reasons, so that seems to limit
> my options.
> 
> 1.  Address_To_Access_Conversions is out (cannot use an incomplete type)
> 2.  I cannot simply do var.all'Address (Again, incomplete type)

Of course you can.

> Is there some way to do this within the Ada standard or am I hamstrung
> due to the incomplete type specification?
> 
> Also, why weren't <, >, <=, >= provided for access types.  Even if the
> representation of an Access type is implementation defined, surely
> those operators could have been defined.

No, in general case it cannot, in the spirit of the semantics that A < B 
if the physical machine address of A < B. Actually even equality cannot 
be for segmented memory but it felt good enough.

> A toned down example:
> 
> generic
>     type Item_Type(<>);
>     type Item_Access is access Item_Type;
> package My_Package is
> 
>     type Some_Type is record
>        Reference : Item_Access := null;
>     end record;
> 
>     function "<" (Left,Right : Some_Type) return Boolean;
> 
> end My_Package;
> 
> package body My_Package is
> 
> function "<" (Left,Right : Some_Type) return Boolean is
> begin
>     return Left.Reference < Right.Reference;  --fails to compile
> end "<";

You have two options:

1. Compare addresses, e.g.

with System; use System;

function "<" (Left,Right : Some_Type) return Boolean is
begin
    return (  Right.Reference /= null
           and then
              (  Left.Reference = null
              or else
                 Left.Reference.all'Address < Right.Reference.all'Address
           )  );
end "<";

2. A better way is to use the native object's order:

generic
    type Item_Type(<>);
    type Item_Access is access Item_Type;
    with function "=" (Left, Right : Item_Type) return Boolean is <>;
    with function "<" (Left, Right : Item_Type) return Boolean is <>;
package My_Package is

function "<" (Left,Right : Some_Type) return Boolean is
begin
    return (  Right.Reference /= null
           and then
              (  Left.Reference = null
              or else
                 Left.Reference.all < Right.Reference.all
           )  );
end "<";

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de