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From: "REH" <spamjunk@stny.rr.com>
Newsgroups: comp.lang.ada
Subject: Re: volatile vs aliased
Date: 6 Oct 2005 06:40:36 -0700
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Date: 2005-10-06T06:40:36-07:00
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Randy Brukardt wrote:
> This seems like a good time to mention that I think the explicit use of
> System.Address in Ada 95 and Ada 200Y code is usually a mistake. Since
> pragma Convention can be used to ensure that general access types have the
> appropriate representation, its rare that Address needs to be used for
> interfacing. (There is only a handful of uses of Address in Claw, for
> example.)
>
> Moreover, when you *do* need to use it, Address_to_Access_Conversions is the
> best way to convert it, not an overlay (which at best blocks optimizations
> and at worst won't even work right).
>
> type T is ...;
>
> procedure Read_Variable (At_Address : in  System.Address;
>                          Into       : out T) is
>    package AAC is new System.Address_to_Access_Conversions (T);
>    V : AAC.Object_Pointer := AAC.To_Pointer (At_Address);
> begin
>     -- perform my own explicit validation of the pointer's contents, perhaps
>     -- using 'Valid; then, copy into Into:
>     Into := V.all;
> end Read_Variable;
>
> But it is better still to declare an appropriate type and never use Address
> in the first place:
>
> type Pointer_T is access all T;
> pragma Convention (C, Pointer_T);
>
> Address clauses should be restricted to mapping to hardware, IMHO.
>
>                           Randy.
Randy,
That's good stuff.  Will it still apply when the address is to a
subprogram?  Can Address_to_Access_Conversions be instantiated with a
subprogram?  That's what I've been trying to do, but I don't know how
to make a generic that takes an access to an arbitrary subprogram type.
 So, I think I am "stuck" using the for X'address specification.

Thanks,
REH