From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: 103376,23ca868289d9f0c
X-Google-Attributes: gid103376,public
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news2.google.com!news4.google.com!news3.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!wns14feed!worldnet.att.net!attbi_s72.POSTED!53ab2750!not-for-mail
From: "Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>
Organization: jrcarter at acm dot org
User-Agent: Thunderbird 1.5.0.7 (Windows/20060909)
MIME-Version: 1.0
Newsgroups: comp.lang.ada
Subject: Re: generic package with procedure paramter gives "not subtype
 conformant
 with declaration"
References: <1159651201.121690.130430@b28g2000cwb.googlegroups.com>
   <eQETg.174544$FQ1.97996@attbi_s71>
   <1159682538.644835.248030@m7g2000cwm.googlegroups.com>
   <3UUTg.1003037$084.701942@attbi_s22>
 <1159738009.962575.108920@i42g2000cwa.googlegroups.com>
In-Reply-To: <1159738009.962575.108920@i42g2000cwa.googlegroups.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Message-ID: <OD%Tg.220137$1i1.136242@attbi_s72>
NNTP-Posting-Host: 12.201.97.213
X-Complaints-To: abuse@mchsi.com
X-Trace: attbi_s72 1159759470 12.201.97.213 (Mon, 02 Oct 2006 03:24:30 GMT)
NNTP-Posting-Date: Mon, 02 Oct 2006 03:24:30 GMT
Date: Mon, 02 Oct 2006 03:24:30 GMT
Xref: g2news2.google.com comp.lang.ada:6833
Date: 2006-10-02T03:24:30+00:00
List-Id: <comp.lang.ada>

cl1 wrote:
> 
> This is a very good concept. Is there a mechanism in ada to allow:
> 
> My_Void_Ptr := My_C_Float'Access;
> 
> where My_C_Float could be My_New_Type or My_Integer, etc.
> I ask, because my code does this and i was using the System.Address as
> a catch all. Once i store the access in the pointer i no longer need to
> know
> what type it is. I just need the reference to pass to the C code. For
> instance:

If the access values come from Ada, it gets a bit more complicated. 
However, C pointers are convertible from one type to another, IIRC, so 
that works for you. You need to declare a convention-C access type for 
the type you use, store the 'access in one of those, then unchecked 
convert that value to the void pointer type. A generic can make that easier:

with Ada.Unchecked_Conversion;
...
type Void_Ptr is ...
...
generic -- Void_Conversion
    type T (<>) is limited private;
package Void_Conversion is
    type T_Ptr is access all T;
    pragma Convention (C, T_Ptr);

    function To_Void_Ptr is new Ada.Unchecked_Conversion
    (Source => T_Ptr, Target => Void_Ptr);
end Void_Conversion;

Then you can do

package C_Float_Convert is new Void_Conversions (T => C_Float);

T_Ptr : constant C_Float_Convert.T_Ptr := My_C_Float'access;

My_Void_Ptr : Void_Ptr := C_Float_Convert.To_Void_Ptr (T_Ptr);

> Now that i know that is the rule. I not only understand what was
> causing
> the problem, but why.

That's usually a good thing.

-- 
Jeff Carter
"Monsieur Arthur King, who has the brain of a duck, you know."
Monty Python & the Holy Grail
09