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,b860b4e8d00468ef
X-Google-Attributes: gid103376,public
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news2.google.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: "Nick Roberts" <nick.roberts@acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Ada access vs C/C++ pointers and references
Date: Thu, 19 Aug 2004 01:21:37 +0100
Message-ID: <opscydybapp4pfvb@bram-2>
References: <b47de02.0408181427.25d82fe1@posting.google.com>
Mime-Version: 1.0
Content-Type: text/plain; format=flowed; delsp=yes; charset=iso-8859-1
Content-Transfer-Encoding: 8bit
X-Trace: news.uni-berlin.de nzXxLXrxhxBMFtXYuTgBPQJ/V4hmuQ2xpVqXELj9BU2pHk4QA=
User-Agent: Opera M2/7.51 (Win32, build 3798)
Xref: g2news1.google.com comp.lang.ada:2824
Date: 2004-08-19T01:21:37+01:00
List-Id: <comp.lang.ada>

On 18 Aug 2004 15:27:47 -0700, Keith H Duggar <duggar@mit.edu> wrote:

> [re array indexing]
> As you can see, method two saves one instruction by storing
> and manipulating a memory address directly.

In a loop, there is well known optimisation known (for reasons which
mystify me) as 'strength reduction', which allows a loop such as:

    for (i=0;i<n;i++) { ... a[i] ... };

to be transformed into the equivalent of:

    for (p=&a[0];p<&a[n];p++) { ... *p ... };

before emitting machine code. So an Ada loop such as:

    for i in 1..N loop
       ... A(i) ...
    end loop;

can be expected to be compiled into machine code which is as
efficient as the code that could be produced by using direct address
manipulation.

> First, is it correct to say that Ada access types are more
> similar to C++ references (T&) than they are to C/C++
> pointers (T*) ?

No. Ada access types are very similar to C and C++ pointers, and are
typically implemented and used in (almost) exactly the same way.

The rough Ada equivalent of C++ references are 'aliased objects'.

> Second, does Ada provide any facilities for direct and raw
> memory access by memory address and pointer arithmetic?

Yes it does. There are standard packages System.Storage_Elements and
System.Address_To_Access_Conversions, which provide facilities for
converting between access types and addresses, and performing integer
arithmetic on addresses, as well as Interfaces.C and
Interfaces.C.Pointers which enable Ada programs to access C strings,
arrays, and pointers.

> Third, if Ada does not provide arithmetic pointers do you
> know if this has played any part in acceptance of Ada for
> systems programming or efficiency critical applications?

I don't know (sorry).

> Fourth, have you experienced or can you think of any cases
> where pointer arithmetic has helped to solve a problem that
> would have been more difficult with Ada access types alone?

Not that I can recall.

-- 
Nick Roberts